Southern England
Southern England
Peter Friend
Illustrated with beautifully detailed photographs throughout, New Naturalist Southern England comprehensively explores the formation of these wonderful landscapes that are so universally admired.Most people share an enthusiasm for beautiful and breathtaking scenery, explored variously through the physical challenge of climbing to the top of the tallest mountains or the joy of viewing the work of a painter; but while easy to admire from a distance, such landscapes are usually difficult to explain in words. Harnessing recent developments in computer technology, the latest New Naturalist volume uses the most up-to-date and accurate maps, diagrams and photographs to analyse the diverse landscapes of Southern England.Peter Friend highlights the many famous and much loved natural landscapes of the southern half of England, ranging from the Chalk Downs to the bays of Cornwall, Devon and Dorset, and provides detailed explanations for the wide variety of natural events and processes that have caused such an exciting range of surroundings.Setting apart the topography that has resulted from natural rather than man-made occurrences, Friend focuses on each region individually, from East Anglia to London and the Thames Valley, and explains the history and development of their land structures through detailed descriptions and colourful diagrams.
Collins New Naturalist Library
108
Southern England
Peter Friend
Editors (#ulink_f10562bc-1341-55d4-bf0e-9e6fe918e733)
SARAH A. CORBET, ScD
PROF. RICHARD WEST, ScD, FRS, FGS
DAVID STREETER, MBE, FIBIOL
JIM FLEGG, OBE, FIHORT
PROF. JONATHAN SILVERTOWN
The aim of this series is to interest the general reader in the wildlife of Britain by recapturing the enquiring spirit of the old naturalists.
The editors believe that the natural pride of the British public in the native flora and fauna, to which must be added concern for their conservation, is best fostered by maintaining a high standard of accuracy combined with clarity of exposition in presenting the results of modern scientific research.
Table of Contents
Cover Page (#u3c0aa122-d385-5c11-9855-f0b69092c134)
Title Page (#uc79fc63d-decc-53f4-a7ec-8f9b32ee7463)
Editors (#u69efca08-d8f3-5d5e-b47d-cd5c3105b42f)
Editor’s Preface (#u7566d457-b797-5829-8640-e44f8b471629)
Picture Credits (#u6afac44d-9de2-5770-b9cd-97dfcba07701)
Author’s Foreword and Acknowledgements (#u41ed23fd-8fa8-58c3-bf6a-9564e883f457)
CHAPTER 1 Looking at Southern England’s Landscapes (#u5c6cc703-5cb5-5819-862c-f0329bf3fead)
CHAPTER 2 Time, Process Southern England’s Landscapes (#u0cb14e9f-2ff6-54ec-9aa9-fb1a8ec7fb44)
CHAPTER 3 Movement of the Earth’s Surface from Within (#ue7c83b75-53b6-5a6c-a9f2-bc327881ada7)
CHAPTER 4 The Southwest Region (#u7edeae7e-1d21-530b-b0bc-f84f0aa0be57)
CHAPTER 5 The South Coast Region (#litres_trial_promo)
CHAPTER 6 The Severn Valley Region (#litres_trial_promo)
CHAPTER 7 London and the Thames Valley Region (#litres_trial_promo)
CHAPTER 8 The East Anglia Region (#litres_trial_promo)
CHAPTER 9 The Making of Southern England (#litres_trial_promo)
Further Reading (#litres_trial_promo)
Index (#litres_trial_promo)
The New Naturalist Library (#litres_trial_promo)
About the Author (#litres_trial_promo)
Copyright (#litres_trial_promo)
About the Publisher (#litres_trial_promo)
Editor’s Preface (#ulink_d39314b2-93a0-555f-af82-eeb074bbd2ce)
L. DUDLEY STAMP’S Britain’s Structure and Scenery was one if the earliest books of The New Naturalist Library, published in 1946. Repeated . later editions in the period to 1986 testified to the success of his approach towards providing a geological framework for understanding Britain’s landscapes and natural history. He began his account in these words: ‘The wealth of a country’s fauna and fauna and flora is not to be measured by numbers of species alone. Its wealth lies rather in variety, and to a naturalist in the British Isles, the fascination of the native fauna and flora is in the great variety to be found in a small space.’ This variety has its foundation in the underlying geology and the landscapes which are derived from the geology, as Dudley Stamp described so well. For some time, it has been the ambition of the Editors to approach this subject again, since our understanding of the geology and associated landscape evolution has increased so very significantly in the last few decades. The author, Peter Friend, has had long experience of active field research in geology and landscape in many parts of the world, as well as having an intimate knowledge of the subject in the British Isles. He has been able to take full advantage of modern developments in computer mapping and colour printing, making it possible to present the subject in a novel fashion, with great clarity, following the New Naturalist tradition emphasising the importance of illustration. The individual treatment of regions and areas of Southern England brings to the fore the significance of geology and landscape for naturalists who have local or wider interests at heart, giving a necessary basis for relating biodiversity to geodiversity. These two aspects of natural history have come to be seen to be widely significant in understanding plant and animal distribution as well as the problems of conservation. The book is therefore a very ti mely addition to the New Naturalist Library.
Picture Credits (#ulink_d5bd70c2-1c48-5314-adcd-374f52fe7d27)
THE PHOTOGRAPHS and other illustrations that form such a key part of this book have come from many sources, and I am grateful to the following organisations and individuals for kindly allowing me to use their material:
Aerofilms (Figs 133, 232, 249, 252, 256, 258, 259, 261, 270, 280, 309, 312)
British Geological Survey (Figs 152, 284)
Cambridge News (Fig. 233)
Cambridge University Collection of Air Photographs (Figs 25, 118, 120, 243, 253, 282, 300, 310)
Cassini Publishing (Figs 26, 27)
Sylvia Cordaiy Photo Library (Fig. 141)
Robert Harding Picture Library (Fig. 313)
English Heritage (Figs 83, 308)
Landform Slides – Ken Gardner (Figs 17, 18, 43, 55, 70, 75, 76, 85, 88, 104, 161, 162, 165, 195)
Landform Slides – John L. Roberts (Fig. 69)
Last Refuge Ltd – Adrian Warren, Dae Sasitorn and Will Brett (Figs 41, 42, 46, 47, 53, 54, 56, 58, 60, 62, 63, 67, 68, 71, 72, 73, 74, 82, 86, 89, 90, 91, 101, 102, 103, 108, 110, 111, 112, 119, 121, 122, 125, 130, 132, 134, 135, 142, 144, 146, 148, 158, 163, 164, 169, 171, 175, 176, 185, 191, 192, 193, 194, 196, 216, 217, 218, 219, 220, 222, 254)
London Aerial Photo Library (Figs 23, 199, 205, 207, 235, 257, 277, 297, 298, 303, 314)
Norfolk Museums and Archaeology Service – Nick Arber (Fig. 16)
Norfolk Museums and Archaeology Service – Derek A. Edwards (Figs 1, 287, 293, 301, 302, 306, 307, 315)
Network Rail (Fig. 145)
Peter Oliver, Herefordshire and Worcestershire Heritage Trust (Fig. 172)
Mike Page (Fig. 311)
Science Photo Library (Fig. 2)
Sedgwick Museum, Cambridge (Fig. 239)
R. C. Selley – Petravin Press (Fig. 202)
Sheila Smart (Fig. 131)
Suffolk County Council (Fig. 255)
Victoria & Albert Museum (Fig. 250)
Illustrations that do not have a source credited in the caption are my own work, or that of the team working with me at the Department of Earth Sciences in Cambridge.
Author’s Foreword and Acknowledgements (#ulink_c0a9d3d2-e3de-52fa-acb0-ac6d8008bc9c)
MOST PEOPLE ENJOY SCENERY. In my case, an enthusiasm for exploring the countryside was learnt early on from my parents, and my career as a geologist has since allowed me to explore landscapes from the Arctic to tropical deserts and jungles. My hope is that this book will help more people to enjoy the countryside by bringing together some of the exciting recent discoveries about our Earth.
Landscapes are easy to look at, but difficult to describe in words. Recent developments in computer technology offer powerful ways of analysing and presenting landscapes using maps, diagrams and photographs, and it is this imagery that forms the core of this book. Developing the imagery has been the main role of a succession of enthusiastic helpers. Lucinda Edes, Emilie Galley, Liesbeth Renders and Helena Warrington brought their skills and enthusiasm to the early days of the project, working out what could be done best. James Sample has more recently further developed the methods of presentation, and has played a key role in bringing this project to fruition. All have helped to make the project enjoyable as well as productive.
The home for this project has been the Department of Earth Sciences at the University of Cambridge. I walked into the Department as a first-year undergraduate more than 50 years ago and, apart from a period in the Scott Polar Research Institute, I have been based here ever since. I have been teaching and exploring the scenery and geology of many parts of the world, including multiple visits to Spitsbergen, Greenland, Spain, India and Pakistan. This has been an exciting period to be working in geology, particularly in Cambridge, because many key advances have been achieved by the people working here. Apart from the great benefit of being part of this research environment, I have enjoyed the support of six successive Heads of Department and many other colleagues, especially our administrator Margaret Johnston and her team. It has been invaluable to have access to the excellent library run by Ruth Banger and Libby Tilley, and the patient computer support of Jun Aizawa, Aidan Foster, Pete Hill and Pete Wilkins.
I would also like to acknowledge my debt to the Cambridge college system, particularly my own college, Darwin. The College has provided me with the congenial friendship of many people from diverse backgrounds, and their skills have helped me to remain a generalist in my interests.
Any work of this sort on the British Isles owes a fundamental debt to the British Geological Survey (BGS), now based at Keyworth near Nottingham. The numerous Survey maps and reports on this country provide a remarkable source of carefully observed and objective information. The BGS has also readily provided advice and discussion of this project, and helped to determine the sort of coverage and level that would be best.
Many other people have made important contributions by providing ideas and information. These include: John R. L. Allen, Julian Andrews, Muriel Arber, Steve Boreham, Becky Briant, Keith Clayton, Tony Cox, Alan Dawn, Colin Forbes, Brian Funnell, Phillip Gibbard, Steve Jones, Gerald Lucy, Dan Mackenzie, Bob Markham, Charles Notcutt, Bernard O’Connor, Richard Preece, Graham Ward and Richard West.
This book is dedicated to the Dr John C. Taylor Foundation, which has provided the financial support for the project, allowing me to work with such a remarkable succession of talented young assistants. More than 40 years ago, John spent two summers exploring the geology of Spitsbergen with me, and we have remained friends ever since. I am extremely grateful for the help of his foundation during the writing of this book.
CHAPTER 1 Looking at Southern England’s Landscapes (#ulink_ba3f8aa6-8516-5500-a71c-901f189eec8f)
FIRST APPROACHES
THE WORD LANDSCAPE has different meanings for different people, and the best way to illustrate the meaning I have adopted in this book is to look at an example. I have chosen a landscape in the northwest corner of Norfolk, part of our East Anglia Region (Fig. 1).
FIG 1. Landscape of the northwest corner of Norfolk. (Copyright Norfolk Museums and Archaeology Service & Derek A. Edwards)
My approach is to focus first on the natural features that we can call landforms, because they have distinctive shapes that directly reflect the processes that formed them. In this Norfolk landscape, coastline landforms are clearly defined but are remarkably varied, ranging from sea cliffs to sandy beaches, gravel spits, wind-blown dunes and salt marshes. Inland, the main features in this photo are the groups of buildings that form the villages and the edge of the town of Hunstanton, and the pattern of fields and woods. All of these are man-made features and are best understood by following the work of landscape historians. My interest is primarily in the natural topography on which these man-made features have developed, because even in this rather flat landscape – and not clearly visible on the photograph – there are gentle hills, valleys and streams that I want to try to understand.
Scale and size in landscapes are important considerations that we will return to frequently. The landscapes that we shall be discussing are generally kilometres to tens of kilometres across, and they are often best examined from the air, or by using computer-based maps with exaggerated vertical scale.
Southern England contains many famous and well-loved natural landscapes, ranging from the Chalk Downs, with their unique flora and fauna, to the rocky promontories and bays of Cornwall, Devon and Dorset. In total topographic contrast, the Fens of East Anglia are regarded by some as representing an extreme absence of any scenery at all, but their remarkable flatness is of interest because they are the result of recent sea-level rise, and of engineering on a remarkable scale. These different landscapes are produced by a wide variety of events and processes; exploring these is the theme of this book.
As we have already seen, landscapes have often been extensively modified by people. The early clearance of woodland and the construction of field boundaries have profoundly changed the scenery and, more recently, the construction of buildings, roads, railways, canals and airports has almost completely covered some areas of Southern England. Figure 2 shows night-time lighting in cities, towns and oil platforms, giving a vivid impression of the present extent and distribution of the larger settlements. It is surprising how varied the population density is, even in crowded Britain. Using the figures for 2002, the population density of the UK overall is 244 people per square kilometre, but this conceals a huge variation: 8 people per square kilometre in the Highland Region of Scotland, 143 for Cornwall, 149 for Norfolk and an amazing 13,609 for Kensington and Chelsea in London.
The main focus of this book is the pattern of large scenic features that have resulted from natural episodes that predate human influence. It is not usually difficult to distinguish the natural from the man-made, and the study of the natural can often explain many aspects of the way our ancestors lived in the landscape. It is possible to uncover the reasons why people have chosen to settle with their families in certain places, why villages have grown by the clustering of houses in particular locations, and why some villages have then grown further and turned into towns and eventually cities. Even the roads, railways and airfields have clearly grown using the valley floors, river crossings, better-drained slopes and plateaus that are part of the natural scenery.
FIG 2. Satellite image over Britain showing artificial lighting at night. (Copyright Planetary Visions Ltd/Science Photo Library)
There is a further enjoyment that people find in landscapes and scenery that is more difficult to understand. Is it just the physical challenge that causes people to walk and climb to the tops of hills, mountains and other viewpoints? Why do people enjoy the work of landscape painters and photographers? Why do so many tourists in cars choose to take ‘scenic’ excursions rather than the shortest routes, and why is the preservation of ‘unspoilt’ or wilderness areas now such a popular cause? It is difficult to understand the various emotions involved, and trying too hard to analyse them may be missing the point. So it seems best to hope simply that this book will help to satisfy some people’s curiosity, and at the same time add to their enjoyment of our natural landscapes.
MAPPING AND ANALYSING SOUTHERN ENGLAND
The detailed discussions of most of the rest of this book have involved dividing Southern England into a number of Areas that form the ‘building blocks’ for the coverage of Southern England (Fig. 3). Each Area is based on a double-page spread of the size used in many of the larger road atlases available for Britain. In this case I have used the Collins Road Atlas, Britain. This means that total coverage of Southern England is provided, and it is easy for the reader to navigate from place to place. At the beginning of each Area description, a location map of the Area and its neighbours is provided. Ordnance Survey (OS) National Grid References are provided for the edges of the Area, in km east and north of the arbitrary OS Grid origin some 80 km west of the Scilly Isles.
FIG 3. Division of Southern England into Regions and Areas.
For convenient reference the Areas – numbered 1 to 16 – are grouped into five Regions. Each Region forms a chapter and starts with a general introduction:
Even the Area building blocks are relatively large, with arbitrary boundaries, and it has generally been helpful to discuss smaller areas within and across these boundaries that are based on natural features of the scenery (Fig. 4). I have called these smaller areas Landscapes, because they are characterised by distinctive features, usually reflecting aspects of the bedrock or distinctive events in their evolution.
These Landscapes correspond closely to area divisions of England that were defined by the Government Countryside Agency (www.countryside.gov.uk). This scheme divides England into 159 ‘character areas’ on the basis of natural features of the scenery along with aspects of its human settlement, past and future development, land use and vegetation and wildlife, so they are likely to be familiar divisions to many readers of the New Naturalist series. Other Government agencies (particularly the Department for Environment, Food and Rural Affairs) that administer the funding of land management use the same character area division.
FIG 4. Examples of the three levels of division adopted in the treatment of Southern England.
Maps displaying patterns of elevation of the countryside above sea level are an important part of the discussions. The elevation data on the maps in this book have been compiled and made available as part of the LANDMAP project, which provides a computer-based digital survey of Britain for research and educational use. LANDMAP Digital Elevation Maps (DEMs) are based on satellite radar survey measurements which divide the land surface into a grid of 25 m by 25 m pixels. The average height of each pixel is then measured to produce a terrain model with a vertical accuracy of about ± 5 m. A standard colour shading scale is used to represent heights, ranging from greens for the lowest ground, through yellows and browns, to greys for the highest ground. It is best to use the full range of colours for each map, no matter what numerical range of heights is involved. This makes it possible to convey the fine detail of slopes etc., whether the map is for the Fens or the high moors. To make it possible to compare between maps using this colour scheme, we have quoted the maximum elevation reached in each Area on each map.
I have used ESRI ARC Geographic Information System (GIS) software in the processing and manipulating of the map data. This software makes it possible to present artificial hill-shading, which makes the topography easier to understand, and to provide maps representing slope patterns in certain areas.
In addition, data on roads, railways, coastlines, town boundaries, rivers, etc. suitable for reproduction at a scale of 1: 200,000 has been made available by the Collins Bartholomew mapping agency. For any further details of the areas covered, it is recommended that Ordnance Survey Landranger (1: 50,000) maps are consulted.
LANDSCAPE CHANGE
We tend to think of rural landscapes as unchanging features of our surroundings, in contrast to the man-made scenery of cities and towns. Yet we all know of local catastrophes, such as a sea cliff collapsing during a storm, or a flooding river removing its bank and wrecking the nearby buildings, and these are the sorts of local events that do result in change. Despite the excitement, individual changes of this sort are small and can usually be regarded as local modifications. However, over time, the accumulated effects of many such modifications can cause whole landscapes to change.
Size and time clearly both play key parts here. The collapsed cliff or eroded river bank will probably be tens to hundreds of metres long at most, while the larger landscape features picked out in this book are tens or even hundreds of kilometres across. Noting the length scales involved in this way is an important way of keeping such differences clearly in mind.
Moreover, while local events such as the destruction of landforms or buildings may be immediately newsworthy, more long-term patterns of change in the natural scenery are rarely apparent during the life spans of people, and even during the hundreds of years of written records. So it becomes necessary to use indirect and circumstantial evidence – to play the detective – to find out what long-term changes have been going on.
An important step in thinking about the natural landscape is to look at it in terms of modifications to complex surfaces defined by the ground. On land, we tend to be most aware of erosional processes removing material, but it is important to realise that the material removed has to end up somewhere – and this will involve its deposition later, on land or in the sea. How much material was removed from the cliff during the storm or from the banks of the river during the flood? Where did the lost material go, and how did it change the landscape when it was deposited at its new destination? Knowledge of these surface modifications can provide a yardstick that allows us to compare different sorts of changes happening over different periods of time and at different scales, and can help us to work out their relative importance, quoting amounts and rates. For example, a flooding river may remove a hundred metres of river bank, modifying the local landscape a little in the process. However, this modification is unlikely to have much impact on the scenery, unless followed many, many times by similar modifications, over centuries to hundreds of thousands of years. In this way a series of such floods can erode and move material that, in the long run, may be of sufficient volume to significantly change the landscape, for example lowering a hill slope or filling a valley bottom.
The majority of- but not all – surface modification processes act to reduce or flatten topography, mainly by eroding the higher features but also by filling in lower ground with sediment. So logically landscapes might always be regarded as tending towards a flat surface. Our understanding of the processes involved suggests that any land area with mountains or hills will be eroded downwards to an increasingly flat surface as time passes, although the rate of erosion will reduce as the topography becomes more and more smooth. Acting against these flattening processes are periodic movements of the ground surface caused by forces within the Earth, producing new mountains and hills, and so creating new landscapes (Fig. 5).
Continuing research into the processes operating within the Earth shows that movements of the Earth’s crust are taking place continuously, even though the rates involved are generally too slow to be noticeable. The discovery that the Earth’s surface consists of a large number of tectonic plates in continuous relative movement was one of the major breakthroughs in the earth sciences, and has fundamentally changed our understanding of the planet. More on this topic will be considered in Chapter 3, but at this point it is important to realise just how slow the rates of movement are: at most a few centimetres per year on average (often compared to the rate at which fingernails grow). Occasionally, movements of centimetres or metres occur within seconds along faults during earthquakes, but the average rate of movement is still rather slow. Most of us living in stable areas are totally unconscious of any movement at all because we are, ourselves, moving slowly with the landscape that we live on. Slow though the movements may be in a particular landscape, so also are the rates of surface modification, and the balance between the two is a delicate one. In much of Southern England modification by surface processes is dominant, but this has not always been the case.
FIG 5. Landscapes are changed by surface modifications (Chapter 2) and solid earth movements (Chapter 3).
Our next chapter deals with the timescales represented in the landscapes of Southern England and the processes that have been modifying them. Chapter 3 deals with the movements from below – from within the Earth’s crust – that are ultimately creating major landscape patterns.
CHAPTER 2 Time, Process Southern England’s Landscapes (#ulink_77560cca-d3bc-5dd8-9cc3-96fb80e32610)
BEDROCK AND SURFACE BLANKET
WALK AROUND THE COUNTRY IN SOUTHERN ENGLAND and the ground beneath your feet is very rarely solid rock. You are walking over soil made of weathered mineral grains and organic debris, along with other relatively soft and granular materials that make up the surface blanket. Beneath the surface blanket lies solid rock, the bedrock of the landscape.
Bedrock forms the bones of the land. From the colour of the soil, to the elevation of the hills, to the types of vegetation present, the landscape is profoundly influenced by the bedrock underlying it. For example, in Southern England the Lower Greensand (a distinctive layer of bedrock of Early Cretaceous age, see page 26) produces soil water with acidic chemical properties. The Lower Greensand was originally deposited as sand over a period of a few million years, more than 100 million years ago. This layer represents a different environment of deposition from the older sediments on which it lies, and was followed by another change of environment which produced the deposits that lie on top of it. Both the preceding and the following bedrock deposits have alkaline chemical properties. In certain regions the bedrock layers have now been brought to the surface of the landscape by erosion and movements within the Earth. The Greensand is harder than the layers above and below it (largely mudstones) and so is generally more resistant to weathering. In some areas the Lower Greensand lies just below the surface blanket and has resisted the general landscape erosion to form a distinct Greensand ridge running across the countryside, characterised by special vegetation adapted to the acidity of the soils.
It is only in cliffs or at man-made excavations such as quarries that we can see bedrock at the surface in most low-lying areas. By using those areas where the bedrock does outcrop at the surface, and the results of drillings (e.g. for wells), we can discover the types and arrangements of rock below any landscape.
THREE DIFFERENT TIMESCALES
More recent past events tend to be better known and of greater interest than distant past events. Figure 6 is plotted on a logarithmic timescale, so that the most recent times are given more space and greater ages are given less and less space.
FIG 6. Three different timescales, plotted to give more space to more recent events.
For the purposes of this book, we can distinguish three overlapping timescales to help us to understand the landscapes of Southern England:
The bedrock timescale (extending from 542 million years ago to about 2 million years ago)
The Ice Age timescale (covering roughly the last 1 million years)
The last 30,000 years timescale
We shall now review each of these, commenting on the sorts of episodes in each that are important in our exploration of Southern England.
THE BEDROCK TIMESCALE
Figure 7 is a simplified version of a generally accepted geological timescale relevant to the landscapes of Southern England. The names of the divisions are universally accepted in the geological world and, unlike the previous diagram, the passage of time is represented on a uniform (linear) timescale. The divisions have been selected, and sometimes grouped, to help in our analysis of the situation in Southern England, and these have been colour-coded for use in the rest of the book.
FIG 7. Bedrock timescale for Southern England.
The rocks at, or just below, the surface of Southern England range in age over hundreds of millions of years, and most of them were formed long before the present scenery began to appear. At the time of their origin, these rocks were deposited in a variety of different environments, mostly when mud or sand materials were transported into and/or around the seas that existed where England is now. Most of the bedrock of Southern England was formed in this way and is said to be of sedimentary origin. The depositional conditions varied from time to time: the climate varied, the geographical pattern of rising and sinking land movements changed, and the supply of mud and sand brought downstream by rivers changed also. Despite these fluctuations, it is possible to generalise the way that sediment has accumulated over an area the size of Southern England, and to offer a succession of layers of different composition, age and average thickness that can provide a general guide. This is shown in Figure 8.
For each of the Regions (and some of the Areas) discussed in Chapters 4 to 8, a rock column, generalised for that particular area, will show the main bedrock layers. Each column will be coloured using the standard colour codes of this book to represent the ages of the layers.
As an example, we will consider another particularly distinctive layer of bedrock, the Chalk, which ranges between 200 and 400 m in thickness. Chalk is visible quite widely at or just below the surface over perhaps a quarter of the area of Southern England (Fig. 9). Chalk is an easily recognised rock because it is made of very small fragments of lime (calcium carbonate) and is usually brilliant white. It formed from fine-grained limey mud deposited on the sea bed, but through many millions of years of burial below other sediments it has been compressed and altered into the hard rock we recognise today. The Chalk is a result of a unique combination of environmental conditions and the presence of particular algal organisms in the history of evolution. It is only found in northwest Europe, and was only formed in Late Cretaceous times.
The presence of Chalk near the surface in Southern England is almost always linked to the presence of hills and slopes in the scenery, clearly showing that Chalk is a tough material that resists landscape erosion more than most of the other rock types available. The Chalk is also noteworthy because it represents the most recent time when most of Southern England was covered uniformly with soft sediment and a shallow sea: in Late Cretaceous times, except for possible islands in the southwest, there was no emergent land across Southern England.
Like all sedimentary bedrock layers, the Chalk initially formed as flat layers or sheets of sediment, extending widely across the floor of the sea. As will be discussed in the next chapter, these sheets of sediment are generally characteristic of stretching movement episodes in the Earth’s surface. Such movements produce areas of collapsed and low-lying land that can accommodate large volumes of sediment, if it is available.
FIG 8. Generalised succession of the bedrock of Southern England, showing a typical thickness for each layer.
FIG 9. The Chalk and its topography. The darker areas represent the chalk uplands.
However, we do not see the Chalk at or near the surface everywhere across Southern England; instead the Chalk forms narrow bands across the land. This is due to later movements affecting the bedrock layers by folding and tilting them, so that some parts were raised (to be later removed by erosion) and other parts were lowered (Fig. 10, A and B). In the millions of years since the sediment layers were laid down, they have been buried, compacted, deformed by various processes, and finally uplifted to form part of the landscape that we know today (deformation processes are treated more fully in Chapter 3). The Chalk layer has been moved and folded as a result of mild compression or convergence, to form a downfold or syncline between the Chilterns and the North Downs, and an upfold or anticline between the North and South Downs (Fig. 10, C). Later, the central part of the anticline was eroded away to produce the bedrock pattern that we recognise today (Fig. 10, D). The vein-like river valleys visible on the elevated Chalk hills of Figure 9 are evidence of this continuing erosion.
FIG 10. Deposition and folding of the Chalk.
LANDSCAPE MODIFICATION BY RIVERS
Weathering of landscape surfaces and the production of soils by the action of rainwater, air and organisms are important factors in shaping landscapes. These processes affect the bedrock when it is very close to the surface, and most of them weaken the material that they work on. This is particularly so when tough silicate rock minerals are altered to soft clay minerals, which are then easily eroded. Freezing and thawing also works to weaken the bedrock as water in cracks freezes and expands, breaking the rocks into fragments.
Whilst weathering is a widespread and general process, most of the other important landscape processes involve the formation of discrete features that we shall call landforms. Rivers result in the formation of a number of important landforms that are described below.
The most important landforms resulting from river processes are the channels of rivers and streams (Fig. 11). When rain falls onto a land surface some of it soaks into the land (forming groundwater), whilst the remainder runs along the surface, collecting in topographical lows and producing stream and river channels. Today, many of Southern England’s river channels tend to be relatively narrow and shallow – only metres or tens of metres in width and less in depth – so they occupy an extremely small percentage of the area that they drain. However, they are still the dominant agents of landscape change, causing downwards and/or sideways erosion as well as acting as conduits to transport the eroded material out of their catchments.
Most river channels develop a sinuous course, becoming curved (or meandering) to varying degrees, or developing a number of channels separated by islands of sediment (becoming braided). The positions of the curves or islands change with time as sediment is shifted downstream, and the position of a river channel will change with time correspondingly.
Because of their ability to erode material and remove the resulting debris, river channels create valleys. The sides of a river valley are referred to as slopes. When a channel cuts downwards the valley sides generally become steeper and slope material (generated by ongoing weathering processes) moves down-slope towards the channel. The material is transported either as small individual fragments or as larger mass flows. Where down-slope movements involve the collapse of large areas of material, the terms landslip or slump are often used. Slope material is then deposited in the channel and removed downstream by the river.
The simplest valleys result from down-cutting by a river or stream to yield a V-shaped profile in cross-section. The gradient of the valley sides depends on the strength of the material that the slopes are composed of in the face of erosion. Stronger materials are more difficult to erode and remove, and so can form steeper slopes than weaker materials. In some areas, the river channel is unable to form valley slopes as the material is too weak to form a noticeable gradient. In the Areas we will be investigating, it is clear that some of the slopes are largely the result of a particularly strong layer in the bedrock resisting erosion as the landscape has developed.
FIG 11. Landforms of rivers.
As the valley develops, its profile can become more complex. In some cases, slopes appear to have retreated across a landscape some distance from the position in which they were initially created by river down-cutting. A river with a wide valley floor is one of the most obvious examples of this, in which movements of the channel across the floor have caused the slopes to retreat as the valley floor has become wider. In some cases, slopes appear to have retreated over many kilometres from the original valley as numerous collapses of the slope took place.
Overall, therefore, the valley profile and the channel course reflect variations in the strength of the material being eroded, and in the strength and flood pattern of the river. Climate changes are likely to have a major effect on the strength of the river by altering the volume of water flowing through the channels. Additionally, the lowering or raising of the channel by Earth movement effects (see Chapter 3) can affect the evolution of the landscape by river processes. For example, both climate change and the vertical movement of the river channel can initiate the formation of river terraces. Different examples of all these river geometries will be discussed in greater detail in the Area descriptions in Chapters 4–8.
Over millions of years, river down-cutting, slope erosion and material transport tend to smooth and lower landscapes until they approximate plains, unless they are raised up again (rejuvenated) by large-scale Earth movements (Chapter 3) or are attacked by a new episode of channel erosion, perhaps due to climate or sea-level change. Southern England generally has a smoothed and lowered landscape, representing hundreds of thousands of years of this river and slope activity.
The branching, map-view patterns of river channels and valleys are an obvious feature of all landscapes. An approach to understanding how this forms is illustrated by a computer-based experiment (Fig. 12) in which a flat surface (plateau or plain) is uplifted along one of its edges, so that it has a uniform slope towards the edge that forms the bottom of the rectangle shown. Rain is then applied uniformly across the surface, causing the formation and down-cutting of channels that erode backwards from the downstream edge. As the experiment continues, the channels and their valleys extend into the uniform sloping surface by headward erosion, resulting in longer valleys, more branches and a greater dissection of the surface by those valleys.
FIG 12. Model showing upstream erosion by tree-like (dendritic) river patterns. (Provided by Dimitri Lague from the work of A. Crave and P. Davy)
As we consider the various Regions and Areas of Southern England, we will summarise the present-day river patterns of each by simplifying the main directions of drainage involved. We will also give an impression of the present-day relative size of the more important rivers by quoting their mean flow rates as estimated in the National River Flow Archive, maintained by the Centre for Ecology and Hydrology at Wallingford.
It seems surprising that today’s often sleepy southern English rivers have been the dominant agent in carving the English landscape. However, even today’s rivers can become surprisingly violent in what are often described as hundred-or thousand-year floods. Floods in the past were certainly more violent at times than those of today, particularly towards the ends of cold episodes, when melting of ice and snow frequently produced floods that we would now regard as very exceptional.
THE ICE AGE TIMESCALE AND LANDSCAPE MODIFICATION
The most recent Ice Age began about 2 million years ago, and is still continuing in Arctic areas. At various times during this period ice has thickly covered most of northwest Europe. Recent research, particularly measurements of oxygen isotopes in polar icecaps and oceanic sediment drill cores, has revealed much of the detail of how the climate has changed during the current Ice Age. It has been discovered that long cold periods have alternated with short warm periods in a complex but rather regular rhythm. Looking at the last half-million years, this alternation has occurred about every 100,000 years, and this is now known to have been a response to regular changes in the way the Earth has rotated and moved in its orbit around the sun. A closer look at the last million years (Fig. 13) reveals that for more than 90 per cent of the time conditions have been colder than those of today. Warm (interglacial) periods, like our present one, have been unusual and short-lived, though they have often left distinctive deposits and organisms.
FIG 13. The last million years of global temperature change. *the Oxygen Isotope Stages are an internationally agreed numbering sequence to label the succession of climatic cold (even numbers) and warm (odd numbers) episodes.
One of the most important cold episodes (glacials), just under half a million years ago, resulted in the Anglian ice sheet. This was up to several hundreds of metres thick and extended from the north southwards, well into Southern England, covering much of East Anglia and the north London area (Fig. 14). As the ice spread slowly southwards, it was constricted between the Chalk hills of Lincolnshire and those of Norfolk. A wide valley, later to become the Wash and the Fens, was filled with ice to a depth well below that of present sea level. As the ice spread outwards from this valley it dumped the rock material it was carrying, including blocks and boulders up to hundreds of metres across, giving some idea of the tremendous power of the ice sheet. The direct evidence for the presence of an ice sheet is material in the surface blanket called till, or boulder clay (Fig. 15). This often rather chaotic mixture of fragments of rock of all sizes (large boulders mixed with sand and mud) lacks the sorting of the fragments by size that would have occurred in flowing water, and so must have been deposited from the melting of ice sheets.
FIG 14. The Anglian ice sheet.
Much of the rest of the surface blanket that accumulated during the last 2 million years was deposited by the rivers that were draining the land or any ice sheets present. As ice sheets have advanced and retreated, so have the rivers changed in their size and in their capacity to carry debris and erode the landscape. Rivers have therefore been much larger in the past as melting winter snow and ice produced torrents of meltwater, laden with sediment, which scoured valleys or dumped large amounts of sediment. The gravel pits scattered along the river valleys and river terraces of Southern England, from which material is removed for building and engineering, are remnants of the beds of old fast-flowing rivers which carried gravel during the cold times.
There are no ice sheets present in the landscape of Figure 16. The scene is typical of most of the Ice Age history (the last 2 million years) of Southern England, in that the ice sheets lie further north. It is summer, snow and ice are lingering, and reindeer, wolves and woolly mammoths are roaming the swampy ground. The river is full of sand and gravel banks, dumped by the violent floods caused by springtime snow-melt. The ground shows ridges of gravel pushed up by freeze-thaw activity, an important process in scenery terms that we discuss below.
FIG 15. Boulder clay or till, West Runton, north Norfolk.
The present-day Arctic has much to tell us about conditions and processes in Southern England during the cold episodes of the Ice Age. Much of the present-day Arctic is ice-sheet-free, but is often characterised by permanently frozen ground (permafrost). When the ground becomes frozen all the cracks and spaces in the surface-blanket materials and uppermost bedrock become filled by ice, so that normal surface drainage cannot occur. In the summer, ice in the very uppermost material may melt and the landscape surface is likely to be wet and swampy. Ice expands on freezing, and so the continuous change between freezing and thawing conditions, both daily and seasonally, can cause the expansion of cracks and the movement of material, with corresponding movements in the surface of these landscapes. This movement can cause many problems in the present-day Arctic by disturbing the foundations of buildings and other structures.
FIG 16. Artist’s impression of Southern England, south of the ice sheet, during the Ice Age. (Copyright Norfolk Museums and Archaeology Service & Nick Arber)
Remarkable polygonal patterns, ranging from centimetres to tens of metres across, are distinctive features of flat Arctic landscapes, resulting from volume changes in the surface blanket on freezing and thawing (Fig. 17). In cross-section the polygon cracks and ridges correspond to downward-narrowing wedges (often visible also in the walls of gravel pits in Southern England). Thaw lakes are also a feature of flat areas under conditions of Arctic frozen ground (Fig. 18). They appear to be linked to the formation of the polygonal features, but can amalgamate to become kilometres across and may periodically discharge their muddy soup of disturbed sediments down even very gentle slopes.
Not only can these frozen ground processes be studied in Arctic areas today, but they have left characteristic traces in many of the landscapes of Southern England. Some examples from Norfolk are illustrated in Chapter 8 (Figs 306 and 307), and these provide specific examples of the result of ancient freeze-thaw processes on a small scale. However, the more we examine the wider features of present-day landscapes across Southern England, the more it becomes clear that most have been considerably modified by the general operation of frozen ground processes during the last 2 million years. These processes are likely to have been responsible for the retreat of significant slopes and even for the lowering of surfaces that have almost no perceptible slope.
FIG 17. Polygonal frozen ground patterns on the Arctic coastal plain near Barrow, Alaska. (Copyright Landform Slides – Ken Gardner)
FIG 18. Thaw lakes, the larger ones several kilometres long, on the Arctic coastal plain near Barrow, Alaska. (Copyright Landform Slides – Ken Gardner)
THE LAST 30,000 YEARS TIMESCALE AND RECENT MODIFICATION
The timescale shown in Figure 19 covers a period during which various episodes have changed the landscapes of Southern England, creating our present-day world. These episodes include the dramatic rise in sea level and landward movement of the coastline caused by the warming of the climate following the last cold episode of the Ice Age. They also include the progressive changing of the countryside by people, leading up to the domination of some landscapes by man-made features.
FIG 19. Time divisions for the last 30,000 years (Late Pleistocene to Holocene).
The last 30,000 years have been warm, on average, relative to the previous 2 million years of the Ice Age. However, the higher level of detail available in this timescale makes it clear that climate change has not been one of uniform warming during this period. Short periods of colder climate, temporarily involving ice-sheet growth in the north of Britain (sometimes called stadials) have alternated with short periods of warmer climate (referred to as interstadials).
SEA-LEVEL CHANGE
The coastline is the most recently created part of the landscape, and the most changeable. This is due, in large part, to the rise in sea level over the last 20,000 years, since the last main cold episode of the Ice Age (the Devensian). Twenty thousand years ago sea level was 120 m lower than it is today because of the great volumes of water that were locked away on land in the world’s ice sheets (Fig. 20). Land extended tens or hundreds of kilometres beyond the present-day coastline, and Southern England was linked to northern France by a large area of land (Fig. 21). Global climate started to warm about 18,000 years ago (Fig. 13) and the world’s ice started to melt, raising global sea level. The North Sea and the Channel gradually flooded, and Britain became an island between 10,500 and 10,000 years ago. This flooding by the sea is known as the Flandrian transgression and was a worldwide episode.
FIG 20. Graph of sea-level rise over the last 18,000 years.
During the period of most rapid sea-level rise (between 12,000 and 8,000 years ago), areas of low-lying land were swamped and some local features of the coastal scenery moved great distances geographically towards their present positions. The sea cliffs, beach barriers, salt marshes, spits and estuaries that can be seen today have only taken up their present positions over the last few thousand years, as sea-level rise slowed.
In the treatment of the Regions and Areas in the rest of this book, maps are presented that distinguish a coastal flooding zone. This presentation is based on the simplifying assumption that the solid Earth movement of Southern England (i.e. any uplift or subsidence, see Chapter 3) has been very small compared with global sea-level changes. The coastal flooding zone is defined as extending between the submarine contour 120 m below present sea level and the contour 20 m above present sea level, and it can be used to identify parts of landscapes which are likely to have been areas of coastline activity in the recent past. Areas of land with an elevation between present sea level and 120 m below sea level correspond to the land submerged during the last 18,000 years of sea-level rise. Areas lying at, or up to, 20 m above present sea level may have been subjected to coastal processes during the highest sea levels of earlier interglacial periods, such as the Ipswichian (see Fig. 13). The coastal flooding zone also defines areas of land that are most likely to become submerged during predicted future rises of sea level.
FIG 21. Two episodes (17,000 and 12,000 years ago) in the rise of sea level around the North Sea area. (Redrawn and simplified from Current Archaeology207, 2006, Gaffney)
Drowned valleys (Figs 22 and 23) are present on the coastlines of Southern England as a result of recent sea-level rise. Formerly, the rivers draining the majority of these valleys would have transported mud and sand to the sea, where it would have been deposited on the sea bed. However, with the rise in sea level mud and sand are now often deposited in the flooded valleys or estuaries instead, and some have developed carpets of sediment, transported down-valley by rivers or brought up-valley by the sea where tides and storms have been effective.
Coastlines with low seaward slopes and a soft surface blanket and/or bedrock may develop beach barriers when flooded by rising sea level. These barriers are ridges of sand or gravel parallel to the general trend of the coastline (Fig. 24). They are created by the impact of storm waves on the gently sloping and soft landscape. They tend to develop a cap of wind-blown sand which is very vulnerable to storm wave erosion, but may eventually become stabilised by vegetation. Behind the barrier a low-lying area of more sheltered conditions develops and regular flooding at high tide may bring in muddy sediment from the sea that can settle and build up salt marshes.
FIG 22. The drowning of a valley by sea-level rise.
FIG 23. Drowned valley of the Deben, Suffolk, viewed from above the sea off Felixstowe Ferry. (Copyright London Aerial Photo Library)
FIG 24. Cross-section of a beach barrier formed as sea level rises over a very gently sloping landscape.
FIG 25. Beach barrier on Scolt Head Island, Norfolk. (Photograph held at Cambridge University Collection of Air Photographs, Unit for Landscape Modelling)
The aerial photograph of part of Scolt Head Island (Fig. 25) in north Norfolk shows the succession of zones parallel to the coastline typical of a recently flooded, gently sloping landscape. On the beach, coast-parallel ridges and hollows (runnels) have been created during recent storms, and are draining water as the photograph was taken at low tide. The crest of the barrier is capped by wind-blown dunes, which have been stabilised by marram grass, but also shows signs of erosion during recent storms. Behind the barrier are salt marshes, generally sheltered from storm waves and developing tidal channels. The salt marshes are forming around the remains of various sand and gravel spits that date from a landscape before the present beach barrier was there. The far side of the salt marsh is marked by a gently curved sea wall built within the last two centuries to reclaim some land by keeping high tides out. Behind that is the boundary between the present flat seaward zone of young sediment and the older terrain, marked by a complex field pattern that is underlain by Chalk bedrock.
DEVELOPMENT BY PEOPLE
My concern in this book is primarily with natural landscapes, and I will tend to comment on the development by people since the Bronze Age only where this relates to the natural features in an interesting way. However, in reviewing the appearance of the whole of Southern England, I have been struck by an intriguing distinction made by some landscape historians: the distinction between ancient and planned countryside (Figs 26–28). I have based my approach on the discussions offered by Oliver Rackham, ecologist and landscape historian, and these are summarised below.
Ancient countryside (Fig. 26) consists of many hamlets, small towns, ancient farms and hedges (of mixed varieties of shrubs and trees), along with roads that are not straight, numerous footpaths and many antiquities.
Planned countryside (Fig. 27) has distinct villages, much larger than the hamlets, along with larger eighteenth- and nineteenth-century farms, hedges of hawthorn and straight roads. Footpaths are less common and the few antiquities that are present are generally prehistoric.
I have re-examined the same areas used by Oliver Rackham as examples of these two countryside types, and compared the early Ordnance Survey maps with maps of the same area generated by me using the data and methods used in this book (see Chapter 1). The shading and ‘hachured’ patterning used in the earlier maps represents the hills and slopes rather clearly – better than the contour representation used in the present-day Ordnance Survey Landranger maps, although these show man-made features much more clearly. My map representation is a compromise in that it represents elevations and slopes using colours and hill-shading, but also allows the patterns of roads and settlements to be seen.
Oliver Rackham’s conclusion is that many of the distinctive features of planned countryside were created by the general parliamentary enclosure of land during the eighteenth and nineteenth centuries. This involved the wholesale conversion of commonly held land with open fields into enclosed fields awarded to individuals and institutions. Many landscape historians have claimed earlier origins for the difference between ancient and planned countryside, believing that historical and cultural differences in the people who settled and developed the two areas played an important role. Variations in the bedrock geology also seem to be important here. For example, the ancient countryside shown in Figure 26 is underlain by strongly deformed Variscan bedrock that has been eroded into small hills and valleys (see Chapter 4).
FIG 26. Example of ancient countryside at the Devon-Somerset border, near Tiverton, with 1809 and recent mapping compared. (Upper part from Cassini Old Series map 181, copyright Cassini Publishing 2007/www.cassinimaps.co.uk)
FIG 27. Example of planned countryside at the Berkshire-Oxfordshire border, around Didcot, with 1830s and recent mapping compared. (Upper part taken from Cassini Old Series maps 164 and 174, copyright Cassini Publishing 2007/www.cassinimaps.co.uk)
FIG 28. Generalised map distinguishing ancient and planned countryside across Southern England.
In contrast, the planned countryside covered by Figure 27 consists of only gently tilted Mesozoic bedrock that has formed a much flatter and more open landscape.
CHAPTER 3 Movement of the Earth’s Surface from Within (#ulink_b8f1aa59-56b9-58ee-ad3a-c811b38a295b)
WIDESPREAD MOVEMENTS OF THE EARTH’S SURFACE
TO UNDERSTAND THE CHANGES and movements affecting the appearance of the landscape on large scales we need to review some geological systems, especially plate tectonics. Many of the large changes that have created landscapes over long periods of time can now be understood using this discovery.
Knowledge of the processes causing the movement of large (10–1,000 km length-scale) areas of the Earth’s surface has been revolutionised by scientific advances made over the last 40 years. During this time, scientists have become convinced that the whole of the Earth’s surface consists of a pattern of interlocking tectonic plates (Fig. 29). The word ‘tectonic’ refers to processes that have built features of the Earth’s crust (Greek: tektōn, a builder). The worldwide plate pattern is confusing – particularly when seen on a flat map – and it is easier to visualise the plates in terms of an interlocking arrangement of panels on the Earth’s spherical surface, broadly like the panels forming the skin of a football.
Tectonic plates are features of the lithosphere, the name given to the ≈125 km thick outer shell of the Earth, distinguished from the material below by the strength of its materials (Greek: lithos, stone). The strength depends upon the composition of the material and also upon its temperature and pressure, both of which tend to increase with depth below the Earth’s surface. In contrast to the mechanically strong lithosphere, the underlying material is weaker and known as the asthenosphere (Greek: asthenos, no-strength). Note that on figure 30 the crustal and outer mantle layers are shown with exaggerated thickness, so that they are visible.
FIG 29. World map showing the present pattern of the largest lithosphere plates.
Most of the strength difference between the lithosphere and the asthenosphere depends on the temperature difference between them. The lithosphere plates are cooler than the underlying material, so they behave in a more rigid way when subjected to the forces generated within the Earth. The asthenosphere is hotter and behaves in a more plastic way, capable of deforming without fracturing and, to some extent, of ‘flowing’. Because of this difference in mechanical properties and the complex internal forces present, the lithosphere plates can move relative to the material below. To visualise the motion of the plates, we can use the idea of lithospheric plates floating on top of the asthenosphere.
Looking at the surface of the Earth (Fig. 29), the largest plates show up as relatively rigid areas of the lithosphere, with interiors that do not experience as much disturbance as their edges. Plates move relative to each other along plate boundaries, in various ways that will be described below. The plate patterns have been worked out by investigating distinctive markers within the plates and at their edges, allowing the relative rates of movement between neighbouring plates to be calculated. These rates are very slow, rarely exceeding a few centimetres per year, but over the millions of years of geological time they can account for thousands of kilometres of relative movement.
It has proved to be much easier to measure plate movements than to work out what has been causing them. However, the general belief today is that the plates move in response to a number of different forces. Heat-driven circulation (convection) occurs within the mantle, but other forces are also at play. Where plates diverge, warm, new material is formed that is elevated above the rest of the plate, providing a pushing force to move the plate laterally, around the surface of the Earth. At convergent boundaries, cold, older material ‘sinks’ into the asthenosphere, providing a pulling force which drags the rest of the plate along behind it. Deep within the Earth, the sinking material melts and is ultimately recycled and brought back to the surface to continue the process.
FIG 30. Diagram of the internal structure of the Earth.
Knowledge of how tectonic plates interact provides the key to understanding the movement history of the Earth’s crust. However, most people are much more familiar with the geographical patterns of land and sea, which do not coincide with the distribution of tectonic plates. From the point of view of landscapes and scenery, coastlines are always going to be key features because they define the limits of the land; we make no attempt in this book to consider submarine scenery in detail.
The upper part of the lithosphere is called the crust. Whereas the distinction between the lithosphere and the asthenosphere is based upon mechanical properties related to temperature and pressure (see above), the distinction between the crust and the lower part of the lithosphere is based upon composition. Broadly speaking, there are two types of crust that can form the upper part of the lithosphere: continental and oceanic. An individual tectonic plate may include just one or both kinds of crust.
Continental crust underlies land areas and also many of the areas covered by shallow seas. Geophysical work shows that this crust is typically about 35 km thick, but may be 80–90 km thick below some high plateaus and mountain ranges. The highest mountains in Britain are barely noticeable on a scale diagram comparing crustal thicknesses (Fig. 31). Continental crust is made of rather less dense materials than the oceanic crust or the mantle, and this lightness is the reason why land surfaces and shallow sea floors are elevated compared to the deep oceans. Much of the continental crust is very old (up to 3–4 billion years), having formed early in the Earth’s life when lighter material separated from denser materials within the Earth and rose to the surface.
Oceanic crust forms the floors of the deep oceans, typically 4 or 5 km below sea level. It is generally 5–10 km thick and is distinctly denser than continental crust. Oceanic crust only forms land where volcanic material has been supplied to it in great quantity (as in the case of Iceland), or where other important local forces in the crust have caused it to rise (as is the case in parts of Cyprus). Oceanic crust is generally relatively young (only 0–200 million years old), because its higher density and lower elevation ensures that it is generally subducted and destroyed at plate boundaries that are convergent (see below).
Figure 29 shows the major pattern of tectonic plates on the Earth today. The Mercator projection of this map distorts shapes, particularly in polar regions, but we can see that there are seven very large plates, identified by the main landmasses located on their surfaces. The Pacific plate lacks continental crust entirely, whereas the other six main plates each contain a large continent (Eurasia, North America, Australia, South America, Africa and Antarctica) as well as oceanic crust. There are a number of other middle-sized plates (e.g. Arabia and India) and large numbers of micro-plates, not shown on the world map.
Figures 29 and 32 also identify the different types of plate boundary, which are distinguished according to the relative motion between the two plates. Convergent plate boundaries involve movement of the plates from each side towards the suture (or central zone) of the boundary. Because the plates are moving towards each other, they become squashed together in the boundary zone. Sometimes one plate is pushed below the other in a process called subduction, which often results in a deep ocean trench and a zone of mountains and/or volcanoes, as well as earthquake activity (Fig. 32). The earthquake that happened on the morning of 26 December 2004 under the sea off western Sumatra was the strongest anywhere in the world for some 40 years. It seized world attention particularly because of the horrifying loss of life caused by the tsunami waves that it generated. This earthquake was the result of a sudden lithosphere movement of several metres on a fault in the convergent subduction zone where the Australian plate has been repeatedly moving below the Eurasian plate.
FIG 31. Scale diagram comparing average thicknesses of oceanic and continental crust and lithosphere.
In other cases the plate boundary is divergent, where the neighbouring plates move apart and new material from deeper within the Earth rises to fill the space created. The new oceanic crust is created by the arrival and cooling of hot volcanic material from below. The mid-Atlantic ridge running through Iceland, with earthquakes and volcanic activity, is one of the nearest examples to Britain of this sort of plate boundary.
Other plate boundaries mainly involve movement parallel to the plate edges and are sometimes called transform boundaries. The Californian coast zone is the classic example but there are many others, such as the transform boundary between the African and Antarctic plates. In some areas, plate movement is at an oblique angle to the suture and there are components of divergence or convergence as well as movement parallel to the boundary.
Britain today sits in the stable interior of the western Eurasian plate, almost equidistant from the divergent mid-Atlantic ridge boundary to the west and the complex convergent boundary to the south where Spain and northwest Africa are colliding. In its earlier history the crust of Britain has been subjected to very direct plate boundary activity: the results of convergent activity in Devonian and Carboniferous times (between 416 and 299 million years ago) are visible at the surface in southwest England, and in Ordovician to Devonian times (between 490 and 360 million years ago) in Wales, northwest England and Scotland.
FIG 32. Diagram illustrating the movement processes of plates (not to scale).
UNDERSTANDING SURFACE MOVEMENTS
We have been considering the large movement systems that originate within the Earth. There are also more local movement systems operating on the Earth’s surface, which are linked to a very variable degree to the large-scale movements of plate tectonics. To explore this complex linkage further, it will be helpful to look now at different processes that may combine to cause particular local movements.
Horizontal movements as part of convergence, divergence or lateral transfer
Tectonic plates are recognised by their rigidity, so there is relatively little horizontal movement between points within the same plate compared to the deformation seen in plate boundary zones. This extreme deformation may involve folding and fracturing of the rock materials, addition of new material from below, or absorption of material into the interior during subduction.
Nonetheless, deformation is not restricted solely to plate boundaries, and does occur to a lesser extent within the plates. In some cases, major structures that originally formed along a plate boundary can become incorporated into the interior of a plate when prolonged collision causes two plates to join. Southern England includes the remains of a former convergent plate boundary and contains many examples of structures of this sort (particularly around Dorset and the Isle of Wight). These structures have often been reactivated long after they first formed in order to accommodate forces along the new plate boundary via deformation within the plate. Conversely, changes of internal stress patterns can sometimes lead to the splitting of a plate into two, forming a new, initially divergent plate boundary. Many of the oil- and gas-containing features of the North Sea floor originated when a belt of divergent rift faults formed across a previously intact plate.
It needs to be stressed that the patterns of deformation (fracturing and folding) due to these plate motions occur at a wide range of different scales, from centimetres to thousands of kilometres. Sometimes they are visible at the scale of an entire plate boundary, such as the enormous Himalayan mountain chain that marks the collision of India with Asia.
The effects of features as large as plate boundaries on landscapes persist over hundreds of millions of years, long after the most active movement has ceased. For example, parts of southwestern England, Wales and the Scottish Highlands are underlain by bedrocks that were formed in convergent boundary zones of the past. The tin and lead mines of Cornwall owe their existence to a 300-million-year-old convergent plate boundary, where an ocean was destroyed as two plates converged and continents collided. The convergence released molten rock that rose in the crust and gradually cooled to form granite, while metals were precipitated in the surrounding crust as ‘lodes’ containing tin and lead (see Chapter 4).
Mapping the patterns of bedrock exposed at the surface often reveals folds and faults that provide key information about the movements that have taken place during the past. Figure 33 provides a key to some of the terms commonly used to classify these structures as a step towards understanding the sorts of movement patterns that they represent. In broad terms, folds tend to indicate some form of local convergent movement, though they may be the result of larger movement patterns of a different kind. Normal faults tend to indicate divergent movements, at least locally, whereas reverse and strike-slip faults tend to indicate convergence. Two broad types of fold are distinguished: synclines are u-shaped downfolds, while anticlines are the opposite – n-shaped upfolds.
Further mapping of folds and faults often reveals complex patterns of changing movements. In the example shown in Figure 34, divergent movements in an area of crust produce plastic deformation in the warmer lower crust, and faulting into a number of discrete blocks in the colder, more brittle, upper crust. This is then followed by an episode of convergent movement that results in closing up the upper crustal blocks and further flow in the plastic lower crust, causing crustal thickening and mountain building at the surface.
Vertical crustal movements as part of other crustal movements
The movement of lithospheric plates is the main cause of convergent and divergent movements affecting thousands of kilometres of the Earth’s surface. As shown in Figures 33 and 34, these horizontal movements are generally accompanied by vertical movements that can produce very large scenic features, such as a mountain belt or a rift valley. In this book we are primarily concerned with scenic features at a more local scale, so we now consider various other processes that may be important in creating vertical crustal movements without contributions from large-scale plate interactions.
FIG 33. The most important types of folds and faults, and the local patterns of forces responsible.
Vertical changes by erosion or deposition
Addition or subtraction of material to the surface of the Earth is happening all the time as sediment is deposited or solid material is eroded. The field of sedimentology is concerned with the wide range of different processes that are involved in the erosion, transport and deposition of material, whether the primary agent of movement is water, ice, mud or wind. An important point is that few of these sedimentary processes relate directly to the large tectonic movements of the Earth’s crust that we have discussed above. Scenery is often produced by erosion of thick deposits that formed in sedimentary basins where material eroded from the surrounding uplands accumulated. One of the characteristic features of these thick deposits is their layered appearance, which is often visible in the scenery. Layering varies from millimetre-scale laminations produced by very small fluctuations in depositional processes, to sheets hundreds of metres thick that extend across an entire sedimentary basin. These thicker sheets are often so distinctive that they are named and mapped as separate geological units representing significant changes in the local environment at the time they were deposited.
FIG 34. Example of a cross-section through the crust, showing how a divergent movement pattern (A) may be modified by later convergent movements (B and C).
Vertical crustal movements resulting from loading or unloading
In addition to the direct raising or lowering of the surface by erosion or deposition, there is a secondary effect due to the unloading or loading of the crust that may take some thousands of years to produce significant effects. As mentioned above, we can visualise the lithosphere as ‘floating’ on the asthenosphere like a boat floating in water. Loading or unloading the surface of the Earth by deposition or erosion will therefore lower or raise the scenery, just as a boat will sit lower or higher in the water depending on its load.
An example of this is the lowering of the area around the Mississippi Delta, loaded by sediment eroded from much of the area of the USA. The Delta region, including New Orleans, is doomed to sink continually as the Mississippi river deposits sediment around its mouth, increasing the crustal load there.
A second example of such loading is provided by the build-up of ice sheets during the Ice Age. The weight of these build-ups depressed the Earth’s surface in the areas involved, and raised beaches in western Scotland provide evidence of the high local sea-levels due partly to this lowering of the crustal surface.
Unloading of the Earth’s surface will cause it to rise. Recent theoretical work on the River Severn suggests that unloading of the crust by erosion may have played a role in raising the Cotswold Hills to the east and an equivalent range of hills in the Welsh Borders (see Chapter 6, Area 9). In western Scotland, as the ice has melted the Earth’s surface has been rising again.
Vertical movements by expansion or contraction
Changing the temperature of the crust and lithosphere is an inevitable result of many of the processes active within the Earth, because they often involve the transfer of heat. In particular, rising plumes of hot material in the Earth’s mantle, often independent of the plate boundaries, are now widely recognised as an explanation for various areas of intense volcanic activity (for example beneath Iceland today). These plumes are often referred to as ‘hot spots’ (see Fig. 32). Heating and cooling leads to expansion or contraction of the lithosphere and can cause the surface to rise or sink, at least locally.
An example of this is the way that Southern England was tilted downwards to the east about 60 million years ago. At about this time, eastern North America moved away from western Europe as the North American and Eurasian plates diverged. The divergence resulted in large volumes of hot material from deep within the Earth being brought to the surface and added to the crust of western Southern England. It is believed that the heating and expansion of the crustal rocks in the west has elevated them above the rocks to the east, giving an eastward tilt to the rock layers and exposing the oldest rocks in the west and the youngest ones in the east. This sequence has important implications for the scenery of England’s south coast (see Chapter 5).
HOW CAN LOCAL SURFACE MOVEMENTS BE DETECTED?
Having just reviewed some of the processes that cause vertical movements of the Earth’s surface, it is useful to consider the practical difficulties of how such movements are measured.
For present-day applications, it seems natural to regard sea level as a datum against which vertical landscape movements can be measured, as long as we remember to allow for tidal and storm variations. However, much work has demonstrated that global sea level has changed rapidly and frequently through time, due to climate fluctuations affecting the size of the polar icecaps and changing the total amount of liquid water present in the oceans and seas. It has also been shown that plate tectonic movements have an important effect on global sea level by changing the size and shape of ocean basins.
Attempts have been made to develop charts showing how sea level, generalised for the whole world, has varied through time. However, it has proved very difficult to distinguish a worldwide signal from local variations, and the dating of the changes is often too uncertain to allow confident correlation between areas.
In sedimentary basins, successful estimates of vertical movements have been made using the thicknesses of sediment layers accumulating over different time intervals in different depths of water. In areas of mountain building, amounts of vertical uplift have been estimated using certain indicator minerals that show the rates of cooling that rocks have experienced as they were brought up to the surface. However, both these approaches are only really possible in areas that have been subjected to movements of the Earth’s crust that are large and continuous enough to completely dominate other possible sources of error.
Local horizontal movements are also difficult to estimate, although fold and/or fault patterns may allow a simple measure in some cases. Movement of sediment across the Earth’s surface by rivers or sea currents can be estimated if mineral grains in the sediment can be tracked back to the areas from which they have come. In the detailed consideration of landscapes in this book, we have to rely on using the widest possible range of types of evidence, carefully distinguishing the times and scales involved. Even then, we are often left with probable movement suggestions rather than certainties.
CHAPTER 4 The Southwest Region (#ulink_34a0c3e0-ec2d-520c-a34b-9f87f0bffe88)
GENERAL INTRODUCTION
MOST OF THE BEDROCK near the surface in the Southwest Region (Fig. 35) is distinctly older than the near-surface bedrock in the rest of Southern England. It therefore provides us with information about earlier episodes, and this is all the more interesting because these episodes involved movements of the crust that created a mountain belt, the only one fully represented in the bedrock story of Southern England. Not only does this add greatly to the interest of the Southwest, but it has resulted in the presence of valuable minerals that have strongly influenced the human history in the Region.
Bedrock foundations and early history
Sedimentation and surface movement before the mountain building
The Southwest Region consists predominantly of bedrock formed between about 415 and 300 million years ago, during Devonian and Carboniferous times. This bedrock records an episode during which some areas of the Earth’s crust rose while others sank, as part of a general buckling of the crust that is the first indication of compression and mountain building (Figs 36 and 37). As the rising areas became significantly elevated they were eroded, shedding sediment into the neighbouring sinking areas that became sedimentary basins. It is these basins that preserve most of the evidence of these events (Fig. 38).
In material that has been further and later deformed, it is difficult to work out the shape of the sinking areas, but many of them were probably elongated or trough-shaped, with the troughs separated by rising ridges that ran roughly east-west, parallel to the general trend of the later mountain belt. The troughs and ridges were caused in the early stages of mountain building by the compression and buckling of the crust. The troughs were generally flooded by the sea, or on the margins of relatively narrow seaways. Muds were the commonest materials to accumulate, although sands were also in plentiful supply. Lime-rich sediments, sometimes with corals and other shelly marine animals, were locally important. There were also periodic episodes of igneous activity that contributed volcanic lavas to the sedimentary successions.
FIG 35. The Southwest Region, showing Areas 1 to 3.
During these episodes of Devonian and Carboniferous basin and ridge activity, the Southwest Region was just one small part of a larger belt of similar activity that extended to the west into Ireland and Canada. Canada was then very much closer, because the Atlantic Ocean is a younger feature that only started to grow (at this latitude) about 100 million years ago, as divergence and spreading occurred along the mid-Atlantic plate boundary. To the south and east, the same belt of activity continued across northern France and into Germany. This broadly east-west trending belt later became the Variscan mountain belt.
Crustal convergence that created the mountain belt
The subsidence and sedimentation were sometimes interrupted by, and generally followed by, episodes of convergence of the Earth’s crust. This was caused by compression or squeezing, broadly in a north-south direction, so that areas of bedrock were folded and pushed closer together, making the east-west trending Variscan belt narrower, as if between the jaws of a vice. The map (Fig. 39) and cross-section (Fig. 40) show how the folds and faults of the region vary locally in their pattern, but can be explained generally by convergent movements in this north-south direction. These continued over at least 100 million years, and occurred along thousands of kilometres of the belt. Mountain-building events such as this occur when tectonic plates collide (as described in Chapter 3) and always have a profound effect upon the scenery in the vicinity of the collision. The Variscan mountain belt is just one of the great mountain-building episodes that have occurred periodically, throughout the Earth’s history.
FIG 36. Timeline diagram showing bedrock deposition and emplacement events in the Southwest Region.
Some of the best evidence for the horizontal crustal shortening comes from examining folds that can be seen in the bedrock at many localities (Figs 41 and 42). Folding of the originally flat layers of the bedrock is a spectacular feature of many southwestern sea cliffs, and the direction of the folding gives a clear indication of the direction of the shortening that resulted. Fractures (faults) also frequently cut the bedrock, and careful mapping makes it possible to recognise that, although some of them are very local features, others turn out to have been flat-lying fractures across which many kilometres of movement have taken place.
FIG 37. Simplified geological map of the Southwest Region.
FIG 38. Typical Devonian sedimentary basin in the Southwest Region.
FIG 39. The major bedrock structures of Southwest England and South Wales.
Another feature of the mountain building is that muddy material – the most abundant sediment in the Southwest Region – was often converted into slates that can typically be split into thin sheets and are said to possess a ‘slatey cleavage’. These rocks are locally referred to as killas, to distinguish them from other rocks with no cleavage, particularly the granites. The conversion into slates took place during the folding and fracturing of the mountain building, when the original muds, rich in clay minerals, were buried deeply below other sediments and then compressed to produce a new layering (or cleavage).
One large feature visible in the bedrock is the ‘Culm fold belt’ or ‘synclinorium’, a large and complex downfold representing horizontal crustal convergence (Figs 39 and 40). The centre of this feature is a belt of bedrock of Carboniferous age that extends between Bude and Exeter running across the centre and north of the Southwest Region. To the north and south of this, older (Devonian) rocks occur at the bedrock surface, forming the margins of the large downfold or syncline (Fig. 37). Culm is an old term much used by miners and European geologists for Carboniferous sediment, and synclinorium is a name for a downfold (or syncline) which contains numerous smaller folds.
In the Lizard area, much of the bedrock consists of a distinctive group of igneous rocks (Figs 39 and 40). These rocks cooled and solidified earlier than the main mountain building, and were mostly formed by intrusion of hot molten rock in a way that is typical of the floor of an ocean basin. The Lizard area provides one of the best examples now visible on land in Britain of material formed originally as ocean-floor crust. In Late Devonian times, as a result of early Variscan convergence, this large area of oceanic crust was forced northwards over and against sedimentary rocks lying just to the north. This shows how, in a large mountain belt, an area of crust (tens of kilometres across), with a distinctive history as the floor of an ocean basin, can be uplifted and incorporated into a mountain belt as its margins are squeezed together.
FIG 40. Schematic cross-section representing major structures of the Variscan mountain belt. The deep structure shown is speculative but shows how crustal shortening seen at the surface may be related to deeper, flat-lying fractures (faults). Located on Figure 39.
FIG 41. Zigzag folding due to horizontal convergence during the Variscan mountain building is spectacularly exposed at Hartland Quay (Area 3). (Copyright Will Brett/www.lastrefuge.co.uk)
FIG 42. Zigzag folding due to horizontal convergence during the Variscan mountain building, this time at Bude (Area 2). (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
Some of the bedrock of this Lizard Complex is called serpentinite, after the common occurrence of the green mineral serpentine in sinuous cracks and veins. This gives the rocks an attractive colour patterning, and the absence of quartz makes them surprisingly easy to work with steel tools, giving rise to a local industry of carving serpentinite ornaments.
Granites and valuable minerals
The granites of Southwest England are an important later feature of the Variscan mountain belt. Granites are igneous rocks with coarse (millimetre across) crystals that have grown, interlocking with each other, as the molten material cooled slowly and solidified at some depth in the Earth’s crust. The minerals of the granite are most commonly quartz (typically about 30 per cent) and feldspar, generally with some other minerals such as mica (Fig. 43). The granite liquid (often called magma) formed as a result of melting deep within the outer layers of the Earth and was then forced upwards, sometimes pushing aside the overlying bedrock and sometimes replacing it by melting. The granites solidified at depths of several hundred metres or more below the surface and are now at, or near, the surface because of landscape erosion. The main granite areas include the Isles of Scilly in the west, followed by Land’s End, Carnmenellis, St Austell, Bodmin Moor and Dartmoor in succession to the east. These distinct granite areas at the surface can be visualised as the tops of fingers extending upwards from a single continuous granite body detectable by gravity surveys at greater depth under the spine of Southwest England (Fig. 44). The deep body extends for some 200 km along the length of the mountain belt.
FIG 43. Polished slab cut in the Dartmoor granite showing typical granite texture. Crystals of quartz (light grey), feldspar (white) and biotite (black) have interlocked as the magma (molten rock) solidified on cooling. (Copyright Landform Slides – Ken Gardner)
FIG 44. Diagram showing the large granite body below the bedrock of Cornwall and Devon, and the way the granite bosses now visible at the surface are upward extensions of this larger body.
Although there was probably some time range in the arrival of different granite bodies in the upper crust, the main episodes took place at the very end of the Carboniferous and during the earliest Permian, roughly 300 million years ago.
The arrival of the granites from below was only one part of the invasion of the upper levels of the bedrock that took place at this time. Widespread mineralisation around the granites has probably been even more important than the arrival of the granites themselves, in terms of human history. The term mineralisation is used to cover the alteration of the solid granite and the surrounding (older) bedrock that has, in some areas, been caused by the movement of very hot and chemically rich water, using the network of cavities and fractures that existed in the rocks. Because of the chemistry of the rocks deep down, many different chemical elements were brought to the upper levels and crystallized there to form new and valuable minerals, or caused alterations of the earlier solid rocks.
FIG 45. Simple diagram of a slice through the Earth’s upper levels, showing how the temperature patterns around a granite body have been responsible for the distribution of minerals containing the more important chemical elements.
The granites probably solidified in the Earth at temperatures of about 850 °C, and most of the mineralisation happened at rather lower temperatures as the rocks cooled (Fig. 45). Tin, wolfram, arsenic and copper minerals formed at between 500 and 300 °C, whereas silver, lead, zinc, uranium, nickel and cobalt minerals formed at between 300 and 200 °C, and iron minerals between 200 and 50 °C.
The tin of Cornwall was a major reason why some of the early inhabitants of mainland Europe were interested in Britain. In fact there is evidence that tin minerals were being gathered here more than 3,000 years ago, during the Bronze Age. In those days, much of the material was collected from young sands and gravels derived from the weathering and erosion of the mineral-bearing rock, unlike later times when mining techniques were developed to extract tin directly from the bedrock.
Some granite areas contain much more mineralisation than others, and the range of minerals and chemical elements that are present varies greatly. This depends on the temperature of the granite emplacement and the chemistry of the fluids accompanying and following the granite. The Land’s End and Carnmenellis granites are particularly rich in tin, and it is around these granites, in areas near to St Ives, Camborne, Redruth and Helston, that most of the mining has been concentrated. The remains of this mining are often clear to see (Fig. 46), but the presence of the minerals themselves does not generally influence the natural scenery.
FIG 46. Tin mine workings near Cape Cornwall, west Cornwall. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
In some of the granites, hot fluids from below altered the mineral feldspar (one of the dominant granite minerals) and turned it into the soft clay mineral kaolinite. The china clay industry has developed round the presence of this mineral, which has usually been extracted from the altered granite by washing it out with powerful water jets. This process has changed the scenery dramatically, particularly around the St Austell granite. For every tonne of useable kaolin, 5 tonnes of waste granite material are produced, and heaps of this waste are obvious scenic features in these areas (Fig. 47). The famous Eden Project at Bodelva, near St Austell, has been constructed inside a large former china clay quarry.
FIG 47. China clay excavations at St Austell. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
The older rocks surrounding each of the granite intrusions generally show evidence of alteration that occurred as the mobile granite worked its way upwards from below. This contact metamorphism, often accompanied by the growth of new minerals, is the result of the transfer of heat and introduction of new chemical components from the granite. It has usually resulted in making the rocks more resistant to later erosion at the surface.
Younger episodes
Sedimentary markers
Between 7 and 20 km to the southwest of Exeter, traversed by the A38 and A380 trunk roads, the Great and Little Haldon Hills are capped by a layer of sediments, assigned to the Upper Greensand, and spanning in age the Early/Late Cretaceous boundary (between about 105 and 95 million years ago). These are the westernmost erosional relicts of a continuous sheet of sediment of this age that extended across much of the rest of Southern England. In the Haldon Hills area, the sandy and fossiliferous material seems to have formed near the coastal margin of an extensive Cretaceous sea.
The Haldon Gravels are distinctive deposits that occur above these Cretaceous sediments. They consist largely of flint pebbles and contain sand and mud between the pebbles. Some of the gravels appear to be the result of removal by solution of the calcareous Late Cretaceous Chalk that can no longer be found in its unaltered state so far west. The flint nodules in the Chalk were then left as a layer of much less soluble pebbles. Some of the gravel appears to have been carried to its present position by rivers or the sea, perhaps also with the incorporation of kaolinite clay from the Dartmoor granite. The age of these gravels appears to be early Tertiary, perhaps about 55 million years.
A few kilometres west of the Haldon Hills, northwest of Torquay, the Bovey Formation of early Tertiary age (Eocene and Oligocene, about 45 to 30 million years ago) occurs in a distinct, fault-bounded basin. The formation is more than 1 km in thickness and consists primarily of the clay mineral kaolinite, deposited as mud by local streams, and associated with minor amounts of sand, gravel and peat-like organic deposits of lignite. This sediment fill continues to be a very important material for ceramics, pipes, tiles etc. ranging from high-quality china clay to lower-quality materials. Most of the sediment appears to have been carried into the basin from the area of the Dartmoor granite and its surroundings. The Bovey Basin formed as a result of subsidence along the northwest-to-southeast trending Sticklepath Fault Zone (Fig. 39) which cuts across the whole of the Southwest Region. This fault zone seems to have been active during the accumulation of sediment in the basin and so, at least in this phase of its history, it was much younger than the Variscan structures of the Southwest generally. Further to the northwest along the same fault zone is the smaller Petrockstowe Basin near Great Torrington (see Fig. 38), and, offshore, under the Bristol Channel is the larger Stabley Bank Basin, east of Lundy Island.
About 6 km southeast of St Ives (see Area 1), near the small village of St Erth, a small area is underlain by some soft sands and muds. When first exposed by quarrying, these sediments provided a rich assemblage of fossils that are thought to have lived some 3 million years ago, in latest Tertiary times. The fossils suggest sea depths of between 60 and 100 m, and are now about 30 m above sea level, so they provide a fragment of evidence from a period when the sea was more than 100 m higher than it is now, relative to the land of Cornwall. As will be mentioned below, this deposit is rather similar in its elevation to the most obvious plateau recognised in many of the inland areas, which may also relate to an episode when the sea stood at this level.
Drainage patterns
On the scale of the whole Southwest Region, the main upland areas are Exmoor in the north and the zone of distinct granite domes in the south, extending from Dartmoor to Land’s End.
The highest point of Exmoor is Dunkery Beacon (519 m). Exmoor has been eroded from Devonian bedrock, and may owe some of its elevation to the greater resistance to erosion of this material compared with the Carboniferous material that forms the bedrock further south. Another possible factor is suggested by the remarkable way that many of the river systems of the southwest drain to the south coast, despite their sources being remarkably close to the north coast (Fig. 48). This is the case for the Exe, flowing from Exmoor southwards via Exeter to Exmouth, and, further west, the Tamar, which begins northeast of Bude and flows southwards past Launceston and Tavistock before discharging into Plymouth Sound. It looks as if this part of the Southwest Region has been tilted southwards as these river systems developed on either side of the high ground of Dartmoor, where the granite resisted erosion. A southerly tilt would also be consistent with a preferential uplift of the Exmoor Hills to the north.
FIG 48. River pathways, mean flow rates (m
/s) at some river stations, main drainage divide (red line) and main granites of the Southwest Region.
The southern areas of hills correspond so clearly with the areas of granite outcrops that there can be little doubt that the greater resistance to erosion of the granite explains their higher elevations. But how long has this erosion been taking place? Emplacement of the granites was over by the end of Carboniferous times (about 300 million years ago) and there is evidence of pebbles in the New Red Sandstone from the Dartmoor granite and from the altered bedrock close by. Although the precise age of the earliest New Red Sandstone is uncertain, it does not appear to be much younger than the age of granite emplacement. However, it appears that the granites were not being significantly eroded in quantity much before Cretaceous times, 200 million years later and about 100 million years ago. Since then, the granites have been eroded into the present patterns of local hills and valleys, but at very variable rates as climate, coverage by the sea and rates of river erosion changed.
Each of the main granite bodies corresponds closely to an area of high ground, and their maximum heights tend to be greater towards the east (44 m for the Isles of Scilly, 247 m for Land’s End, 252 m for Carnmenellis, 312 m for St Austell, 420 m for Bodmin and 621 m for Dartmoor). This gradient is overall only about 3 m per km. The geophysical data on the large, deep granite body (Fig. 44) recognised below the surface granite bodies do not provide independent evidence for a slope of this sort deep down. Some tilting of the landscape downwards towards the west may have occurred, or the slope may simply reflect the greater proximity of the western granite bodies to the sea and repeated episodes of marine erosion.
Ice Age episodes
Ice sheets do not appear to have covered the present land of the Southwest Region to any important extent during any of the major cold episodes of the Ice Age. In the Isles of Scilly, material deposited directly from a grounded ice sheet has been recognised and is thought to be Devensian (last cold phase) in age (Fig. 49). Various giant boulders derived from metamorphic sources are a notable feature of some localities on the North Devon coast, some of which appear to have come from Scotland. However, it is not clear whether they were transported to their present locations by a large ice sheet or by floating ice.
In spite of the lack of an actual ice sheet, the repeated cold episodes of the Ice Age must have had a considerable effect upon the weathering style of the bedrock, for example influencing the granite tors, mobilising material to move down slopes and changing drainage patterns and the surface blanket of soft materials.
FIG 49. Map showing the greatest extent of the last main (Devensian) ice sheet across England and Wales.
AREA 1: WEST CORNWALL
A remarkable feature of the peninsula of West Cornwall (Figs 50 and 51), as it narrows towards Land’s End, is the contrast between the spectacular coastal scenery and the scenery inland. The rocky coastal cliffs and sharply indented coves reflect West Cornwall’s exposure to the prevailing Atlantic storms, and contrast starkly with the inland scenery of rolling – though often rocky -hillsides, carved into a network of small valleys and streams.
The main features of the inland landscape appear to have formed over millions of years, and ultimately reflect the bedrock pattern that has been inherited from the Variscan mountain building that ended 300 million years ago. In contrast, the coastal landscape is clearly much younger, and much of it has been produced by changes in sea level that have occurred since the last main cold phase of the Ice Age, some 10,000 years ago. There is some evidence of earlier sea levels but this is more difficult to evaluate, as it has generally been removed by more recent erosional events.
FIG 50. Location map for Area 1.
I have divided West Cornwall into three Landscapes (A to C), each with distinctive bedrock geology (Fig. 52).
Landscape A: Granite areas
The Isles of Scilly (A1; Fig. 53) are formed by the westernmost significant granite bodies of southwestern England. They lie some 45 km southwest of Land’s End, scattered over an area approximately 20 km by 15 km. Most of the 150 islands are little more than bare outcrops of granite, sometimes largely submerged at high tide. The landscape is windswept and mainly treeless, with heathlands where the ground has not been cultivated. Historically the islanders eked out a precarious existence from crofting, until the nineteenth century, when shipbuilding and the growing of flowers became economic. Today most of the cultivated land consists of small fields of flowers edged with evergreen hedges, and horticultural work, along with tourism, has become the mainstay of the economy.
The smaller islands are often arranged in rows, separated by ‘sounds’ (areas of shallow water) that tend to have a northwest-southeast orientation. These sounds must have been valleys before they were drowned by the recent (Flandrian) sea-level rise. Their orientation is similar to that of the valleys and faults of the Land’s End granite, discussed more fully below. Numerous sandy bays and beaches reflect the granite weathering and the transport of the weathered sediment, by storms and tides, to more sheltered parts of the island landscape.
FIG 51. Natural and man-made features of Area 1.
In the general section of this chapter it has been mentioned that the northern Scillies appear to have been invaded by ice late in the history of the last (Devensian) cold phase of the Ice Age (Fig. 49), and this is surprising in view of their southerly location. It appears that when the Devensian ice sheet had grown to its greatest extent, an elongate tongue of ice, perhaps some 150 km wide, extended for nearly 500 km from the Irish and Welsh ice sheets to the edge of the Atlantic continental shelf. This tongue became so large because it was vigorously fed by ice from the high ground of Ireland to the west, and the Lake District of England and the mountains of Wales to the east. The ice extended across the mouth of the Bristol Channel, well clear of the present north Cornwall coastline, before leaving ice-laid sediment on the northern fringe of the Isles of Scilly. South of the island areas that were covered by ice, the granite has been weathered locally into tors.
FIG 52. Area 1, showing Landscapes A to C and specific localities mentioned in the text. Major divisions of Landscape A are identified by A1, A2, A3 etc., and localities are shown as a1, a2, a3 etc.
Land’s End is the westernmost tip of mainland England. The local cliffs are made of granite and clearly show vertical sets of fractures, probably formed when the granite was cooling and contracting (Figs 54 and 55). Apart from the fractures, the granite is massive compared with the strongly layered and deformed rocks into which the main granites were intruded. Most of the northerly inland areas are exposed and windswept moorland, though arable farming for early vegetables has developed in the valleys to the south. The valleys eroded in the Land’s End granite are distinct and often oriented very clearly in a northwest-southeast direction. This orientation is parallel to a large number of faults which appear to have first formed late in the Variscan mountain-building episode. However, they must also have been active much later, after the intrusion of the main granite, because its margin is locally offset by faults with this trend. The movement of superheated water along these fault systems has resulted in mineralisation of the bedrock, altering its resistance to erosion so that valley incision has taken place preferentially in this direction. Tors are largely absent from the Land’s End, Godolphin, Carnmenellis and St Austell granite areas, while they are common weathering features on Bodmin Moor and Dartmoor. This probably reflects a difference in the weathering and uplift histories of the different granite bodies.
FIG 53. The Isles of Scilly, looking east towards Bryher, Tresco and St Martins. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
The Land’s End granite (A2) forms the bedrock of most of the far southwestern peninsula, which is largely ringed by cliffs. To the east of the granite, St Ives Bay on the north coast and Mount’s Bay on the south coast show how much more readily eroded the Devonian killas is in comparison. Along the north coast of the Land’s End peninsula, the killas is preserved as a screen of land, rarely more than a kilometre in width, but clearly showing distinctive layering, as seen at Gurnard’s Head (a3; Fig. 56). Present coastal erosion may have been slowed at this point by the greater strength of the Devonian where it has been altered close to the granite. Just north of Land’s End point, Whitesands Bay (a1) is one of the only sandy bays to face the open sea to the west. The bay lacks any significant stream system that could have supplied sand to the beach, so it seems most likely that the sand has been carried into this bay by the storms which so often attack this exposed coast.
FIG 54. Land’s End peninsula from the air, looking eastwards. Note the lack of clear layering in the granite bedrock and the steep fracture surfaces (joints) that have controlled the form of the cliffs. (Copyright Dae Sasitorn & Adrian Warren / www.lastrefuge.co.uk)
A distinct, though irregular, platform at 100–150 m above sea level rings the area of the Land’s End granite, and tends to be followed by local roads. This may have been formed during an early episode of coastal erosion, when sea level was standing at this height relative to the land (Fig. 57). Some evidence for its age is mentioned below. Its irregularity probably reflects local valley erosion that has taken place since its formation.
FIG 55. Land’s End cliffs, looking westwards. Again, note the vertical jointing. (Copyright Landform Slides – Ken Gardner)
FIG 56. Gurnard’s Head (Fig. 52, a3), west of St Ives, showing the coastline along the northern edge of the Land’s End granite. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
FIG 57. Slope map showing the southwestern part of Area 1. Slopes greater than 5 degrees are coloured red, and the main granite areas and the Lizard Complex are outlined. Topographic cross-sections illustrate wave-cut platforms that are presently inland and show the lack of topography on the Lizard Plateau.
The next main granite bedrock area to the east underlies the Carnmenellis area (A4), although there are other smaller granite areas, such as St Michael’s Mount (Fig. 58), across the bay from Penzance, and the intermediate-sized Godolphin granite (A3), some 8 km to the east. These smaller granite areas show the range in size of ‘feeders’ that branched off from the major granite body that underlies the whole Southwest Region (Fig. 44). In all cases the granite bedrock corresponds to high ground in the landscape – evidence of the greater resistance of the granite in the face of repeated landscape erosion. Derelict mine engine houses litter the landscape, especially northwards near Camborne and Redruth, once prosperous tin-mining centres. To the south, the landscape is more sheltered and fertile, allowing better farming. Trees are rare because of their past cutting for fuel for the mining industry, as well as because of the general exposure of the landscape to the weather.
FIG 58. St Michael’s Mount. (Copyright Dae Sasitorn & Adrian Warren/ www.lastrefuge.co.uk)
The same northwest-to-southeast valley pattern that has just been mentioned in the Land’s End granite is also apparent in the area around the Carnmenellis granite, and appears to be the result of preferential stream erosion parallel to the faults trending in this direction (Fig. 59).
FIG 59. Sketch map showing the orientation of some of the main faults in West Cornwall.
Another similarity to Land’s End is the widespread topographic platform at about 140 m above sea level. This platform is particularly clear north of the Carnmenellis granite but is also obvious in the Godolphin granite (Fig. 57). In the Porthmeor and Camborne areas the platform is particularly distinctive, and the continuity of its landward slope is clear on the slope map. It has generally been assumed that these platforms were cut by storm waves when the sea stood at this level about 3 million years ago. At this time, West Cornwall would have consisted of granite islands, like the present Isles of Scilly, while the surrounding Devonian bedrock (killas) was submerged.
The western part of the St Austell granite (A5) lies within Area 1, and again its resistance to landscape erosion is shown by the high ground that it occupies. The remarkable feature of this granite is the way it has been altered by the circulation of hot fluids. Much of the feldspar in this granite has been altered to the mineral kaolinite, which is a member of the clay mineral group that is the key component of china clay. The result of this is that the St Austell granite has been quarried, particularly in its western part within Area 1. The kaolinite has been extracted from the rotted granite by high-pressure water jets, which leave large volumes of quartz and feldspar grains that are heaped up in enormous and obvious spoil heaps.
Most of the original heathland and moorland on the St Austell granite has been destroyed by the mining industry. More recently, the Eden project redevelopment of one quarry complex (in Area 2) has brought many visitors to the area.
Landscape B: The Lizard
Lizard Point is the most southerly headland in Britain, part of a wider Lizard landscape comprising a flat heathland plateau bounded by dramatic cliffs and small coves (Fig. 60). Notice how steep many of the sea cliffs are, and that they show little in the way of well-developed, regular layering or fracturing. Unlike the other upland areas of Cornwall, the Lizard is not underlain by granite. As mentioned in the general section of this chapter, some of the area is underlain by serpentinite, a distinctive, decorative rock that was originally part of the Earth’s mantle, below the crust and many kilometres below the surface (see Chapter 3). Other parts of the Lizard bedrock were originally basalt lavas and minor sheet-like intrusions along with small amounts of sediments, all similar to successions elsewhere that appear to have formed in or below the Earth’s oceanic crust. During the Variscan mountain building, this mixture of distinctive bedrock types appears to have been squeezed up amongst the strongly compressed Devonian killas. Today, the exceptional bedrock chemistry of the unusual Lizard rocks is the reason why the peninsula has such a variety of rare plant habitats. Much of the peninsula is a National Nature Reserve (NNR) or owned by the National Trust.
As in Carnmenellis and Land’s End, a wave-cut platform has been identified on the Lizard, although its level is rather lower. In fact, the platform actually forms the Lizard Plateau and is remarkably flat, the ground surface varying between 60 and 100 m above sea level over large areas. This relative flatness probably reflects the rather uniform composition of the rock materials involved, and their uniform resistance to weathering and erosion.
The coast of the Lizard Peninsula is formed almost entirely of steep cliffs, particularly around its southwestern perimeter. A few small beaches do occur in sheltered locations, such as at Coverack (b1), and picturesque fishing villages are scattered along the east side of the peninsula around small coves and gullies.
FIG 60. The Lizard coastline. Note the contrast between the jagged coastal cliffs and the flat inland landscape. (Copyright Dae Sasitorn & Adrian Warren/ www.lastrefuge.co.uk)
Landscape C: Cornish killas
Most of the bedrock of West Cornwall is Devonian sediment, folded, faulted and – locally – altered during the Variscan mountain-building episode (see the general section of this chapter). The Devonian sediments, known to miners and quarrymen as killas, have been less resistant to landscape weathering and erosion than the granites (A) and the Lizard Complex (B), and so have been preferentially eroded to form lower landscapes. All the major bays and estuaries of this Area, such as St Ives Bay (c4) and the Carrick Roads at Falmouth (c7), are situated in killas areas for this reason. The Variscan folding and faulting that deformed the killas has also locally influenced the directions of valleys and their slopes, which have picked out variations in the killas layering, giving an east-west grain to the landscape (Fig. 61).
FIG 61. Slope map of the eastern part of West Cornwall. The main granite bedrock areas are outlined and important boundaries in the Devonian bedrock indicate the direction of the Variscan folding. Note the circular china-clay workings that are visible in the St Austell granite (A5).
FIG 62. Complex landscape of the North Cornwall coast, looking eastwards from Crantock Beach, over the Pentire Ridge towards Newquay (Fig. 52, c2) and Watergate Bay. (Copyright Dae Sasitorn & Adrian Warren/ www.lastrefuge.co.uk)
FIG 63. Headlands, bays and beaches of the Newquay area (Fig. 52, c2), looking eastwards from a point 2 km west of Figure 62. Crantock Beach is visible in the middle distance. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
The Flandrian sea-level rise, which ended only 5,000 years ago, has also left its mark on West Cornwall. The most obvious legacy is the extensive array of tidal estuaries at the mouths of the main rivers, which are flooded river valleys or rias. The most striking example is the series of branched rias around Falmouth known as the Carrick Roads (c7). These extend northwards across half of the width of West Cornwall and have had an obvious major influence on the road and rail transport pattern of the area. Major branch rias to the west, north and east around the Carrick Roads divide this part of the Cornish landscape into numerous isolated peninsulas. The inland valleys of the killas areas tend to be deeply incised with little widening, and the branching patterns of these valleys are very clear on the slope map. The rias are obviously the direct result of the drowning of valleys of this form by the Flandrian sea-level rise.
The coastline of the killas landscape of West Cornwall is extremely varied: small, sandy coves alternating with rocky promontories and high cliffs are typical of this part of the north coast (Figs 62 and 63). This irregular coastline is due to local variation in the type and strength of the killas bedrock, with weaker units (often slates) eroding to small bays while the more resistant rocks (often limestones or quartzites) form the headlands.
The sandy bays of north Cornwall (c1, Padstow and the River Camel Estuary; c2, Newquay Bay; c3, Perranporth and Perran beach; c4, St Ives Bay) are famous for surfing, due to the splendid waves that roll in from the Atlantic Ocean. Apart from Padstow Bay (c1), at the mouth of the River Camel, most of the north Cornwall beaches are not obviously linked to river sources of sand and so must have been filled by sand transported from offshore sources by storm waves. At many famous surfing beaches, such as Perranporth (c3), sand banks built up by winter storms can be eroded in the summer, resulting in dangerous currents sweeping out to sea. The wind-blown dunes of the Penhale Sands, north of Perranporth, are a spectacular example of the way that gales from the west can move beach sand up to 2 km inland. Because of the variation in the killas bedrock, some headlands are long and the inlets are narrow enough to develop fast tidal flows, capable of forming large, regular ripples as seen in the foreground of Figure 62.
On the south coast, storm-built sandy beaches have also formed, for example at Newlyn (c8), Praa Sands (c5) and at the mouth of Helston valley south of Porthleven (c6). Further east, the coastline is much more sheltered and the scenery is dominated by the drowned valleys and quiet inlets of the Carrick Roads (c7).
AREA 2: EAST CORNWALL AND SOUTH DEVON
This Area straddles the boundary between Cornwall and Devon (Fig. 64). In terms of the coastlines of the Southwest, it includes a small stretch of the north coast near Tintagel, and a large section of the south coast from St Austell, via Plymouth and Start Point, to Torquay and Exmouth (Fig. 65).
In the general section of this chapter, the early geological history of the Southwest Region as a whole has been outlined, particularly the evolution of the Variscan mountain belt. Younger episodes in the region have also been discussed, involving river and valley erosion of the landscape, the effects of the Ice Age and the changes in the coastline that have resulted from the most recent (Flandrian) rise in sea level.
In the sections below we shall consider more local features of the scenery in this Area, dividing it into four distinct Landscapes (labelled A to D), each underlain by a different kind of bedrock (Fig. 66).
FIG 64. Location map for Area 2.
FIG 65. Natural and man-made features of Area 2.
FIG 66. Area 2, showing Landscape A to D and localities (a1, a2 etc.) mentioned in the text.
Landscape A: Granite areas
Bodmin Moor (A6) and Dartmoor (A7) are the most southerly large upland areas in England and, in each case, the granite bedrock has resisted landscape erosion to produce the high ground. The highest point of elevation in this Landscape is High Willhays (a1) at 621 m above sea level on Dartmoor. Evidence of the ongoing nature of this landscape erosion is provided by the contrast between the high moorland, with bogs, steep valleys and exposed tors, on one hand, and the surrounding low farmland on the other.
In the general section of this chapter I have outlined some main features of the southwest granites, such as their resistance to erosion and the valuable minerals associated with them. They have also provided excellent strong building stone for the buildings of the Region.
As in the granite areas of West Cornwall (Area 1), mineral mining activities have had a strong impact on the area, and derelict tin mine buildings are scattered over much of the landscape. The china-clay industry has also produced significant changes to the scenery, one of the most remarkable sites being the workings 3 km northeast of St Austell (a2). These pits now house the famous Eden Project, an educational charity providing a ‘Living Theatre of People and Plants’ and attracting over a million visitors each year (Fig. 67).
FIG 67. The Eden Project is situated in a former china-clay pit. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
The present-day pattern of streams and their valleys has evolved from ancestral streams and valleys that carved most of the inland scenery over millions of years. In the general section of this chapter, we have seen the remarkable way that the drainages of the rivers Tamar and Exe flow southwards across most of the Southwest Region to the sea, divided by the high ground of Dartmoor. A general tilt of the Region to the south, and the resistance of the granite domes to stream and valley erosion, appear to have been important factors. Closer examination of the drainage patterns shows that the streams and valleys of the Bodmin Moor granite tend to radiate out from near its centre, but that the distinctly larger Dartmoor granite has eroded down to form two drainage divides, one in the north and one in the south. This may simply be a matter of the different size of these two granite areas, which has allowed a more complex drainage pattern to develop through time over Dartmoor.
The parallel groups of incised valleys that are common in the granites of Area 1 are not clearly developed on Bodmin Moor and not visible at all on Dartmoor. It is intriguing that the fault system that was responsible for the parallel valleys further to the west is not present in these larger eastern granites. This may tell us something about the greater depth of weathering and erosion experienced by the eastern granites.
There are a number of gorges resulting from the deep incision of rivers and streams into the granites and their surrounding materials. Around the Dartmoor granite, the valleys of the River Dart to the east and the Lydford Gorge to the west (a3) are examples of these. South of the Bodmin Moor granite, the River Fowey also has a spectacular and well-known gorge at the Golitha Falls (a4).
Tors are remarkable features of both the Dartmoor and Bodmin Moor granite areas (Fig. 68). They provide a focus for visitors in granite scenery that is often otherwise rather featureless and empty, and there are well over a hundred tors on Dartmoor alone. Tors tend to look like heaps of granite blocks, but a closer inspection shows that they are not jumbled but rather blocks that ‘belong’ next to their neighbours. These linked blocks are relict volumes of a much larger volume of granite, most of which has disintegrated and been removed by weathering. Tors are very much features of granite weathering, suggesting that the coarse interlocking crystal texture and general lack of layering have caused these remarkable landforms to appear.
Many tors occur on the most elevated parts of the scenery, looking like man-made cairns. Others occur on the slopes of valleys, but it is clear that tors will only form where down-slope processes, driven by gravity, can remove the weathering debris from around them. Cracks in the granite (technically called joints) give tors much of their distinctive appearance: near-vertical joints produce towers and pillars, while roughly horizontal joints give the rocks a layered, blocky appearance (Fig. 69). Most of the joints seem to have formed during the arrival of the granite material from below (intrusion), either due to contraction from cooling of the newly solid material, or due to other stresses acting shortly after solidification. The flat-lying joints (horizontal on hill tops, and parallel to slopes elsewhere) may also be due to the erosion of the present scenery, allowing the granite to expand and fracture as the weight of the overlying material is removed.
FIG 68. Hay Tor, Dartmoor, looking southeast. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
FIG 69. Hound Tor, Dartmoor. (Copyright Landform Slides – John L. Roberts)
The slopes round tors tend to be covered with loose granite blocks (often referred to as clitter), generally angular and obviously derived from the tors (Fig. 70). Finer-grained, crystal-size gravel or sand of quartz and feldspar is another weathering product and is locally called growan or sometimes head. It is clear that much of the alteration of the granite that has resulted in the appearance of the tors must have been strongly influenced by the climate, vegetation and soil-forming conditions existing at different times and in different scenic settings. Much of this has been compared to the weathering and down-slope movement that is seen in high-latitude cold climates today, and so is explained as a result of the cold climate conditions experienced repeatedly during the Ice Age. However, weathering of granites is much faster today in the warm, tropical jungle areas of the world, compared to drier, cooler and less vegetated conditions. Early episodes of weathering of the Southwest Region granites may have taken place under the warm, tropical conditions that are indicated by early Tertiary fossil deposits elsewhere in England.
FIG 70. Mass-flow terrace, looking westward from Cox Tor, Dartmoor. The terrace is interpreted as being the result of down-slope movement under alternating freeze-thaw conditions. (Copyright Landform Slides – Ken Gardner)
Rock basins are low-lying hollows in the granite topography draped with granite weathering products. Sometimes these are dry and their floors are simply coated with granite weathering materials. In other places the hollows are covered by peat, which is often a feature of the higher and wetter parts of the granite hills. Under very wet conditions, the hollows contain deep bogs or mires, with a reputation for being bottomless! How these low hollows were excavated is a puzzle.
Topographic platforms, cut by storm waves during times of high sea level, have been claimed to be present around the Dartmoor and Bodmin granite areas, although they are not as distinctive as those discussed on the Land’s End and Carnmenellis granites of Area 1. The Area 2 platforms are at heights of between 200 and 300 m above sea level, but in the absence of dated deposits similar to the St Erth beds of Area 1, their relevance to sea-level changes is open to doubt. Indeed, as mentioned above, terraces have been recognised around the Dartmoor granite that are thought to be the result of down-slope mass movement under freeze-thaw conditions, rather than due to sea-level changes.
Landscape B: Killas and other Devonian bedrock
Apart from the granites, Devonian sediments make up the bedrock of the southern and central part of Area 2. They consist largely of slates and mudstones with some sandstones, and are known generally as killas to distinguish them from the granites and other younger, less altered sediments. In a few localities around Plymouth (b1) there are Devonian limestones, similar to the well-known limestones around Torquay (d1) and Chudleigh (d9). The settings in which these Devonian sediments may have formed are illustrated in Figure 38, in the general section of this chapter.
The youth of the coastal scenery of this Landscape combines with the vigour of many of the processes operating to make it much more distinctive and dramatic than the inland scenery. In the west of Area 2, cliffs characterise the Cornish section of the south coast and often intersect deeply incised valleys that are clearly older features (Fig. 71).
Cornwall and Devon are separated from each other in this Area by the River Tamar, and this meets the south coast in a large drowned valley system around which Plymouth (b1) has grown (Fig. 72). Plymouth Sound is one of the best natural harbours in the Southwest and the historical naval importance of this city is the result. Similar, but smaller, valley systems (sometimes called rias) are common all along this stretch of coast, as they are further west in Area 1. Flooded valleys form the estuaries of the River Fowey, east of St Austell, and farther east still at Salcombe (b3) and Dartmouth (b8).
FIG 71. Polperro, on the south Cornwall coast. (Copyright Dae Sasitorn & Adrian Warren/ www.lastrefuge.co.uk)
FIG 72. The Tamar and Brunel Bridges, between Plymouth (Devon), to the right, and Saltash (Cornwall). (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
The headlands from Bolt Tail (b2) to Start Point (b5) are made of some of the most highly altered and probably oldest bedrock in Devon, although the age of their deposition as sediments is not known. They have been changed locally to mica-rich and hornblende-rich schists that must have been altered (metamorphosed) several kilometres below the surface, before being pushed upwards during the Variscan mountain-building event. The local resistance of these schists to erosion has led to a particularly intricate pattern of small but sharp headlands and tight small bays. The slope map (Fig. 78) reveals a strong east-west orientation of slopes in this area that must be a reflection of folding in the bedrock. Three separate coast platforms, the highest at about 7 m above present sea level, are very clear at Sharpers Head (b4). Each platform represents an episode in the retreat and relative lowering of the sea before the latest Flandrian rise.
FIG 73. Slapton Sands (Fig. 66, b7). (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
Just north of Start Point (b5) lies the ruined village of Hallsands (b6), which vividly illustrates the damage people can unwittingly do in changing features of the coastal zone. From 1897 to 1902 over half a million tonnes of gravel were removed from the bay off Hallsands to construct an extension to the dockyard at Plymouth. Engineers believed that natural storm currents offshore would replenish the material they had taken, but this did not happen. Instead, the removal of the shingle left the beach open to intense storm erosion, and in January 1917, some 15 years later, the lower part of the village and a sizeable section of coastline were removed by a combination of storm and tide conditions.
Further north, the 3.5 km long barrier beach of Slapton Sands (b7) is another shingle barrier kept active by storm waves from the southeast (Fig. 73). The freshwater lagoon behind it, Slapton Ley, is a nature reserve, home to many rare species of plants and animals. It is under threat from erosion and breaching of the shingle barrier, causing flooding by salt water, and from silting up because of ploughing and deforestation of the inland landscape.
This Landscape of Area 2 also includes a short section of the north Cornish coast around Tintagel (b9), which was an important trading centre on this difficult coast and became the site of a twelfth-century Norman castle, linked to the legends of King Arthur (Fig. 74). The coastline here is often sheer and rugged, and the bedrock contains sharp folds, fracture surfaces and multiple surfaces of mica-rich cleavage, providing evidence of extreme compression during the Variscan mountain building. Many of the cliffs are flat-topped, because erosion has been controlled by relatively flat-lying surfaces in the bedrock, which contrasts sharply with the hog’s-back or whaleback cliffs of other coastal stretches.
Landscape C: The Carboniferous Culm of Devon
The northern landscape of Area 2 is underlain by Carboniferous bedrock (locally known as the Culm) which occupies a complex downfold in this part of the eroded Variscan mountain belt. The coastal bedrock here contains many beautiful examples of folding, for example at Boscastle (c3), famous for the flash flood that did so much damage in August 2004. Spectacular folding is also clearly visible at Millook Haven (c2; Fig. 75), and at Crackington Haven (c1; Fig. 76). In both cases, the convergence directions represented by the folds are near vertical, suggesting that the Variscan folding may have involved a later tilting of an earlier fold set.
FIG 74. Tintagel Head (Fig. 66, b9). (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
FIG 75. Chevron folding of Carboniferous sandstones and mudstones, Millook Haven (Fig. 66, c2). (Copyright Landform Slides – Ken Gardner)
FIG 76. Overturned fold in Crackington Formation, Culm Measures, Crackington Haven (Fig. 66, c1). (Copyright Landform Slides – Ken Gardner)
Landscape D: New Red Sandstone and younger bedrock
Along the eastern edge of Area 2, relatively unfolded New Red Sandstone of Permian and Triassic age rests on the folded Devonian and Carboniferous sediments of Landscapes B and C. The junction of the younger material with the older was formed when the younger sediment was deposited on the eroded margins of the Variscan hills. The New Red Sandstone occurs in a wide, north-south trending belt, extending southwards as far as Torquay (d1) and Paignton, and with fingers extending westwards to the north of Dartmoor (Fig. 37). Along the coast, from Exmouth (d4) southwards via Dawlish (d3) and Teignmouth (d2), the New Red Sandstone has been quarried and penetrated by the tunnels of the main coastal railway to the Southwest. The sandstone forms dramatic red cliffs, and marks the western edge of the World Heritage Site that extends to the east along the coast of Dorset.
The New Red Sandstone consists of sandstones, gravels and mudstones that formed as alluvial fans, desert dunes and in short-lived lakes along the edge of an irregular hilly landscape of older bedrock. The characteristic red colour so typical of many Devon soils has largely been derived from these New Red Sandstone rocks. In the Exeter area (d5), roads have been spectacularly cut through the New Red sediments, and also through some scattered deposits of volcanic rock, mainly lava. These lavas have been highly altered and have not resisted weathering at the surface any more than the sediments of the succession, so they have not had much influence upon the scenery.
An intriguing feature of the New Red Sandstone is the way the original landscape on which it formed is reappearing as the present landscape erodes. For example, the Crediton Basin (d6), north of Dartmoor, is now obvious as a remarkably finger-like strip of sediment, only 2–3 km across north to south, but extending almost 40 km west to east (Fig. 37). Detailed examination of the New Red sediment in this basin shows that it was deposited as the fill of a long, narrow valley, with material being derived from north and south as well as along its length from the west. The valley formed parallel to the folds and faults of the earlier Carboniferous bedrock on each side of it, and must have been cut first by river erosion in Permian times, controlled by the earlier folds and faults that were formed during the Variscan mountain convergence. A few kilometres further north, the Tiverton Basin in Area 3 has a similar west-to-east trend, though it is more open and less elongate.
From Exeter (d5) to Torquay (d1), the base of the New Red Sandstone reveals topography of hollows eroded westwards into a higher ground of Devonian and Carboniferous bedrock. The New Red Sandstone pattern is of alluvial fans radiating downstream, but generally draining towards the east, and it bears a striking general similarity to the present drainage and scenery of the area (Fig. 77). During Variscan convergence, the Devonian limestones were moved into their present pattern by flat-lying faults, and this was then followed by the intrusion of the Dartmoor granite, which may help to explain why the New Red valleys here were shorter than those preserved as the Crediton (d6) and Tiverton basins.
FIG 77. Reconstruction of the Permian topography and drainage, looking southwards towards the location of present-day Torquay (Fig. 66, d1). The current coastline is indicated purely for reference; there is no evidence for a sea in Permian times where the sea now is.
I have already described the importance of the Devonian limestones in creating topography in the Torquay area (d1) and around Torbay generally. This material is an important part of the bedrock in its own right, and has been quarried widely as a building stone. It was a popular stone for ornaments and furniture, particularly in Victorian times, when cut and polished fossil corals featured in many of the washstands that were produced in the period.
In Torquay the Kent’s Cavern complex of caves, formed by solution of Devonian limestones, is an important archeological site, preserving evidence of Heidelberg and Neanderthal man from deposits about 450,000 years old. These were deposited during the Anglian cold phase of the Ice Age, when ice sheets spread across East Anglia and into the Thames valley, though not into the Southwest. The remains of cave bears, hyenas and sabre-tooth cats have also been found in the cave complex, as well as those of modern humans.
Конец ознакомительного фрагмента.
Текст предоставлен ООО «ЛитРес».
Прочитайте эту книгу целиком, купив полную легальную версию (https://www.litres.ru/peter-friend/southern-england/) на ЛитРес.
Безопасно оплатить книгу можно банковской картой Visa, MasterCard, Maestro, со счета мобильного телефона, с платежного терминала, в салоне МТС или Связной, через PayPal, WebMoney, Яндекс.Деньги, QIWI Кошелек, бонусными картами или другим удобным Вам способом.
Peter Friend
Illustrated with beautifully detailed photographs throughout, New Naturalist Southern England comprehensively explores the formation of these wonderful landscapes that are so universally admired.Most people share an enthusiasm for beautiful and breathtaking scenery, explored variously through the physical challenge of climbing to the top of the tallest mountains or the joy of viewing the work of a painter; but while easy to admire from a distance, such landscapes are usually difficult to explain in words. Harnessing recent developments in computer technology, the latest New Naturalist volume uses the most up-to-date and accurate maps, diagrams and photographs to analyse the diverse landscapes of Southern England.Peter Friend highlights the many famous and much loved natural landscapes of the southern half of England, ranging from the Chalk Downs to the bays of Cornwall, Devon and Dorset, and provides detailed explanations for the wide variety of natural events and processes that have caused such an exciting range of surroundings.Setting apart the topography that has resulted from natural rather than man-made occurrences, Friend focuses on each region individually, from East Anglia to London and the Thames Valley, and explains the history and development of their land structures through detailed descriptions and colourful diagrams.
Collins New Naturalist Library
108
Southern England
Peter Friend
Editors (#ulink_f10562bc-1341-55d4-bf0e-9e6fe918e733)
SARAH A. CORBET, ScD
PROF. RICHARD WEST, ScD, FRS, FGS
DAVID STREETER, MBE, FIBIOL
JIM FLEGG, OBE, FIHORT
PROF. JONATHAN SILVERTOWN
The aim of this series is to interest the general reader in the wildlife of Britain by recapturing the enquiring spirit of the old naturalists.
The editors believe that the natural pride of the British public in the native flora and fauna, to which must be added concern for their conservation, is best fostered by maintaining a high standard of accuracy combined with clarity of exposition in presenting the results of modern scientific research.
Table of Contents
Cover Page (#u3c0aa122-d385-5c11-9855-f0b69092c134)
Title Page (#uc79fc63d-decc-53f4-a7ec-8f9b32ee7463)
Editors (#u69efca08-d8f3-5d5e-b47d-cd5c3105b42f)
Editor’s Preface (#u7566d457-b797-5829-8640-e44f8b471629)
Picture Credits (#u6afac44d-9de2-5770-b9cd-97dfcba07701)
Author’s Foreword and Acknowledgements (#u41ed23fd-8fa8-58c3-bf6a-9564e883f457)
CHAPTER 1 Looking at Southern England’s Landscapes (#u5c6cc703-5cb5-5819-862c-f0329bf3fead)
CHAPTER 2 Time, Process Southern England’s Landscapes (#u0cb14e9f-2ff6-54ec-9aa9-fb1a8ec7fb44)
CHAPTER 3 Movement of the Earth’s Surface from Within (#ue7c83b75-53b6-5a6c-a9f2-bc327881ada7)
CHAPTER 4 The Southwest Region (#u7edeae7e-1d21-530b-b0bc-f84f0aa0be57)
CHAPTER 5 The South Coast Region (#litres_trial_promo)
CHAPTER 6 The Severn Valley Region (#litres_trial_promo)
CHAPTER 7 London and the Thames Valley Region (#litres_trial_promo)
CHAPTER 8 The East Anglia Region (#litres_trial_promo)
CHAPTER 9 The Making of Southern England (#litres_trial_promo)
Further Reading (#litres_trial_promo)
Index (#litres_trial_promo)
The New Naturalist Library (#litres_trial_promo)
About the Author (#litres_trial_promo)
Copyright (#litres_trial_promo)
About the Publisher (#litres_trial_promo)
Editor’s Preface (#ulink_d39314b2-93a0-555f-af82-eeb074bbd2ce)
L. DUDLEY STAMP’S Britain’s Structure and Scenery was one if the earliest books of The New Naturalist Library, published in 1946. Repeated . later editions in the period to 1986 testified to the success of his approach towards providing a geological framework for understanding Britain’s landscapes and natural history. He began his account in these words: ‘The wealth of a country’s fauna and fauna and flora is not to be measured by numbers of species alone. Its wealth lies rather in variety, and to a naturalist in the British Isles, the fascination of the native fauna and flora is in the great variety to be found in a small space.’ This variety has its foundation in the underlying geology and the landscapes which are derived from the geology, as Dudley Stamp described so well. For some time, it has been the ambition of the Editors to approach this subject again, since our understanding of the geology and associated landscape evolution has increased so very significantly in the last few decades. The author, Peter Friend, has had long experience of active field research in geology and landscape in many parts of the world, as well as having an intimate knowledge of the subject in the British Isles. He has been able to take full advantage of modern developments in computer mapping and colour printing, making it possible to present the subject in a novel fashion, with great clarity, following the New Naturalist tradition emphasising the importance of illustration. The individual treatment of regions and areas of Southern England brings to the fore the significance of geology and landscape for naturalists who have local or wider interests at heart, giving a necessary basis for relating biodiversity to geodiversity. These two aspects of natural history have come to be seen to be widely significant in understanding plant and animal distribution as well as the problems of conservation. The book is therefore a very ti mely addition to the New Naturalist Library.
Picture Credits (#ulink_d5bd70c2-1c48-5314-adcd-374f52fe7d27)
THE PHOTOGRAPHS and other illustrations that form such a key part of this book have come from many sources, and I am grateful to the following organisations and individuals for kindly allowing me to use their material:
Aerofilms (Figs 133, 232, 249, 252, 256, 258, 259, 261, 270, 280, 309, 312)
British Geological Survey (Figs 152, 284)
Cambridge News (Fig. 233)
Cambridge University Collection of Air Photographs (Figs 25, 118, 120, 243, 253, 282, 300, 310)
Cassini Publishing (Figs 26, 27)
Sylvia Cordaiy Photo Library (Fig. 141)
Robert Harding Picture Library (Fig. 313)
English Heritage (Figs 83, 308)
Landform Slides – Ken Gardner (Figs 17, 18, 43, 55, 70, 75, 76, 85, 88, 104, 161, 162, 165, 195)
Landform Slides – John L. Roberts (Fig. 69)
Last Refuge Ltd – Adrian Warren, Dae Sasitorn and Will Brett (Figs 41, 42, 46, 47, 53, 54, 56, 58, 60, 62, 63, 67, 68, 71, 72, 73, 74, 82, 86, 89, 90, 91, 101, 102, 103, 108, 110, 111, 112, 119, 121, 122, 125, 130, 132, 134, 135, 142, 144, 146, 148, 158, 163, 164, 169, 171, 175, 176, 185, 191, 192, 193, 194, 196, 216, 217, 218, 219, 220, 222, 254)
London Aerial Photo Library (Figs 23, 199, 205, 207, 235, 257, 277, 297, 298, 303, 314)
Norfolk Museums and Archaeology Service – Nick Arber (Fig. 16)
Norfolk Museums and Archaeology Service – Derek A. Edwards (Figs 1, 287, 293, 301, 302, 306, 307, 315)
Network Rail (Fig. 145)
Peter Oliver, Herefordshire and Worcestershire Heritage Trust (Fig. 172)
Mike Page (Fig. 311)
Science Photo Library (Fig. 2)
Sedgwick Museum, Cambridge (Fig. 239)
R. C. Selley – Petravin Press (Fig. 202)
Sheila Smart (Fig. 131)
Suffolk County Council (Fig. 255)
Victoria & Albert Museum (Fig. 250)
Illustrations that do not have a source credited in the caption are my own work, or that of the team working with me at the Department of Earth Sciences in Cambridge.
Author’s Foreword and Acknowledgements (#ulink_c0a9d3d2-e3de-52fa-acb0-ac6d8008bc9c)
MOST PEOPLE ENJOY SCENERY. In my case, an enthusiasm for exploring the countryside was learnt early on from my parents, and my career as a geologist has since allowed me to explore landscapes from the Arctic to tropical deserts and jungles. My hope is that this book will help more people to enjoy the countryside by bringing together some of the exciting recent discoveries about our Earth.
Landscapes are easy to look at, but difficult to describe in words. Recent developments in computer technology offer powerful ways of analysing and presenting landscapes using maps, diagrams and photographs, and it is this imagery that forms the core of this book. Developing the imagery has been the main role of a succession of enthusiastic helpers. Lucinda Edes, Emilie Galley, Liesbeth Renders and Helena Warrington brought their skills and enthusiasm to the early days of the project, working out what could be done best. James Sample has more recently further developed the methods of presentation, and has played a key role in bringing this project to fruition. All have helped to make the project enjoyable as well as productive.
The home for this project has been the Department of Earth Sciences at the University of Cambridge. I walked into the Department as a first-year undergraduate more than 50 years ago and, apart from a period in the Scott Polar Research Institute, I have been based here ever since. I have been teaching and exploring the scenery and geology of many parts of the world, including multiple visits to Spitsbergen, Greenland, Spain, India and Pakistan. This has been an exciting period to be working in geology, particularly in Cambridge, because many key advances have been achieved by the people working here. Apart from the great benefit of being part of this research environment, I have enjoyed the support of six successive Heads of Department and many other colleagues, especially our administrator Margaret Johnston and her team. It has been invaluable to have access to the excellent library run by Ruth Banger and Libby Tilley, and the patient computer support of Jun Aizawa, Aidan Foster, Pete Hill and Pete Wilkins.
I would also like to acknowledge my debt to the Cambridge college system, particularly my own college, Darwin. The College has provided me with the congenial friendship of many people from diverse backgrounds, and their skills have helped me to remain a generalist in my interests.
Any work of this sort on the British Isles owes a fundamental debt to the British Geological Survey (BGS), now based at Keyworth near Nottingham. The numerous Survey maps and reports on this country provide a remarkable source of carefully observed and objective information. The BGS has also readily provided advice and discussion of this project, and helped to determine the sort of coverage and level that would be best.
Many other people have made important contributions by providing ideas and information. These include: John R. L. Allen, Julian Andrews, Muriel Arber, Steve Boreham, Becky Briant, Keith Clayton, Tony Cox, Alan Dawn, Colin Forbes, Brian Funnell, Phillip Gibbard, Steve Jones, Gerald Lucy, Dan Mackenzie, Bob Markham, Charles Notcutt, Bernard O’Connor, Richard Preece, Graham Ward and Richard West.
This book is dedicated to the Dr John C. Taylor Foundation, which has provided the financial support for the project, allowing me to work with such a remarkable succession of talented young assistants. More than 40 years ago, John spent two summers exploring the geology of Spitsbergen with me, and we have remained friends ever since. I am extremely grateful for the help of his foundation during the writing of this book.
CHAPTER 1 Looking at Southern England’s Landscapes (#ulink_ba3f8aa6-8516-5500-a71c-901f189eec8f)
FIRST APPROACHES
THE WORD LANDSCAPE has different meanings for different people, and the best way to illustrate the meaning I have adopted in this book is to look at an example. I have chosen a landscape in the northwest corner of Norfolk, part of our East Anglia Region (Fig. 1).
FIG 1. Landscape of the northwest corner of Norfolk. (Copyright Norfolk Museums and Archaeology Service & Derek A. Edwards)
My approach is to focus first on the natural features that we can call landforms, because they have distinctive shapes that directly reflect the processes that formed them. In this Norfolk landscape, coastline landforms are clearly defined but are remarkably varied, ranging from sea cliffs to sandy beaches, gravel spits, wind-blown dunes and salt marshes. Inland, the main features in this photo are the groups of buildings that form the villages and the edge of the town of Hunstanton, and the pattern of fields and woods. All of these are man-made features and are best understood by following the work of landscape historians. My interest is primarily in the natural topography on which these man-made features have developed, because even in this rather flat landscape – and not clearly visible on the photograph – there are gentle hills, valleys and streams that I want to try to understand.
Scale and size in landscapes are important considerations that we will return to frequently. The landscapes that we shall be discussing are generally kilometres to tens of kilometres across, and they are often best examined from the air, or by using computer-based maps with exaggerated vertical scale.
Southern England contains many famous and well-loved natural landscapes, ranging from the Chalk Downs, with their unique flora and fauna, to the rocky promontories and bays of Cornwall, Devon and Dorset. In total topographic contrast, the Fens of East Anglia are regarded by some as representing an extreme absence of any scenery at all, but their remarkable flatness is of interest because they are the result of recent sea-level rise, and of engineering on a remarkable scale. These different landscapes are produced by a wide variety of events and processes; exploring these is the theme of this book.
As we have already seen, landscapes have often been extensively modified by people. The early clearance of woodland and the construction of field boundaries have profoundly changed the scenery and, more recently, the construction of buildings, roads, railways, canals and airports has almost completely covered some areas of Southern England. Figure 2 shows night-time lighting in cities, towns and oil platforms, giving a vivid impression of the present extent and distribution of the larger settlements. It is surprising how varied the population density is, even in crowded Britain. Using the figures for 2002, the population density of the UK overall is 244 people per square kilometre, but this conceals a huge variation: 8 people per square kilometre in the Highland Region of Scotland, 143 for Cornwall, 149 for Norfolk and an amazing 13,609 for Kensington and Chelsea in London.
The main focus of this book is the pattern of large scenic features that have resulted from natural episodes that predate human influence. It is not usually difficult to distinguish the natural from the man-made, and the study of the natural can often explain many aspects of the way our ancestors lived in the landscape. It is possible to uncover the reasons why people have chosen to settle with their families in certain places, why villages have grown by the clustering of houses in particular locations, and why some villages have then grown further and turned into towns and eventually cities. Even the roads, railways and airfields have clearly grown using the valley floors, river crossings, better-drained slopes and plateaus that are part of the natural scenery.
FIG 2. Satellite image over Britain showing artificial lighting at night. (Copyright Planetary Visions Ltd/Science Photo Library)
There is a further enjoyment that people find in landscapes and scenery that is more difficult to understand. Is it just the physical challenge that causes people to walk and climb to the tops of hills, mountains and other viewpoints? Why do people enjoy the work of landscape painters and photographers? Why do so many tourists in cars choose to take ‘scenic’ excursions rather than the shortest routes, and why is the preservation of ‘unspoilt’ or wilderness areas now such a popular cause? It is difficult to understand the various emotions involved, and trying too hard to analyse them may be missing the point. So it seems best to hope simply that this book will help to satisfy some people’s curiosity, and at the same time add to their enjoyment of our natural landscapes.
MAPPING AND ANALYSING SOUTHERN ENGLAND
The detailed discussions of most of the rest of this book have involved dividing Southern England into a number of Areas that form the ‘building blocks’ for the coverage of Southern England (Fig. 3). Each Area is based on a double-page spread of the size used in many of the larger road atlases available for Britain. In this case I have used the Collins Road Atlas, Britain. This means that total coverage of Southern England is provided, and it is easy for the reader to navigate from place to place. At the beginning of each Area description, a location map of the Area and its neighbours is provided. Ordnance Survey (OS) National Grid References are provided for the edges of the Area, in km east and north of the arbitrary OS Grid origin some 80 km west of the Scilly Isles.
FIG 3. Division of Southern England into Regions and Areas.
For convenient reference the Areas – numbered 1 to 16 – are grouped into five Regions. Each Region forms a chapter and starts with a general introduction:
Even the Area building blocks are relatively large, with arbitrary boundaries, and it has generally been helpful to discuss smaller areas within and across these boundaries that are based on natural features of the scenery (Fig. 4). I have called these smaller areas Landscapes, because they are characterised by distinctive features, usually reflecting aspects of the bedrock or distinctive events in their evolution.
These Landscapes correspond closely to area divisions of England that were defined by the Government Countryside Agency (www.countryside.gov.uk). This scheme divides England into 159 ‘character areas’ on the basis of natural features of the scenery along with aspects of its human settlement, past and future development, land use and vegetation and wildlife, so they are likely to be familiar divisions to many readers of the New Naturalist series. Other Government agencies (particularly the Department for Environment, Food and Rural Affairs) that administer the funding of land management use the same character area division.
FIG 4. Examples of the three levels of division adopted in the treatment of Southern England.
Maps displaying patterns of elevation of the countryside above sea level are an important part of the discussions. The elevation data on the maps in this book have been compiled and made available as part of the LANDMAP project, which provides a computer-based digital survey of Britain for research and educational use. LANDMAP Digital Elevation Maps (DEMs) are based on satellite radar survey measurements which divide the land surface into a grid of 25 m by 25 m pixels. The average height of each pixel is then measured to produce a terrain model with a vertical accuracy of about ± 5 m. A standard colour shading scale is used to represent heights, ranging from greens for the lowest ground, through yellows and browns, to greys for the highest ground. It is best to use the full range of colours for each map, no matter what numerical range of heights is involved. This makes it possible to convey the fine detail of slopes etc., whether the map is for the Fens or the high moors. To make it possible to compare between maps using this colour scheme, we have quoted the maximum elevation reached in each Area on each map.
I have used ESRI ARC Geographic Information System (GIS) software in the processing and manipulating of the map data. This software makes it possible to present artificial hill-shading, which makes the topography easier to understand, and to provide maps representing slope patterns in certain areas.
In addition, data on roads, railways, coastlines, town boundaries, rivers, etc. suitable for reproduction at a scale of 1: 200,000 has been made available by the Collins Bartholomew mapping agency. For any further details of the areas covered, it is recommended that Ordnance Survey Landranger (1: 50,000) maps are consulted.
LANDSCAPE CHANGE
We tend to think of rural landscapes as unchanging features of our surroundings, in contrast to the man-made scenery of cities and towns. Yet we all know of local catastrophes, such as a sea cliff collapsing during a storm, or a flooding river removing its bank and wrecking the nearby buildings, and these are the sorts of local events that do result in change. Despite the excitement, individual changes of this sort are small and can usually be regarded as local modifications. However, over time, the accumulated effects of many such modifications can cause whole landscapes to change.
Size and time clearly both play key parts here. The collapsed cliff or eroded river bank will probably be tens to hundreds of metres long at most, while the larger landscape features picked out in this book are tens or even hundreds of kilometres across. Noting the length scales involved in this way is an important way of keeping such differences clearly in mind.
Moreover, while local events such as the destruction of landforms or buildings may be immediately newsworthy, more long-term patterns of change in the natural scenery are rarely apparent during the life spans of people, and even during the hundreds of years of written records. So it becomes necessary to use indirect and circumstantial evidence – to play the detective – to find out what long-term changes have been going on.
An important step in thinking about the natural landscape is to look at it in terms of modifications to complex surfaces defined by the ground. On land, we tend to be most aware of erosional processes removing material, but it is important to realise that the material removed has to end up somewhere – and this will involve its deposition later, on land or in the sea. How much material was removed from the cliff during the storm or from the banks of the river during the flood? Where did the lost material go, and how did it change the landscape when it was deposited at its new destination? Knowledge of these surface modifications can provide a yardstick that allows us to compare different sorts of changes happening over different periods of time and at different scales, and can help us to work out their relative importance, quoting amounts and rates. For example, a flooding river may remove a hundred metres of river bank, modifying the local landscape a little in the process. However, this modification is unlikely to have much impact on the scenery, unless followed many, many times by similar modifications, over centuries to hundreds of thousands of years. In this way a series of such floods can erode and move material that, in the long run, may be of sufficient volume to significantly change the landscape, for example lowering a hill slope or filling a valley bottom.
The majority of- but not all – surface modification processes act to reduce or flatten topography, mainly by eroding the higher features but also by filling in lower ground with sediment. So logically landscapes might always be regarded as tending towards a flat surface. Our understanding of the processes involved suggests that any land area with mountains or hills will be eroded downwards to an increasingly flat surface as time passes, although the rate of erosion will reduce as the topography becomes more and more smooth. Acting against these flattening processes are periodic movements of the ground surface caused by forces within the Earth, producing new mountains and hills, and so creating new landscapes (Fig. 5).
Continuing research into the processes operating within the Earth shows that movements of the Earth’s crust are taking place continuously, even though the rates involved are generally too slow to be noticeable. The discovery that the Earth’s surface consists of a large number of tectonic plates in continuous relative movement was one of the major breakthroughs in the earth sciences, and has fundamentally changed our understanding of the planet. More on this topic will be considered in Chapter 3, but at this point it is important to realise just how slow the rates of movement are: at most a few centimetres per year on average (often compared to the rate at which fingernails grow). Occasionally, movements of centimetres or metres occur within seconds along faults during earthquakes, but the average rate of movement is still rather slow. Most of us living in stable areas are totally unconscious of any movement at all because we are, ourselves, moving slowly with the landscape that we live on. Slow though the movements may be in a particular landscape, so also are the rates of surface modification, and the balance between the two is a delicate one. In much of Southern England modification by surface processes is dominant, but this has not always been the case.
FIG 5. Landscapes are changed by surface modifications (Chapter 2) and solid earth movements (Chapter 3).
Our next chapter deals with the timescales represented in the landscapes of Southern England and the processes that have been modifying them. Chapter 3 deals with the movements from below – from within the Earth’s crust – that are ultimately creating major landscape patterns.
CHAPTER 2 Time, Process Southern England’s Landscapes (#ulink_77560cca-d3bc-5dd8-9cc3-96fb80e32610)
BEDROCK AND SURFACE BLANKET
WALK AROUND THE COUNTRY IN SOUTHERN ENGLAND and the ground beneath your feet is very rarely solid rock. You are walking over soil made of weathered mineral grains and organic debris, along with other relatively soft and granular materials that make up the surface blanket. Beneath the surface blanket lies solid rock, the bedrock of the landscape.
Bedrock forms the bones of the land. From the colour of the soil, to the elevation of the hills, to the types of vegetation present, the landscape is profoundly influenced by the bedrock underlying it. For example, in Southern England the Lower Greensand (a distinctive layer of bedrock of Early Cretaceous age, see page 26) produces soil water with acidic chemical properties. The Lower Greensand was originally deposited as sand over a period of a few million years, more than 100 million years ago. This layer represents a different environment of deposition from the older sediments on which it lies, and was followed by another change of environment which produced the deposits that lie on top of it. Both the preceding and the following bedrock deposits have alkaline chemical properties. In certain regions the bedrock layers have now been brought to the surface of the landscape by erosion and movements within the Earth. The Greensand is harder than the layers above and below it (largely mudstones) and so is generally more resistant to weathering. In some areas the Lower Greensand lies just below the surface blanket and has resisted the general landscape erosion to form a distinct Greensand ridge running across the countryside, characterised by special vegetation adapted to the acidity of the soils.
It is only in cliffs or at man-made excavations such as quarries that we can see bedrock at the surface in most low-lying areas. By using those areas where the bedrock does outcrop at the surface, and the results of drillings (e.g. for wells), we can discover the types and arrangements of rock below any landscape.
THREE DIFFERENT TIMESCALES
More recent past events tend to be better known and of greater interest than distant past events. Figure 6 is plotted on a logarithmic timescale, so that the most recent times are given more space and greater ages are given less and less space.
FIG 6. Three different timescales, plotted to give more space to more recent events.
For the purposes of this book, we can distinguish three overlapping timescales to help us to understand the landscapes of Southern England:
The bedrock timescale (extending from 542 million years ago to about 2 million years ago)
The Ice Age timescale (covering roughly the last 1 million years)
The last 30,000 years timescale
We shall now review each of these, commenting on the sorts of episodes in each that are important in our exploration of Southern England.
THE BEDROCK TIMESCALE
Figure 7 is a simplified version of a generally accepted geological timescale relevant to the landscapes of Southern England. The names of the divisions are universally accepted in the geological world and, unlike the previous diagram, the passage of time is represented on a uniform (linear) timescale. The divisions have been selected, and sometimes grouped, to help in our analysis of the situation in Southern England, and these have been colour-coded for use in the rest of the book.
FIG 7. Bedrock timescale for Southern England.
The rocks at, or just below, the surface of Southern England range in age over hundreds of millions of years, and most of them were formed long before the present scenery began to appear. At the time of their origin, these rocks were deposited in a variety of different environments, mostly when mud or sand materials were transported into and/or around the seas that existed where England is now. Most of the bedrock of Southern England was formed in this way and is said to be of sedimentary origin. The depositional conditions varied from time to time: the climate varied, the geographical pattern of rising and sinking land movements changed, and the supply of mud and sand brought downstream by rivers changed also. Despite these fluctuations, it is possible to generalise the way that sediment has accumulated over an area the size of Southern England, and to offer a succession of layers of different composition, age and average thickness that can provide a general guide. This is shown in Figure 8.
For each of the Regions (and some of the Areas) discussed in Chapters 4 to 8, a rock column, generalised for that particular area, will show the main bedrock layers. Each column will be coloured using the standard colour codes of this book to represent the ages of the layers.
As an example, we will consider another particularly distinctive layer of bedrock, the Chalk, which ranges between 200 and 400 m in thickness. Chalk is visible quite widely at or just below the surface over perhaps a quarter of the area of Southern England (Fig. 9). Chalk is an easily recognised rock because it is made of very small fragments of lime (calcium carbonate) and is usually brilliant white. It formed from fine-grained limey mud deposited on the sea bed, but through many millions of years of burial below other sediments it has been compressed and altered into the hard rock we recognise today. The Chalk is a result of a unique combination of environmental conditions and the presence of particular algal organisms in the history of evolution. It is only found in northwest Europe, and was only formed in Late Cretaceous times.
The presence of Chalk near the surface in Southern England is almost always linked to the presence of hills and slopes in the scenery, clearly showing that Chalk is a tough material that resists landscape erosion more than most of the other rock types available. The Chalk is also noteworthy because it represents the most recent time when most of Southern England was covered uniformly with soft sediment and a shallow sea: in Late Cretaceous times, except for possible islands in the southwest, there was no emergent land across Southern England.
Like all sedimentary bedrock layers, the Chalk initially formed as flat layers or sheets of sediment, extending widely across the floor of the sea. As will be discussed in the next chapter, these sheets of sediment are generally characteristic of stretching movement episodes in the Earth’s surface. Such movements produce areas of collapsed and low-lying land that can accommodate large volumes of sediment, if it is available.
FIG 8. Generalised succession of the bedrock of Southern England, showing a typical thickness for each layer.
FIG 9. The Chalk and its topography. The darker areas represent the chalk uplands.
However, we do not see the Chalk at or near the surface everywhere across Southern England; instead the Chalk forms narrow bands across the land. This is due to later movements affecting the bedrock layers by folding and tilting them, so that some parts were raised (to be later removed by erosion) and other parts were lowered (Fig. 10, A and B). In the millions of years since the sediment layers were laid down, they have been buried, compacted, deformed by various processes, and finally uplifted to form part of the landscape that we know today (deformation processes are treated more fully in Chapter 3). The Chalk layer has been moved and folded as a result of mild compression or convergence, to form a downfold or syncline between the Chilterns and the North Downs, and an upfold or anticline between the North and South Downs (Fig. 10, C). Later, the central part of the anticline was eroded away to produce the bedrock pattern that we recognise today (Fig. 10, D). The vein-like river valleys visible on the elevated Chalk hills of Figure 9 are evidence of this continuing erosion.
FIG 10. Deposition and folding of the Chalk.
LANDSCAPE MODIFICATION BY RIVERS
Weathering of landscape surfaces and the production of soils by the action of rainwater, air and organisms are important factors in shaping landscapes. These processes affect the bedrock when it is very close to the surface, and most of them weaken the material that they work on. This is particularly so when tough silicate rock minerals are altered to soft clay minerals, which are then easily eroded. Freezing and thawing also works to weaken the bedrock as water in cracks freezes and expands, breaking the rocks into fragments.
Whilst weathering is a widespread and general process, most of the other important landscape processes involve the formation of discrete features that we shall call landforms. Rivers result in the formation of a number of important landforms that are described below.
The most important landforms resulting from river processes are the channels of rivers and streams (Fig. 11). When rain falls onto a land surface some of it soaks into the land (forming groundwater), whilst the remainder runs along the surface, collecting in topographical lows and producing stream and river channels. Today, many of Southern England’s river channels tend to be relatively narrow and shallow – only metres or tens of metres in width and less in depth – so they occupy an extremely small percentage of the area that they drain. However, they are still the dominant agents of landscape change, causing downwards and/or sideways erosion as well as acting as conduits to transport the eroded material out of their catchments.
Most river channels develop a sinuous course, becoming curved (or meandering) to varying degrees, or developing a number of channels separated by islands of sediment (becoming braided). The positions of the curves or islands change with time as sediment is shifted downstream, and the position of a river channel will change with time correspondingly.
Because of their ability to erode material and remove the resulting debris, river channels create valleys. The sides of a river valley are referred to as slopes. When a channel cuts downwards the valley sides generally become steeper and slope material (generated by ongoing weathering processes) moves down-slope towards the channel. The material is transported either as small individual fragments or as larger mass flows. Where down-slope movements involve the collapse of large areas of material, the terms landslip or slump are often used. Slope material is then deposited in the channel and removed downstream by the river.
The simplest valleys result from down-cutting by a river or stream to yield a V-shaped profile in cross-section. The gradient of the valley sides depends on the strength of the material that the slopes are composed of in the face of erosion. Stronger materials are more difficult to erode and remove, and so can form steeper slopes than weaker materials. In some areas, the river channel is unable to form valley slopes as the material is too weak to form a noticeable gradient. In the Areas we will be investigating, it is clear that some of the slopes are largely the result of a particularly strong layer in the bedrock resisting erosion as the landscape has developed.
FIG 11. Landforms of rivers.
As the valley develops, its profile can become more complex. In some cases, slopes appear to have retreated across a landscape some distance from the position in which they were initially created by river down-cutting. A river with a wide valley floor is one of the most obvious examples of this, in which movements of the channel across the floor have caused the slopes to retreat as the valley floor has become wider. In some cases, slopes appear to have retreated over many kilometres from the original valley as numerous collapses of the slope took place.
Overall, therefore, the valley profile and the channel course reflect variations in the strength of the material being eroded, and in the strength and flood pattern of the river. Climate changes are likely to have a major effect on the strength of the river by altering the volume of water flowing through the channels. Additionally, the lowering or raising of the channel by Earth movement effects (see Chapter 3) can affect the evolution of the landscape by river processes. For example, both climate change and the vertical movement of the river channel can initiate the formation of river terraces. Different examples of all these river geometries will be discussed in greater detail in the Area descriptions in Chapters 4–8.
Over millions of years, river down-cutting, slope erosion and material transport tend to smooth and lower landscapes until they approximate plains, unless they are raised up again (rejuvenated) by large-scale Earth movements (Chapter 3) or are attacked by a new episode of channel erosion, perhaps due to climate or sea-level change. Southern England generally has a smoothed and lowered landscape, representing hundreds of thousands of years of this river and slope activity.
The branching, map-view patterns of river channels and valleys are an obvious feature of all landscapes. An approach to understanding how this forms is illustrated by a computer-based experiment (Fig. 12) in which a flat surface (plateau or plain) is uplifted along one of its edges, so that it has a uniform slope towards the edge that forms the bottom of the rectangle shown. Rain is then applied uniformly across the surface, causing the formation and down-cutting of channels that erode backwards from the downstream edge. As the experiment continues, the channels and their valleys extend into the uniform sloping surface by headward erosion, resulting in longer valleys, more branches and a greater dissection of the surface by those valleys.
FIG 12. Model showing upstream erosion by tree-like (dendritic) river patterns. (Provided by Dimitri Lague from the work of A. Crave and P. Davy)
As we consider the various Regions and Areas of Southern England, we will summarise the present-day river patterns of each by simplifying the main directions of drainage involved. We will also give an impression of the present-day relative size of the more important rivers by quoting their mean flow rates as estimated in the National River Flow Archive, maintained by the Centre for Ecology and Hydrology at Wallingford.
It seems surprising that today’s often sleepy southern English rivers have been the dominant agent in carving the English landscape. However, even today’s rivers can become surprisingly violent in what are often described as hundred-or thousand-year floods. Floods in the past were certainly more violent at times than those of today, particularly towards the ends of cold episodes, when melting of ice and snow frequently produced floods that we would now regard as very exceptional.
THE ICE AGE TIMESCALE AND LANDSCAPE MODIFICATION
The most recent Ice Age began about 2 million years ago, and is still continuing in Arctic areas. At various times during this period ice has thickly covered most of northwest Europe. Recent research, particularly measurements of oxygen isotopes in polar icecaps and oceanic sediment drill cores, has revealed much of the detail of how the climate has changed during the current Ice Age. It has been discovered that long cold periods have alternated with short warm periods in a complex but rather regular rhythm. Looking at the last half-million years, this alternation has occurred about every 100,000 years, and this is now known to have been a response to regular changes in the way the Earth has rotated and moved in its orbit around the sun. A closer look at the last million years (Fig. 13) reveals that for more than 90 per cent of the time conditions have been colder than those of today. Warm (interglacial) periods, like our present one, have been unusual and short-lived, though they have often left distinctive deposits and organisms.
FIG 13. The last million years of global temperature change. *the Oxygen Isotope Stages are an internationally agreed numbering sequence to label the succession of climatic cold (even numbers) and warm (odd numbers) episodes.
One of the most important cold episodes (glacials), just under half a million years ago, resulted in the Anglian ice sheet. This was up to several hundreds of metres thick and extended from the north southwards, well into Southern England, covering much of East Anglia and the north London area (Fig. 14). As the ice spread slowly southwards, it was constricted between the Chalk hills of Lincolnshire and those of Norfolk. A wide valley, later to become the Wash and the Fens, was filled with ice to a depth well below that of present sea level. As the ice spread outwards from this valley it dumped the rock material it was carrying, including blocks and boulders up to hundreds of metres across, giving some idea of the tremendous power of the ice sheet. The direct evidence for the presence of an ice sheet is material in the surface blanket called till, or boulder clay (Fig. 15). This often rather chaotic mixture of fragments of rock of all sizes (large boulders mixed with sand and mud) lacks the sorting of the fragments by size that would have occurred in flowing water, and so must have been deposited from the melting of ice sheets.
FIG 14. The Anglian ice sheet.
Much of the rest of the surface blanket that accumulated during the last 2 million years was deposited by the rivers that were draining the land or any ice sheets present. As ice sheets have advanced and retreated, so have the rivers changed in their size and in their capacity to carry debris and erode the landscape. Rivers have therefore been much larger in the past as melting winter snow and ice produced torrents of meltwater, laden with sediment, which scoured valleys or dumped large amounts of sediment. The gravel pits scattered along the river valleys and river terraces of Southern England, from which material is removed for building and engineering, are remnants of the beds of old fast-flowing rivers which carried gravel during the cold times.
There are no ice sheets present in the landscape of Figure 16. The scene is typical of most of the Ice Age history (the last 2 million years) of Southern England, in that the ice sheets lie further north. It is summer, snow and ice are lingering, and reindeer, wolves and woolly mammoths are roaming the swampy ground. The river is full of sand and gravel banks, dumped by the violent floods caused by springtime snow-melt. The ground shows ridges of gravel pushed up by freeze-thaw activity, an important process in scenery terms that we discuss below.
FIG 15. Boulder clay or till, West Runton, north Norfolk.
The present-day Arctic has much to tell us about conditions and processes in Southern England during the cold episodes of the Ice Age. Much of the present-day Arctic is ice-sheet-free, but is often characterised by permanently frozen ground (permafrost). When the ground becomes frozen all the cracks and spaces in the surface-blanket materials and uppermost bedrock become filled by ice, so that normal surface drainage cannot occur. In the summer, ice in the very uppermost material may melt and the landscape surface is likely to be wet and swampy. Ice expands on freezing, and so the continuous change between freezing and thawing conditions, both daily and seasonally, can cause the expansion of cracks and the movement of material, with corresponding movements in the surface of these landscapes. This movement can cause many problems in the present-day Arctic by disturbing the foundations of buildings and other structures.
FIG 16. Artist’s impression of Southern England, south of the ice sheet, during the Ice Age. (Copyright Norfolk Museums and Archaeology Service & Nick Arber)
Remarkable polygonal patterns, ranging from centimetres to tens of metres across, are distinctive features of flat Arctic landscapes, resulting from volume changes in the surface blanket on freezing and thawing (Fig. 17). In cross-section the polygon cracks and ridges correspond to downward-narrowing wedges (often visible also in the walls of gravel pits in Southern England). Thaw lakes are also a feature of flat areas under conditions of Arctic frozen ground (Fig. 18). They appear to be linked to the formation of the polygonal features, but can amalgamate to become kilometres across and may periodically discharge their muddy soup of disturbed sediments down even very gentle slopes.
Not only can these frozen ground processes be studied in Arctic areas today, but they have left characteristic traces in many of the landscapes of Southern England. Some examples from Norfolk are illustrated in Chapter 8 (Figs 306 and 307), and these provide specific examples of the result of ancient freeze-thaw processes on a small scale. However, the more we examine the wider features of present-day landscapes across Southern England, the more it becomes clear that most have been considerably modified by the general operation of frozen ground processes during the last 2 million years. These processes are likely to have been responsible for the retreat of significant slopes and even for the lowering of surfaces that have almost no perceptible slope.
FIG 17. Polygonal frozen ground patterns on the Arctic coastal plain near Barrow, Alaska. (Copyright Landform Slides – Ken Gardner)
FIG 18. Thaw lakes, the larger ones several kilometres long, on the Arctic coastal plain near Barrow, Alaska. (Copyright Landform Slides – Ken Gardner)
THE LAST 30,000 YEARS TIMESCALE AND RECENT MODIFICATION
The timescale shown in Figure 19 covers a period during which various episodes have changed the landscapes of Southern England, creating our present-day world. These episodes include the dramatic rise in sea level and landward movement of the coastline caused by the warming of the climate following the last cold episode of the Ice Age. They also include the progressive changing of the countryside by people, leading up to the domination of some landscapes by man-made features.
FIG 19. Time divisions for the last 30,000 years (Late Pleistocene to Holocene).
The last 30,000 years have been warm, on average, relative to the previous 2 million years of the Ice Age. However, the higher level of detail available in this timescale makes it clear that climate change has not been one of uniform warming during this period. Short periods of colder climate, temporarily involving ice-sheet growth in the north of Britain (sometimes called stadials) have alternated with short periods of warmer climate (referred to as interstadials).
SEA-LEVEL CHANGE
The coastline is the most recently created part of the landscape, and the most changeable. This is due, in large part, to the rise in sea level over the last 20,000 years, since the last main cold episode of the Ice Age (the Devensian). Twenty thousand years ago sea level was 120 m lower than it is today because of the great volumes of water that were locked away on land in the world’s ice sheets (Fig. 20). Land extended tens or hundreds of kilometres beyond the present-day coastline, and Southern England was linked to northern France by a large area of land (Fig. 21). Global climate started to warm about 18,000 years ago (Fig. 13) and the world’s ice started to melt, raising global sea level. The North Sea and the Channel gradually flooded, and Britain became an island between 10,500 and 10,000 years ago. This flooding by the sea is known as the Flandrian transgression and was a worldwide episode.
FIG 20. Graph of sea-level rise over the last 18,000 years.
During the period of most rapid sea-level rise (between 12,000 and 8,000 years ago), areas of low-lying land were swamped and some local features of the coastal scenery moved great distances geographically towards their present positions. The sea cliffs, beach barriers, salt marshes, spits and estuaries that can be seen today have only taken up their present positions over the last few thousand years, as sea-level rise slowed.
In the treatment of the Regions and Areas in the rest of this book, maps are presented that distinguish a coastal flooding zone. This presentation is based on the simplifying assumption that the solid Earth movement of Southern England (i.e. any uplift or subsidence, see Chapter 3) has been very small compared with global sea-level changes. The coastal flooding zone is defined as extending between the submarine contour 120 m below present sea level and the contour 20 m above present sea level, and it can be used to identify parts of landscapes which are likely to have been areas of coastline activity in the recent past. Areas of land with an elevation between present sea level and 120 m below sea level correspond to the land submerged during the last 18,000 years of sea-level rise. Areas lying at, or up to, 20 m above present sea level may have been subjected to coastal processes during the highest sea levels of earlier interglacial periods, such as the Ipswichian (see Fig. 13). The coastal flooding zone also defines areas of land that are most likely to become submerged during predicted future rises of sea level.
FIG 21. Two episodes (17,000 and 12,000 years ago) in the rise of sea level around the North Sea area. (Redrawn and simplified from Current Archaeology207, 2006, Gaffney)
Drowned valleys (Figs 22 and 23) are present on the coastlines of Southern England as a result of recent sea-level rise. Formerly, the rivers draining the majority of these valleys would have transported mud and sand to the sea, where it would have been deposited on the sea bed. However, with the rise in sea level mud and sand are now often deposited in the flooded valleys or estuaries instead, and some have developed carpets of sediment, transported down-valley by rivers or brought up-valley by the sea where tides and storms have been effective.
Coastlines with low seaward slopes and a soft surface blanket and/or bedrock may develop beach barriers when flooded by rising sea level. These barriers are ridges of sand or gravel parallel to the general trend of the coastline (Fig. 24). They are created by the impact of storm waves on the gently sloping and soft landscape. They tend to develop a cap of wind-blown sand which is very vulnerable to storm wave erosion, but may eventually become stabilised by vegetation. Behind the barrier a low-lying area of more sheltered conditions develops and regular flooding at high tide may bring in muddy sediment from the sea that can settle and build up salt marshes.
FIG 22. The drowning of a valley by sea-level rise.
FIG 23. Drowned valley of the Deben, Suffolk, viewed from above the sea off Felixstowe Ferry. (Copyright London Aerial Photo Library)
FIG 24. Cross-section of a beach barrier formed as sea level rises over a very gently sloping landscape.
FIG 25. Beach barrier on Scolt Head Island, Norfolk. (Photograph held at Cambridge University Collection of Air Photographs, Unit for Landscape Modelling)
The aerial photograph of part of Scolt Head Island (Fig. 25) in north Norfolk shows the succession of zones parallel to the coastline typical of a recently flooded, gently sloping landscape. On the beach, coast-parallel ridges and hollows (runnels) have been created during recent storms, and are draining water as the photograph was taken at low tide. The crest of the barrier is capped by wind-blown dunes, which have been stabilised by marram grass, but also shows signs of erosion during recent storms. Behind the barrier are salt marshes, generally sheltered from storm waves and developing tidal channels. The salt marshes are forming around the remains of various sand and gravel spits that date from a landscape before the present beach barrier was there. The far side of the salt marsh is marked by a gently curved sea wall built within the last two centuries to reclaim some land by keeping high tides out. Behind that is the boundary between the present flat seaward zone of young sediment and the older terrain, marked by a complex field pattern that is underlain by Chalk bedrock.
DEVELOPMENT BY PEOPLE
My concern in this book is primarily with natural landscapes, and I will tend to comment on the development by people since the Bronze Age only where this relates to the natural features in an interesting way. However, in reviewing the appearance of the whole of Southern England, I have been struck by an intriguing distinction made by some landscape historians: the distinction between ancient and planned countryside (Figs 26–28). I have based my approach on the discussions offered by Oliver Rackham, ecologist and landscape historian, and these are summarised below.
Ancient countryside (Fig. 26) consists of many hamlets, small towns, ancient farms and hedges (of mixed varieties of shrubs and trees), along with roads that are not straight, numerous footpaths and many antiquities.
Planned countryside (Fig. 27) has distinct villages, much larger than the hamlets, along with larger eighteenth- and nineteenth-century farms, hedges of hawthorn and straight roads. Footpaths are less common and the few antiquities that are present are generally prehistoric.
I have re-examined the same areas used by Oliver Rackham as examples of these two countryside types, and compared the early Ordnance Survey maps with maps of the same area generated by me using the data and methods used in this book (see Chapter 1). The shading and ‘hachured’ patterning used in the earlier maps represents the hills and slopes rather clearly – better than the contour representation used in the present-day Ordnance Survey Landranger maps, although these show man-made features much more clearly. My map representation is a compromise in that it represents elevations and slopes using colours and hill-shading, but also allows the patterns of roads and settlements to be seen.
Oliver Rackham’s conclusion is that many of the distinctive features of planned countryside were created by the general parliamentary enclosure of land during the eighteenth and nineteenth centuries. This involved the wholesale conversion of commonly held land with open fields into enclosed fields awarded to individuals and institutions. Many landscape historians have claimed earlier origins for the difference between ancient and planned countryside, believing that historical and cultural differences in the people who settled and developed the two areas played an important role. Variations in the bedrock geology also seem to be important here. For example, the ancient countryside shown in Figure 26 is underlain by strongly deformed Variscan bedrock that has been eroded into small hills and valleys (see Chapter 4).
FIG 26. Example of ancient countryside at the Devon-Somerset border, near Tiverton, with 1809 and recent mapping compared. (Upper part from Cassini Old Series map 181, copyright Cassini Publishing 2007/www.cassinimaps.co.uk)
FIG 27. Example of planned countryside at the Berkshire-Oxfordshire border, around Didcot, with 1830s and recent mapping compared. (Upper part taken from Cassini Old Series maps 164 and 174, copyright Cassini Publishing 2007/www.cassinimaps.co.uk)
FIG 28. Generalised map distinguishing ancient and planned countryside across Southern England.
In contrast, the planned countryside covered by Figure 27 consists of only gently tilted Mesozoic bedrock that has formed a much flatter and more open landscape.
CHAPTER 3 Movement of the Earth’s Surface from Within (#ulink_b8f1aa59-56b9-58ee-ad3a-c811b38a295b)
WIDESPREAD MOVEMENTS OF THE EARTH’S SURFACE
TO UNDERSTAND THE CHANGES and movements affecting the appearance of the landscape on large scales we need to review some geological systems, especially plate tectonics. Many of the large changes that have created landscapes over long periods of time can now be understood using this discovery.
Knowledge of the processes causing the movement of large (10–1,000 km length-scale) areas of the Earth’s surface has been revolutionised by scientific advances made over the last 40 years. During this time, scientists have become convinced that the whole of the Earth’s surface consists of a pattern of interlocking tectonic plates (Fig. 29). The word ‘tectonic’ refers to processes that have built features of the Earth’s crust (Greek: tektōn, a builder). The worldwide plate pattern is confusing – particularly when seen on a flat map – and it is easier to visualise the plates in terms of an interlocking arrangement of panels on the Earth’s spherical surface, broadly like the panels forming the skin of a football.
Tectonic plates are features of the lithosphere, the name given to the ≈125 km thick outer shell of the Earth, distinguished from the material below by the strength of its materials (Greek: lithos, stone). The strength depends upon the composition of the material and also upon its temperature and pressure, both of which tend to increase with depth below the Earth’s surface. In contrast to the mechanically strong lithosphere, the underlying material is weaker and known as the asthenosphere (Greek: asthenos, no-strength). Note that on figure 30 the crustal and outer mantle layers are shown with exaggerated thickness, so that they are visible.
FIG 29. World map showing the present pattern of the largest lithosphere plates.
Most of the strength difference between the lithosphere and the asthenosphere depends on the temperature difference between them. The lithosphere plates are cooler than the underlying material, so they behave in a more rigid way when subjected to the forces generated within the Earth. The asthenosphere is hotter and behaves in a more plastic way, capable of deforming without fracturing and, to some extent, of ‘flowing’. Because of this difference in mechanical properties and the complex internal forces present, the lithosphere plates can move relative to the material below. To visualise the motion of the plates, we can use the idea of lithospheric plates floating on top of the asthenosphere.
Looking at the surface of the Earth (Fig. 29), the largest plates show up as relatively rigid areas of the lithosphere, with interiors that do not experience as much disturbance as their edges. Plates move relative to each other along plate boundaries, in various ways that will be described below. The plate patterns have been worked out by investigating distinctive markers within the plates and at their edges, allowing the relative rates of movement between neighbouring plates to be calculated. These rates are very slow, rarely exceeding a few centimetres per year, but over the millions of years of geological time they can account for thousands of kilometres of relative movement.
It has proved to be much easier to measure plate movements than to work out what has been causing them. However, the general belief today is that the plates move in response to a number of different forces. Heat-driven circulation (convection) occurs within the mantle, but other forces are also at play. Where plates diverge, warm, new material is formed that is elevated above the rest of the plate, providing a pushing force to move the plate laterally, around the surface of the Earth. At convergent boundaries, cold, older material ‘sinks’ into the asthenosphere, providing a pulling force which drags the rest of the plate along behind it. Deep within the Earth, the sinking material melts and is ultimately recycled and brought back to the surface to continue the process.
FIG 30. Diagram of the internal structure of the Earth.
Knowledge of how tectonic plates interact provides the key to understanding the movement history of the Earth’s crust. However, most people are much more familiar with the geographical patterns of land and sea, which do not coincide with the distribution of tectonic plates. From the point of view of landscapes and scenery, coastlines are always going to be key features because they define the limits of the land; we make no attempt in this book to consider submarine scenery in detail.
The upper part of the lithosphere is called the crust. Whereas the distinction between the lithosphere and the asthenosphere is based upon mechanical properties related to temperature and pressure (see above), the distinction between the crust and the lower part of the lithosphere is based upon composition. Broadly speaking, there are two types of crust that can form the upper part of the lithosphere: continental and oceanic. An individual tectonic plate may include just one or both kinds of crust.
Continental crust underlies land areas and also many of the areas covered by shallow seas. Geophysical work shows that this crust is typically about 35 km thick, but may be 80–90 km thick below some high plateaus and mountain ranges. The highest mountains in Britain are barely noticeable on a scale diagram comparing crustal thicknesses (Fig. 31). Continental crust is made of rather less dense materials than the oceanic crust or the mantle, and this lightness is the reason why land surfaces and shallow sea floors are elevated compared to the deep oceans. Much of the continental crust is very old (up to 3–4 billion years), having formed early in the Earth’s life when lighter material separated from denser materials within the Earth and rose to the surface.
Oceanic crust forms the floors of the deep oceans, typically 4 or 5 km below sea level. It is generally 5–10 km thick and is distinctly denser than continental crust. Oceanic crust only forms land where volcanic material has been supplied to it in great quantity (as in the case of Iceland), or where other important local forces in the crust have caused it to rise (as is the case in parts of Cyprus). Oceanic crust is generally relatively young (only 0–200 million years old), because its higher density and lower elevation ensures that it is generally subducted and destroyed at plate boundaries that are convergent (see below).
Figure 29 shows the major pattern of tectonic plates on the Earth today. The Mercator projection of this map distorts shapes, particularly in polar regions, but we can see that there are seven very large plates, identified by the main landmasses located on their surfaces. The Pacific plate lacks continental crust entirely, whereas the other six main plates each contain a large continent (Eurasia, North America, Australia, South America, Africa and Antarctica) as well as oceanic crust. There are a number of other middle-sized plates (e.g. Arabia and India) and large numbers of micro-plates, not shown on the world map.
Figures 29 and 32 also identify the different types of plate boundary, which are distinguished according to the relative motion between the two plates. Convergent plate boundaries involve movement of the plates from each side towards the suture (or central zone) of the boundary. Because the plates are moving towards each other, they become squashed together in the boundary zone. Sometimes one plate is pushed below the other in a process called subduction, which often results in a deep ocean trench and a zone of mountains and/or volcanoes, as well as earthquake activity (Fig. 32). The earthquake that happened on the morning of 26 December 2004 under the sea off western Sumatra was the strongest anywhere in the world for some 40 years. It seized world attention particularly because of the horrifying loss of life caused by the tsunami waves that it generated. This earthquake was the result of a sudden lithosphere movement of several metres on a fault in the convergent subduction zone where the Australian plate has been repeatedly moving below the Eurasian plate.
FIG 31. Scale diagram comparing average thicknesses of oceanic and continental crust and lithosphere.
In other cases the plate boundary is divergent, where the neighbouring plates move apart and new material from deeper within the Earth rises to fill the space created. The new oceanic crust is created by the arrival and cooling of hot volcanic material from below. The mid-Atlantic ridge running through Iceland, with earthquakes and volcanic activity, is one of the nearest examples to Britain of this sort of plate boundary.
Other plate boundaries mainly involve movement parallel to the plate edges and are sometimes called transform boundaries. The Californian coast zone is the classic example but there are many others, such as the transform boundary between the African and Antarctic plates. In some areas, plate movement is at an oblique angle to the suture and there are components of divergence or convergence as well as movement parallel to the boundary.
Britain today sits in the stable interior of the western Eurasian plate, almost equidistant from the divergent mid-Atlantic ridge boundary to the west and the complex convergent boundary to the south where Spain and northwest Africa are colliding. In its earlier history the crust of Britain has been subjected to very direct plate boundary activity: the results of convergent activity in Devonian and Carboniferous times (between 416 and 299 million years ago) are visible at the surface in southwest England, and in Ordovician to Devonian times (between 490 and 360 million years ago) in Wales, northwest England and Scotland.
FIG 32. Diagram illustrating the movement processes of plates (not to scale).
UNDERSTANDING SURFACE MOVEMENTS
We have been considering the large movement systems that originate within the Earth. There are also more local movement systems operating on the Earth’s surface, which are linked to a very variable degree to the large-scale movements of plate tectonics. To explore this complex linkage further, it will be helpful to look now at different processes that may combine to cause particular local movements.
Horizontal movements as part of convergence, divergence or lateral transfer
Tectonic plates are recognised by their rigidity, so there is relatively little horizontal movement between points within the same plate compared to the deformation seen in plate boundary zones. This extreme deformation may involve folding and fracturing of the rock materials, addition of new material from below, or absorption of material into the interior during subduction.
Nonetheless, deformation is not restricted solely to plate boundaries, and does occur to a lesser extent within the plates. In some cases, major structures that originally formed along a plate boundary can become incorporated into the interior of a plate when prolonged collision causes two plates to join. Southern England includes the remains of a former convergent plate boundary and contains many examples of structures of this sort (particularly around Dorset and the Isle of Wight). These structures have often been reactivated long after they first formed in order to accommodate forces along the new plate boundary via deformation within the plate. Conversely, changes of internal stress patterns can sometimes lead to the splitting of a plate into two, forming a new, initially divergent plate boundary. Many of the oil- and gas-containing features of the North Sea floor originated when a belt of divergent rift faults formed across a previously intact plate.
It needs to be stressed that the patterns of deformation (fracturing and folding) due to these plate motions occur at a wide range of different scales, from centimetres to thousands of kilometres. Sometimes they are visible at the scale of an entire plate boundary, such as the enormous Himalayan mountain chain that marks the collision of India with Asia.
The effects of features as large as plate boundaries on landscapes persist over hundreds of millions of years, long after the most active movement has ceased. For example, parts of southwestern England, Wales and the Scottish Highlands are underlain by bedrocks that were formed in convergent boundary zones of the past. The tin and lead mines of Cornwall owe their existence to a 300-million-year-old convergent plate boundary, where an ocean was destroyed as two plates converged and continents collided. The convergence released molten rock that rose in the crust and gradually cooled to form granite, while metals were precipitated in the surrounding crust as ‘lodes’ containing tin and lead (see Chapter 4).
Mapping the patterns of bedrock exposed at the surface often reveals folds and faults that provide key information about the movements that have taken place during the past. Figure 33 provides a key to some of the terms commonly used to classify these structures as a step towards understanding the sorts of movement patterns that they represent. In broad terms, folds tend to indicate some form of local convergent movement, though they may be the result of larger movement patterns of a different kind. Normal faults tend to indicate divergent movements, at least locally, whereas reverse and strike-slip faults tend to indicate convergence. Two broad types of fold are distinguished: synclines are u-shaped downfolds, while anticlines are the opposite – n-shaped upfolds.
Further mapping of folds and faults often reveals complex patterns of changing movements. In the example shown in Figure 34, divergent movements in an area of crust produce plastic deformation in the warmer lower crust, and faulting into a number of discrete blocks in the colder, more brittle, upper crust. This is then followed by an episode of convergent movement that results in closing up the upper crustal blocks and further flow in the plastic lower crust, causing crustal thickening and mountain building at the surface.
Vertical crustal movements as part of other crustal movements
The movement of lithospheric plates is the main cause of convergent and divergent movements affecting thousands of kilometres of the Earth’s surface. As shown in Figures 33 and 34, these horizontal movements are generally accompanied by vertical movements that can produce very large scenic features, such as a mountain belt or a rift valley. In this book we are primarily concerned with scenic features at a more local scale, so we now consider various other processes that may be important in creating vertical crustal movements without contributions from large-scale plate interactions.
FIG 33. The most important types of folds and faults, and the local patterns of forces responsible.
Vertical changes by erosion or deposition
Addition or subtraction of material to the surface of the Earth is happening all the time as sediment is deposited or solid material is eroded. The field of sedimentology is concerned with the wide range of different processes that are involved in the erosion, transport and deposition of material, whether the primary agent of movement is water, ice, mud or wind. An important point is that few of these sedimentary processes relate directly to the large tectonic movements of the Earth’s crust that we have discussed above. Scenery is often produced by erosion of thick deposits that formed in sedimentary basins where material eroded from the surrounding uplands accumulated. One of the characteristic features of these thick deposits is their layered appearance, which is often visible in the scenery. Layering varies from millimetre-scale laminations produced by very small fluctuations in depositional processes, to sheets hundreds of metres thick that extend across an entire sedimentary basin. These thicker sheets are often so distinctive that they are named and mapped as separate geological units representing significant changes in the local environment at the time they were deposited.
FIG 34. Example of a cross-section through the crust, showing how a divergent movement pattern (A) may be modified by later convergent movements (B and C).
Vertical crustal movements resulting from loading or unloading
In addition to the direct raising or lowering of the surface by erosion or deposition, there is a secondary effect due to the unloading or loading of the crust that may take some thousands of years to produce significant effects. As mentioned above, we can visualise the lithosphere as ‘floating’ on the asthenosphere like a boat floating in water. Loading or unloading the surface of the Earth by deposition or erosion will therefore lower or raise the scenery, just as a boat will sit lower or higher in the water depending on its load.
An example of this is the lowering of the area around the Mississippi Delta, loaded by sediment eroded from much of the area of the USA. The Delta region, including New Orleans, is doomed to sink continually as the Mississippi river deposits sediment around its mouth, increasing the crustal load there.
A second example of such loading is provided by the build-up of ice sheets during the Ice Age. The weight of these build-ups depressed the Earth’s surface in the areas involved, and raised beaches in western Scotland provide evidence of the high local sea-levels due partly to this lowering of the crustal surface.
Unloading of the Earth’s surface will cause it to rise. Recent theoretical work on the River Severn suggests that unloading of the crust by erosion may have played a role in raising the Cotswold Hills to the east and an equivalent range of hills in the Welsh Borders (see Chapter 6, Area 9). In western Scotland, as the ice has melted the Earth’s surface has been rising again.
Vertical movements by expansion or contraction
Changing the temperature of the crust and lithosphere is an inevitable result of many of the processes active within the Earth, because they often involve the transfer of heat. In particular, rising plumes of hot material in the Earth’s mantle, often independent of the plate boundaries, are now widely recognised as an explanation for various areas of intense volcanic activity (for example beneath Iceland today). These plumes are often referred to as ‘hot spots’ (see Fig. 32). Heating and cooling leads to expansion or contraction of the lithosphere and can cause the surface to rise or sink, at least locally.
An example of this is the way that Southern England was tilted downwards to the east about 60 million years ago. At about this time, eastern North America moved away from western Europe as the North American and Eurasian plates diverged. The divergence resulted in large volumes of hot material from deep within the Earth being brought to the surface and added to the crust of western Southern England. It is believed that the heating and expansion of the crustal rocks in the west has elevated them above the rocks to the east, giving an eastward tilt to the rock layers and exposing the oldest rocks in the west and the youngest ones in the east. This sequence has important implications for the scenery of England’s south coast (see Chapter 5).
HOW CAN LOCAL SURFACE MOVEMENTS BE DETECTED?
Having just reviewed some of the processes that cause vertical movements of the Earth’s surface, it is useful to consider the practical difficulties of how such movements are measured.
For present-day applications, it seems natural to regard sea level as a datum against which vertical landscape movements can be measured, as long as we remember to allow for tidal and storm variations. However, much work has demonstrated that global sea level has changed rapidly and frequently through time, due to climate fluctuations affecting the size of the polar icecaps and changing the total amount of liquid water present in the oceans and seas. It has also been shown that plate tectonic movements have an important effect on global sea level by changing the size and shape of ocean basins.
Attempts have been made to develop charts showing how sea level, generalised for the whole world, has varied through time. However, it has proved very difficult to distinguish a worldwide signal from local variations, and the dating of the changes is often too uncertain to allow confident correlation between areas.
In sedimentary basins, successful estimates of vertical movements have been made using the thicknesses of sediment layers accumulating over different time intervals in different depths of water. In areas of mountain building, amounts of vertical uplift have been estimated using certain indicator minerals that show the rates of cooling that rocks have experienced as they were brought up to the surface. However, both these approaches are only really possible in areas that have been subjected to movements of the Earth’s crust that are large and continuous enough to completely dominate other possible sources of error.
Local horizontal movements are also difficult to estimate, although fold and/or fault patterns may allow a simple measure in some cases. Movement of sediment across the Earth’s surface by rivers or sea currents can be estimated if mineral grains in the sediment can be tracked back to the areas from which they have come. In the detailed consideration of landscapes in this book, we have to rely on using the widest possible range of types of evidence, carefully distinguishing the times and scales involved. Even then, we are often left with probable movement suggestions rather than certainties.
CHAPTER 4 The Southwest Region (#ulink_34a0c3e0-ec2d-520c-a34b-9f87f0bffe88)
GENERAL INTRODUCTION
MOST OF THE BEDROCK near the surface in the Southwest Region (Fig. 35) is distinctly older than the near-surface bedrock in the rest of Southern England. It therefore provides us with information about earlier episodes, and this is all the more interesting because these episodes involved movements of the crust that created a mountain belt, the only one fully represented in the bedrock story of Southern England. Not only does this add greatly to the interest of the Southwest, but it has resulted in the presence of valuable minerals that have strongly influenced the human history in the Region.
Bedrock foundations and early history
Sedimentation and surface movement before the mountain building
The Southwest Region consists predominantly of bedrock formed between about 415 and 300 million years ago, during Devonian and Carboniferous times. This bedrock records an episode during which some areas of the Earth’s crust rose while others sank, as part of a general buckling of the crust that is the first indication of compression and mountain building (Figs 36 and 37). As the rising areas became significantly elevated they were eroded, shedding sediment into the neighbouring sinking areas that became sedimentary basins. It is these basins that preserve most of the evidence of these events (Fig. 38).
In material that has been further and later deformed, it is difficult to work out the shape of the sinking areas, but many of them were probably elongated or trough-shaped, with the troughs separated by rising ridges that ran roughly east-west, parallel to the general trend of the later mountain belt. The troughs and ridges were caused in the early stages of mountain building by the compression and buckling of the crust. The troughs were generally flooded by the sea, or on the margins of relatively narrow seaways. Muds were the commonest materials to accumulate, although sands were also in plentiful supply. Lime-rich sediments, sometimes with corals and other shelly marine animals, were locally important. There were also periodic episodes of igneous activity that contributed volcanic lavas to the sedimentary successions.
FIG 35. The Southwest Region, showing Areas 1 to 3.
During these episodes of Devonian and Carboniferous basin and ridge activity, the Southwest Region was just one small part of a larger belt of similar activity that extended to the west into Ireland and Canada. Canada was then very much closer, because the Atlantic Ocean is a younger feature that only started to grow (at this latitude) about 100 million years ago, as divergence and spreading occurred along the mid-Atlantic plate boundary. To the south and east, the same belt of activity continued across northern France and into Germany. This broadly east-west trending belt later became the Variscan mountain belt.
Crustal convergence that created the mountain belt
The subsidence and sedimentation were sometimes interrupted by, and generally followed by, episodes of convergence of the Earth’s crust. This was caused by compression or squeezing, broadly in a north-south direction, so that areas of bedrock were folded and pushed closer together, making the east-west trending Variscan belt narrower, as if between the jaws of a vice. The map (Fig. 39) and cross-section (Fig. 40) show how the folds and faults of the region vary locally in their pattern, but can be explained generally by convergent movements in this north-south direction. These continued over at least 100 million years, and occurred along thousands of kilometres of the belt. Mountain-building events such as this occur when tectonic plates collide (as described in Chapter 3) and always have a profound effect upon the scenery in the vicinity of the collision. The Variscan mountain belt is just one of the great mountain-building episodes that have occurred periodically, throughout the Earth’s history.
FIG 36. Timeline diagram showing bedrock deposition and emplacement events in the Southwest Region.
Some of the best evidence for the horizontal crustal shortening comes from examining folds that can be seen in the bedrock at many localities (Figs 41 and 42). Folding of the originally flat layers of the bedrock is a spectacular feature of many southwestern sea cliffs, and the direction of the folding gives a clear indication of the direction of the shortening that resulted. Fractures (faults) also frequently cut the bedrock, and careful mapping makes it possible to recognise that, although some of them are very local features, others turn out to have been flat-lying fractures across which many kilometres of movement have taken place.
FIG 37. Simplified geological map of the Southwest Region.
FIG 38. Typical Devonian sedimentary basin in the Southwest Region.
FIG 39. The major bedrock structures of Southwest England and South Wales.
Another feature of the mountain building is that muddy material – the most abundant sediment in the Southwest Region – was often converted into slates that can typically be split into thin sheets and are said to possess a ‘slatey cleavage’. These rocks are locally referred to as killas, to distinguish them from other rocks with no cleavage, particularly the granites. The conversion into slates took place during the folding and fracturing of the mountain building, when the original muds, rich in clay minerals, were buried deeply below other sediments and then compressed to produce a new layering (or cleavage).
One large feature visible in the bedrock is the ‘Culm fold belt’ or ‘synclinorium’, a large and complex downfold representing horizontal crustal convergence (Figs 39 and 40). The centre of this feature is a belt of bedrock of Carboniferous age that extends between Bude and Exeter running across the centre and north of the Southwest Region. To the north and south of this, older (Devonian) rocks occur at the bedrock surface, forming the margins of the large downfold or syncline (Fig. 37). Culm is an old term much used by miners and European geologists for Carboniferous sediment, and synclinorium is a name for a downfold (or syncline) which contains numerous smaller folds.
In the Lizard area, much of the bedrock consists of a distinctive group of igneous rocks (Figs 39 and 40). These rocks cooled and solidified earlier than the main mountain building, and were mostly formed by intrusion of hot molten rock in a way that is typical of the floor of an ocean basin. The Lizard area provides one of the best examples now visible on land in Britain of material formed originally as ocean-floor crust. In Late Devonian times, as a result of early Variscan convergence, this large area of oceanic crust was forced northwards over and against sedimentary rocks lying just to the north. This shows how, in a large mountain belt, an area of crust (tens of kilometres across), with a distinctive history as the floor of an ocean basin, can be uplifted and incorporated into a mountain belt as its margins are squeezed together.
FIG 40. Schematic cross-section representing major structures of the Variscan mountain belt. The deep structure shown is speculative but shows how crustal shortening seen at the surface may be related to deeper, flat-lying fractures (faults). Located on Figure 39.
FIG 41. Zigzag folding due to horizontal convergence during the Variscan mountain building is spectacularly exposed at Hartland Quay (Area 3). (Copyright Will Brett/www.lastrefuge.co.uk)
FIG 42. Zigzag folding due to horizontal convergence during the Variscan mountain building, this time at Bude (Area 2). (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
Some of the bedrock of this Lizard Complex is called serpentinite, after the common occurrence of the green mineral serpentine in sinuous cracks and veins. This gives the rocks an attractive colour patterning, and the absence of quartz makes them surprisingly easy to work with steel tools, giving rise to a local industry of carving serpentinite ornaments.
Granites and valuable minerals
The granites of Southwest England are an important later feature of the Variscan mountain belt. Granites are igneous rocks with coarse (millimetre across) crystals that have grown, interlocking with each other, as the molten material cooled slowly and solidified at some depth in the Earth’s crust. The minerals of the granite are most commonly quartz (typically about 30 per cent) and feldspar, generally with some other minerals such as mica (Fig. 43). The granite liquid (often called magma) formed as a result of melting deep within the outer layers of the Earth and was then forced upwards, sometimes pushing aside the overlying bedrock and sometimes replacing it by melting. The granites solidified at depths of several hundred metres or more below the surface and are now at, or near, the surface because of landscape erosion. The main granite areas include the Isles of Scilly in the west, followed by Land’s End, Carnmenellis, St Austell, Bodmin Moor and Dartmoor in succession to the east. These distinct granite areas at the surface can be visualised as the tops of fingers extending upwards from a single continuous granite body detectable by gravity surveys at greater depth under the spine of Southwest England (Fig. 44). The deep body extends for some 200 km along the length of the mountain belt.
FIG 43. Polished slab cut in the Dartmoor granite showing typical granite texture. Crystals of quartz (light grey), feldspar (white) and biotite (black) have interlocked as the magma (molten rock) solidified on cooling. (Copyright Landform Slides – Ken Gardner)
FIG 44. Diagram showing the large granite body below the bedrock of Cornwall and Devon, and the way the granite bosses now visible at the surface are upward extensions of this larger body.
Although there was probably some time range in the arrival of different granite bodies in the upper crust, the main episodes took place at the very end of the Carboniferous and during the earliest Permian, roughly 300 million years ago.
The arrival of the granites from below was only one part of the invasion of the upper levels of the bedrock that took place at this time. Widespread mineralisation around the granites has probably been even more important than the arrival of the granites themselves, in terms of human history. The term mineralisation is used to cover the alteration of the solid granite and the surrounding (older) bedrock that has, in some areas, been caused by the movement of very hot and chemically rich water, using the network of cavities and fractures that existed in the rocks. Because of the chemistry of the rocks deep down, many different chemical elements were brought to the upper levels and crystallized there to form new and valuable minerals, or caused alterations of the earlier solid rocks.
FIG 45. Simple diagram of a slice through the Earth’s upper levels, showing how the temperature patterns around a granite body have been responsible for the distribution of minerals containing the more important chemical elements.
The granites probably solidified in the Earth at temperatures of about 850 °C, and most of the mineralisation happened at rather lower temperatures as the rocks cooled (Fig. 45). Tin, wolfram, arsenic and copper minerals formed at between 500 and 300 °C, whereas silver, lead, zinc, uranium, nickel and cobalt minerals formed at between 300 and 200 °C, and iron minerals between 200 and 50 °C.
The tin of Cornwall was a major reason why some of the early inhabitants of mainland Europe were interested in Britain. In fact there is evidence that tin minerals were being gathered here more than 3,000 years ago, during the Bronze Age. In those days, much of the material was collected from young sands and gravels derived from the weathering and erosion of the mineral-bearing rock, unlike later times when mining techniques were developed to extract tin directly from the bedrock.
Some granite areas contain much more mineralisation than others, and the range of minerals and chemical elements that are present varies greatly. This depends on the temperature of the granite emplacement and the chemistry of the fluids accompanying and following the granite. The Land’s End and Carnmenellis granites are particularly rich in tin, and it is around these granites, in areas near to St Ives, Camborne, Redruth and Helston, that most of the mining has been concentrated. The remains of this mining are often clear to see (Fig. 46), but the presence of the minerals themselves does not generally influence the natural scenery.
FIG 46. Tin mine workings near Cape Cornwall, west Cornwall. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
In some of the granites, hot fluids from below altered the mineral feldspar (one of the dominant granite minerals) and turned it into the soft clay mineral kaolinite. The china clay industry has developed round the presence of this mineral, which has usually been extracted from the altered granite by washing it out with powerful water jets. This process has changed the scenery dramatically, particularly around the St Austell granite. For every tonne of useable kaolin, 5 tonnes of waste granite material are produced, and heaps of this waste are obvious scenic features in these areas (Fig. 47). The famous Eden Project at Bodelva, near St Austell, has been constructed inside a large former china clay quarry.
FIG 47. China clay excavations at St Austell. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
The older rocks surrounding each of the granite intrusions generally show evidence of alteration that occurred as the mobile granite worked its way upwards from below. This contact metamorphism, often accompanied by the growth of new minerals, is the result of the transfer of heat and introduction of new chemical components from the granite. It has usually resulted in making the rocks more resistant to later erosion at the surface.
Younger episodes
Sedimentary markers
Between 7 and 20 km to the southwest of Exeter, traversed by the A38 and A380 trunk roads, the Great and Little Haldon Hills are capped by a layer of sediments, assigned to the Upper Greensand, and spanning in age the Early/Late Cretaceous boundary (between about 105 and 95 million years ago). These are the westernmost erosional relicts of a continuous sheet of sediment of this age that extended across much of the rest of Southern England. In the Haldon Hills area, the sandy and fossiliferous material seems to have formed near the coastal margin of an extensive Cretaceous sea.
The Haldon Gravels are distinctive deposits that occur above these Cretaceous sediments. They consist largely of flint pebbles and contain sand and mud between the pebbles. Some of the gravels appear to be the result of removal by solution of the calcareous Late Cretaceous Chalk that can no longer be found in its unaltered state so far west. The flint nodules in the Chalk were then left as a layer of much less soluble pebbles. Some of the gravel appears to have been carried to its present position by rivers or the sea, perhaps also with the incorporation of kaolinite clay from the Dartmoor granite. The age of these gravels appears to be early Tertiary, perhaps about 55 million years.
A few kilometres west of the Haldon Hills, northwest of Torquay, the Bovey Formation of early Tertiary age (Eocene and Oligocene, about 45 to 30 million years ago) occurs in a distinct, fault-bounded basin. The formation is more than 1 km in thickness and consists primarily of the clay mineral kaolinite, deposited as mud by local streams, and associated with minor amounts of sand, gravel and peat-like organic deposits of lignite. This sediment fill continues to be a very important material for ceramics, pipes, tiles etc. ranging from high-quality china clay to lower-quality materials. Most of the sediment appears to have been carried into the basin from the area of the Dartmoor granite and its surroundings. The Bovey Basin formed as a result of subsidence along the northwest-to-southeast trending Sticklepath Fault Zone (Fig. 39) which cuts across the whole of the Southwest Region. This fault zone seems to have been active during the accumulation of sediment in the basin and so, at least in this phase of its history, it was much younger than the Variscan structures of the Southwest generally. Further to the northwest along the same fault zone is the smaller Petrockstowe Basin near Great Torrington (see Fig. 38), and, offshore, under the Bristol Channel is the larger Stabley Bank Basin, east of Lundy Island.
About 6 km southeast of St Ives (see Area 1), near the small village of St Erth, a small area is underlain by some soft sands and muds. When first exposed by quarrying, these sediments provided a rich assemblage of fossils that are thought to have lived some 3 million years ago, in latest Tertiary times. The fossils suggest sea depths of between 60 and 100 m, and are now about 30 m above sea level, so they provide a fragment of evidence from a period when the sea was more than 100 m higher than it is now, relative to the land of Cornwall. As will be mentioned below, this deposit is rather similar in its elevation to the most obvious plateau recognised in many of the inland areas, which may also relate to an episode when the sea stood at this level.
Drainage patterns
On the scale of the whole Southwest Region, the main upland areas are Exmoor in the north and the zone of distinct granite domes in the south, extending from Dartmoor to Land’s End.
The highest point of Exmoor is Dunkery Beacon (519 m). Exmoor has been eroded from Devonian bedrock, and may owe some of its elevation to the greater resistance to erosion of this material compared with the Carboniferous material that forms the bedrock further south. Another possible factor is suggested by the remarkable way that many of the river systems of the southwest drain to the south coast, despite their sources being remarkably close to the north coast (Fig. 48). This is the case for the Exe, flowing from Exmoor southwards via Exeter to Exmouth, and, further west, the Tamar, which begins northeast of Bude and flows southwards past Launceston and Tavistock before discharging into Plymouth Sound. It looks as if this part of the Southwest Region has been tilted southwards as these river systems developed on either side of the high ground of Dartmoor, where the granite resisted erosion. A southerly tilt would also be consistent with a preferential uplift of the Exmoor Hills to the north.
FIG 48. River pathways, mean flow rates (m
/s) at some river stations, main drainage divide (red line) and main granites of the Southwest Region.
The southern areas of hills correspond so clearly with the areas of granite outcrops that there can be little doubt that the greater resistance to erosion of the granite explains their higher elevations. But how long has this erosion been taking place? Emplacement of the granites was over by the end of Carboniferous times (about 300 million years ago) and there is evidence of pebbles in the New Red Sandstone from the Dartmoor granite and from the altered bedrock close by. Although the precise age of the earliest New Red Sandstone is uncertain, it does not appear to be much younger than the age of granite emplacement. However, it appears that the granites were not being significantly eroded in quantity much before Cretaceous times, 200 million years later and about 100 million years ago. Since then, the granites have been eroded into the present patterns of local hills and valleys, but at very variable rates as climate, coverage by the sea and rates of river erosion changed.
Each of the main granite bodies corresponds closely to an area of high ground, and their maximum heights tend to be greater towards the east (44 m for the Isles of Scilly, 247 m for Land’s End, 252 m for Carnmenellis, 312 m for St Austell, 420 m for Bodmin and 621 m for Dartmoor). This gradient is overall only about 3 m per km. The geophysical data on the large, deep granite body (Fig. 44) recognised below the surface granite bodies do not provide independent evidence for a slope of this sort deep down. Some tilting of the landscape downwards towards the west may have occurred, or the slope may simply reflect the greater proximity of the western granite bodies to the sea and repeated episodes of marine erosion.
Ice Age episodes
Ice sheets do not appear to have covered the present land of the Southwest Region to any important extent during any of the major cold episodes of the Ice Age. In the Isles of Scilly, material deposited directly from a grounded ice sheet has been recognised and is thought to be Devensian (last cold phase) in age (Fig. 49). Various giant boulders derived from metamorphic sources are a notable feature of some localities on the North Devon coast, some of which appear to have come from Scotland. However, it is not clear whether they were transported to their present locations by a large ice sheet or by floating ice.
In spite of the lack of an actual ice sheet, the repeated cold episodes of the Ice Age must have had a considerable effect upon the weathering style of the bedrock, for example influencing the granite tors, mobilising material to move down slopes and changing drainage patterns and the surface blanket of soft materials.
FIG 49. Map showing the greatest extent of the last main (Devensian) ice sheet across England and Wales.
AREA 1: WEST CORNWALL
A remarkable feature of the peninsula of West Cornwall (Figs 50 and 51), as it narrows towards Land’s End, is the contrast between the spectacular coastal scenery and the scenery inland. The rocky coastal cliffs and sharply indented coves reflect West Cornwall’s exposure to the prevailing Atlantic storms, and contrast starkly with the inland scenery of rolling – though often rocky -hillsides, carved into a network of small valleys and streams.
The main features of the inland landscape appear to have formed over millions of years, and ultimately reflect the bedrock pattern that has been inherited from the Variscan mountain building that ended 300 million years ago. In contrast, the coastal landscape is clearly much younger, and much of it has been produced by changes in sea level that have occurred since the last main cold phase of the Ice Age, some 10,000 years ago. There is some evidence of earlier sea levels but this is more difficult to evaluate, as it has generally been removed by more recent erosional events.
FIG 50. Location map for Area 1.
I have divided West Cornwall into three Landscapes (A to C), each with distinctive bedrock geology (Fig. 52).
Landscape A: Granite areas
The Isles of Scilly (A1; Fig. 53) are formed by the westernmost significant granite bodies of southwestern England. They lie some 45 km southwest of Land’s End, scattered over an area approximately 20 km by 15 km. Most of the 150 islands are little more than bare outcrops of granite, sometimes largely submerged at high tide. The landscape is windswept and mainly treeless, with heathlands where the ground has not been cultivated. Historically the islanders eked out a precarious existence from crofting, until the nineteenth century, when shipbuilding and the growing of flowers became economic. Today most of the cultivated land consists of small fields of flowers edged with evergreen hedges, and horticultural work, along with tourism, has become the mainstay of the economy.
The smaller islands are often arranged in rows, separated by ‘sounds’ (areas of shallow water) that tend to have a northwest-southeast orientation. These sounds must have been valleys before they were drowned by the recent (Flandrian) sea-level rise. Their orientation is similar to that of the valleys and faults of the Land’s End granite, discussed more fully below. Numerous sandy bays and beaches reflect the granite weathering and the transport of the weathered sediment, by storms and tides, to more sheltered parts of the island landscape.
FIG 51. Natural and man-made features of Area 1.
In the general section of this chapter it has been mentioned that the northern Scillies appear to have been invaded by ice late in the history of the last (Devensian) cold phase of the Ice Age (Fig. 49), and this is surprising in view of their southerly location. It appears that when the Devensian ice sheet had grown to its greatest extent, an elongate tongue of ice, perhaps some 150 km wide, extended for nearly 500 km from the Irish and Welsh ice sheets to the edge of the Atlantic continental shelf. This tongue became so large because it was vigorously fed by ice from the high ground of Ireland to the west, and the Lake District of England and the mountains of Wales to the east. The ice extended across the mouth of the Bristol Channel, well clear of the present north Cornwall coastline, before leaving ice-laid sediment on the northern fringe of the Isles of Scilly. South of the island areas that were covered by ice, the granite has been weathered locally into tors.
FIG 52. Area 1, showing Landscapes A to C and specific localities mentioned in the text. Major divisions of Landscape A are identified by A1, A2, A3 etc., and localities are shown as a1, a2, a3 etc.
Land’s End is the westernmost tip of mainland England. The local cliffs are made of granite and clearly show vertical sets of fractures, probably formed when the granite was cooling and contracting (Figs 54 and 55). Apart from the fractures, the granite is massive compared with the strongly layered and deformed rocks into which the main granites were intruded. Most of the northerly inland areas are exposed and windswept moorland, though arable farming for early vegetables has developed in the valleys to the south. The valleys eroded in the Land’s End granite are distinct and often oriented very clearly in a northwest-southeast direction. This orientation is parallel to a large number of faults which appear to have first formed late in the Variscan mountain-building episode. However, they must also have been active much later, after the intrusion of the main granite, because its margin is locally offset by faults with this trend. The movement of superheated water along these fault systems has resulted in mineralisation of the bedrock, altering its resistance to erosion so that valley incision has taken place preferentially in this direction. Tors are largely absent from the Land’s End, Godolphin, Carnmenellis and St Austell granite areas, while they are common weathering features on Bodmin Moor and Dartmoor. This probably reflects a difference in the weathering and uplift histories of the different granite bodies.
FIG 53. The Isles of Scilly, looking east towards Bryher, Tresco and St Martins. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
The Land’s End granite (A2) forms the bedrock of most of the far southwestern peninsula, which is largely ringed by cliffs. To the east of the granite, St Ives Bay on the north coast and Mount’s Bay on the south coast show how much more readily eroded the Devonian killas is in comparison. Along the north coast of the Land’s End peninsula, the killas is preserved as a screen of land, rarely more than a kilometre in width, but clearly showing distinctive layering, as seen at Gurnard’s Head (a3; Fig. 56). Present coastal erosion may have been slowed at this point by the greater strength of the Devonian where it has been altered close to the granite. Just north of Land’s End point, Whitesands Bay (a1) is one of the only sandy bays to face the open sea to the west. The bay lacks any significant stream system that could have supplied sand to the beach, so it seems most likely that the sand has been carried into this bay by the storms which so often attack this exposed coast.
FIG 54. Land’s End peninsula from the air, looking eastwards. Note the lack of clear layering in the granite bedrock and the steep fracture surfaces (joints) that have controlled the form of the cliffs. (Copyright Dae Sasitorn & Adrian Warren / www.lastrefuge.co.uk)
A distinct, though irregular, platform at 100–150 m above sea level rings the area of the Land’s End granite, and tends to be followed by local roads. This may have been formed during an early episode of coastal erosion, when sea level was standing at this height relative to the land (Fig. 57). Some evidence for its age is mentioned below. Its irregularity probably reflects local valley erosion that has taken place since its formation.
FIG 55. Land’s End cliffs, looking westwards. Again, note the vertical jointing. (Copyright Landform Slides – Ken Gardner)
FIG 56. Gurnard’s Head (Fig. 52, a3), west of St Ives, showing the coastline along the northern edge of the Land’s End granite. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
FIG 57. Slope map showing the southwestern part of Area 1. Slopes greater than 5 degrees are coloured red, and the main granite areas and the Lizard Complex are outlined. Topographic cross-sections illustrate wave-cut platforms that are presently inland and show the lack of topography on the Lizard Plateau.
The next main granite bedrock area to the east underlies the Carnmenellis area (A4), although there are other smaller granite areas, such as St Michael’s Mount (Fig. 58), across the bay from Penzance, and the intermediate-sized Godolphin granite (A3), some 8 km to the east. These smaller granite areas show the range in size of ‘feeders’ that branched off from the major granite body that underlies the whole Southwest Region (Fig. 44). In all cases the granite bedrock corresponds to high ground in the landscape – evidence of the greater resistance of the granite in the face of repeated landscape erosion. Derelict mine engine houses litter the landscape, especially northwards near Camborne and Redruth, once prosperous tin-mining centres. To the south, the landscape is more sheltered and fertile, allowing better farming. Trees are rare because of their past cutting for fuel for the mining industry, as well as because of the general exposure of the landscape to the weather.
FIG 58. St Michael’s Mount. (Copyright Dae Sasitorn & Adrian Warren/ www.lastrefuge.co.uk)
The same northwest-to-southeast valley pattern that has just been mentioned in the Land’s End granite is also apparent in the area around the Carnmenellis granite, and appears to be the result of preferential stream erosion parallel to the faults trending in this direction (Fig. 59).
FIG 59. Sketch map showing the orientation of some of the main faults in West Cornwall.
Another similarity to Land’s End is the widespread topographic platform at about 140 m above sea level. This platform is particularly clear north of the Carnmenellis granite but is also obvious in the Godolphin granite (Fig. 57). In the Porthmeor and Camborne areas the platform is particularly distinctive, and the continuity of its landward slope is clear on the slope map. It has generally been assumed that these platforms were cut by storm waves when the sea stood at this level about 3 million years ago. At this time, West Cornwall would have consisted of granite islands, like the present Isles of Scilly, while the surrounding Devonian bedrock (killas) was submerged.
The western part of the St Austell granite (A5) lies within Area 1, and again its resistance to landscape erosion is shown by the high ground that it occupies. The remarkable feature of this granite is the way it has been altered by the circulation of hot fluids. Much of the feldspar in this granite has been altered to the mineral kaolinite, which is a member of the clay mineral group that is the key component of china clay. The result of this is that the St Austell granite has been quarried, particularly in its western part within Area 1. The kaolinite has been extracted from the rotted granite by high-pressure water jets, which leave large volumes of quartz and feldspar grains that are heaped up in enormous and obvious spoil heaps.
Most of the original heathland and moorland on the St Austell granite has been destroyed by the mining industry. More recently, the Eden project redevelopment of one quarry complex (in Area 2) has brought many visitors to the area.
Landscape B: The Lizard
Lizard Point is the most southerly headland in Britain, part of a wider Lizard landscape comprising a flat heathland plateau bounded by dramatic cliffs and small coves (Fig. 60). Notice how steep many of the sea cliffs are, and that they show little in the way of well-developed, regular layering or fracturing. Unlike the other upland areas of Cornwall, the Lizard is not underlain by granite. As mentioned in the general section of this chapter, some of the area is underlain by serpentinite, a distinctive, decorative rock that was originally part of the Earth’s mantle, below the crust and many kilometres below the surface (see Chapter 3). Other parts of the Lizard bedrock were originally basalt lavas and minor sheet-like intrusions along with small amounts of sediments, all similar to successions elsewhere that appear to have formed in or below the Earth’s oceanic crust. During the Variscan mountain building, this mixture of distinctive bedrock types appears to have been squeezed up amongst the strongly compressed Devonian killas. Today, the exceptional bedrock chemistry of the unusual Lizard rocks is the reason why the peninsula has such a variety of rare plant habitats. Much of the peninsula is a National Nature Reserve (NNR) or owned by the National Trust.
As in Carnmenellis and Land’s End, a wave-cut platform has been identified on the Lizard, although its level is rather lower. In fact, the platform actually forms the Lizard Plateau and is remarkably flat, the ground surface varying between 60 and 100 m above sea level over large areas. This relative flatness probably reflects the rather uniform composition of the rock materials involved, and their uniform resistance to weathering and erosion.
The coast of the Lizard Peninsula is formed almost entirely of steep cliffs, particularly around its southwestern perimeter. A few small beaches do occur in sheltered locations, such as at Coverack (b1), and picturesque fishing villages are scattered along the east side of the peninsula around small coves and gullies.
FIG 60. The Lizard coastline. Note the contrast between the jagged coastal cliffs and the flat inland landscape. (Copyright Dae Sasitorn & Adrian Warren/ www.lastrefuge.co.uk)
Landscape C: Cornish killas
Most of the bedrock of West Cornwall is Devonian sediment, folded, faulted and – locally – altered during the Variscan mountain-building episode (see the general section of this chapter). The Devonian sediments, known to miners and quarrymen as killas, have been less resistant to landscape weathering and erosion than the granites (A) and the Lizard Complex (B), and so have been preferentially eroded to form lower landscapes. All the major bays and estuaries of this Area, such as St Ives Bay (c4) and the Carrick Roads at Falmouth (c7), are situated in killas areas for this reason. The Variscan folding and faulting that deformed the killas has also locally influenced the directions of valleys and their slopes, which have picked out variations in the killas layering, giving an east-west grain to the landscape (Fig. 61).
FIG 61. Slope map of the eastern part of West Cornwall. The main granite bedrock areas are outlined and important boundaries in the Devonian bedrock indicate the direction of the Variscan folding. Note the circular china-clay workings that are visible in the St Austell granite (A5).
FIG 62. Complex landscape of the North Cornwall coast, looking eastwards from Crantock Beach, over the Pentire Ridge towards Newquay (Fig. 52, c2) and Watergate Bay. (Copyright Dae Sasitorn & Adrian Warren/ www.lastrefuge.co.uk)
FIG 63. Headlands, bays and beaches of the Newquay area (Fig. 52, c2), looking eastwards from a point 2 km west of Figure 62. Crantock Beach is visible in the middle distance. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
The Flandrian sea-level rise, which ended only 5,000 years ago, has also left its mark on West Cornwall. The most obvious legacy is the extensive array of tidal estuaries at the mouths of the main rivers, which are flooded river valleys or rias. The most striking example is the series of branched rias around Falmouth known as the Carrick Roads (c7). These extend northwards across half of the width of West Cornwall and have had an obvious major influence on the road and rail transport pattern of the area. Major branch rias to the west, north and east around the Carrick Roads divide this part of the Cornish landscape into numerous isolated peninsulas. The inland valleys of the killas areas tend to be deeply incised with little widening, and the branching patterns of these valleys are very clear on the slope map. The rias are obviously the direct result of the drowning of valleys of this form by the Flandrian sea-level rise.
The coastline of the killas landscape of West Cornwall is extremely varied: small, sandy coves alternating with rocky promontories and high cliffs are typical of this part of the north coast (Figs 62 and 63). This irregular coastline is due to local variation in the type and strength of the killas bedrock, with weaker units (often slates) eroding to small bays while the more resistant rocks (often limestones or quartzites) form the headlands.
The sandy bays of north Cornwall (c1, Padstow and the River Camel Estuary; c2, Newquay Bay; c3, Perranporth and Perran beach; c4, St Ives Bay) are famous for surfing, due to the splendid waves that roll in from the Atlantic Ocean. Apart from Padstow Bay (c1), at the mouth of the River Camel, most of the north Cornwall beaches are not obviously linked to river sources of sand and so must have been filled by sand transported from offshore sources by storm waves. At many famous surfing beaches, such as Perranporth (c3), sand banks built up by winter storms can be eroded in the summer, resulting in dangerous currents sweeping out to sea. The wind-blown dunes of the Penhale Sands, north of Perranporth, are a spectacular example of the way that gales from the west can move beach sand up to 2 km inland. Because of the variation in the killas bedrock, some headlands are long and the inlets are narrow enough to develop fast tidal flows, capable of forming large, regular ripples as seen in the foreground of Figure 62.
On the south coast, storm-built sandy beaches have also formed, for example at Newlyn (c8), Praa Sands (c5) and at the mouth of Helston valley south of Porthleven (c6). Further east, the coastline is much more sheltered and the scenery is dominated by the drowned valleys and quiet inlets of the Carrick Roads (c7).
AREA 2: EAST CORNWALL AND SOUTH DEVON
This Area straddles the boundary between Cornwall and Devon (Fig. 64). In terms of the coastlines of the Southwest, it includes a small stretch of the north coast near Tintagel, and a large section of the south coast from St Austell, via Plymouth and Start Point, to Torquay and Exmouth (Fig. 65).
In the general section of this chapter, the early geological history of the Southwest Region as a whole has been outlined, particularly the evolution of the Variscan mountain belt. Younger episodes in the region have also been discussed, involving river and valley erosion of the landscape, the effects of the Ice Age and the changes in the coastline that have resulted from the most recent (Flandrian) rise in sea level.
In the sections below we shall consider more local features of the scenery in this Area, dividing it into four distinct Landscapes (labelled A to D), each underlain by a different kind of bedrock (Fig. 66).
FIG 64. Location map for Area 2.
FIG 65. Natural and man-made features of Area 2.
FIG 66. Area 2, showing Landscape A to D and localities (a1, a2 etc.) mentioned in the text.
Landscape A: Granite areas
Bodmin Moor (A6) and Dartmoor (A7) are the most southerly large upland areas in England and, in each case, the granite bedrock has resisted landscape erosion to produce the high ground. The highest point of elevation in this Landscape is High Willhays (a1) at 621 m above sea level on Dartmoor. Evidence of the ongoing nature of this landscape erosion is provided by the contrast between the high moorland, with bogs, steep valleys and exposed tors, on one hand, and the surrounding low farmland on the other.
In the general section of this chapter I have outlined some main features of the southwest granites, such as their resistance to erosion and the valuable minerals associated with them. They have also provided excellent strong building stone for the buildings of the Region.
As in the granite areas of West Cornwall (Area 1), mineral mining activities have had a strong impact on the area, and derelict tin mine buildings are scattered over much of the landscape. The china-clay industry has also produced significant changes to the scenery, one of the most remarkable sites being the workings 3 km northeast of St Austell (a2). These pits now house the famous Eden Project, an educational charity providing a ‘Living Theatre of People and Plants’ and attracting over a million visitors each year (Fig. 67).
FIG 67. The Eden Project is situated in a former china-clay pit. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
The present-day pattern of streams and their valleys has evolved from ancestral streams and valleys that carved most of the inland scenery over millions of years. In the general section of this chapter, we have seen the remarkable way that the drainages of the rivers Tamar and Exe flow southwards across most of the Southwest Region to the sea, divided by the high ground of Dartmoor. A general tilt of the Region to the south, and the resistance of the granite domes to stream and valley erosion, appear to have been important factors. Closer examination of the drainage patterns shows that the streams and valleys of the Bodmin Moor granite tend to radiate out from near its centre, but that the distinctly larger Dartmoor granite has eroded down to form two drainage divides, one in the north and one in the south. This may simply be a matter of the different size of these two granite areas, which has allowed a more complex drainage pattern to develop through time over Dartmoor.
The parallel groups of incised valleys that are common in the granites of Area 1 are not clearly developed on Bodmin Moor and not visible at all on Dartmoor. It is intriguing that the fault system that was responsible for the parallel valleys further to the west is not present in these larger eastern granites. This may tell us something about the greater depth of weathering and erosion experienced by the eastern granites.
There are a number of gorges resulting from the deep incision of rivers and streams into the granites and their surrounding materials. Around the Dartmoor granite, the valleys of the River Dart to the east and the Lydford Gorge to the west (a3) are examples of these. South of the Bodmin Moor granite, the River Fowey also has a spectacular and well-known gorge at the Golitha Falls (a4).
Tors are remarkable features of both the Dartmoor and Bodmin Moor granite areas (Fig. 68). They provide a focus for visitors in granite scenery that is often otherwise rather featureless and empty, and there are well over a hundred tors on Dartmoor alone. Tors tend to look like heaps of granite blocks, but a closer inspection shows that they are not jumbled but rather blocks that ‘belong’ next to their neighbours. These linked blocks are relict volumes of a much larger volume of granite, most of which has disintegrated and been removed by weathering. Tors are very much features of granite weathering, suggesting that the coarse interlocking crystal texture and general lack of layering have caused these remarkable landforms to appear.
Many tors occur on the most elevated parts of the scenery, looking like man-made cairns. Others occur on the slopes of valleys, but it is clear that tors will only form where down-slope processes, driven by gravity, can remove the weathering debris from around them. Cracks in the granite (technically called joints) give tors much of their distinctive appearance: near-vertical joints produce towers and pillars, while roughly horizontal joints give the rocks a layered, blocky appearance (Fig. 69). Most of the joints seem to have formed during the arrival of the granite material from below (intrusion), either due to contraction from cooling of the newly solid material, or due to other stresses acting shortly after solidification. The flat-lying joints (horizontal on hill tops, and parallel to slopes elsewhere) may also be due to the erosion of the present scenery, allowing the granite to expand and fracture as the weight of the overlying material is removed.
FIG 68. Hay Tor, Dartmoor, looking southeast. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
FIG 69. Hound Tor, Dartmoor. (Copyright Landform Slides – John L. Roberts)
The slopes round tors tend to be covered with loose granite blocks (often referred to as clitter), generally angular and obviously derived from the tors (Fig. 70). Finer-grained, crystal-size gravel or sand of quartz and feldspar is another weathering product and is locally called growan or sometimes head. It is clear that much of the alteration of the granite that has resulted in the appearance of the tors must have been strongly influenced by the climate, vegetation and soil-forming conditions existing at different times and in different scenic settings. Much of this has been compared to the weathering and down-slope movement that is seen in high-latitude cold climates today, and so is explained as a result of the cold climate conditions experienced repeatedly during the Ice Age. However, weathering of granites is much faster today in the warm, tropical jungle areas of the world, compared to drier, cooler and less vegetated conditions. Early episodes of weathering of the Southwest Region granites may have taken place under the warm, tropical conditions that are indicated by early Tertiary fossil deposits elsewhere in England.
FIG 70. Mass-flow terrace, looking westward from Cox Tor, Dartmoor. The terrace is interpreted as being the result of down-slope movement under alternating freeze-thaw conditions. (Copyright Landform Slides – Ken Gardner)
Rock basins are low-lying hollows in the granite topography draped with granite weathering products. Sometimes these are dry and their floors are simply coated with granite weathering materials. In other places the hollows are covered by peat, which is often a feature of the higher and wetter parts of the granite hills. Under very wet conditions, the hollows contain deep bogs or mires, with a reputation for being bottomless! How these low hollows were excavated is a puzzle.
Topographic platforms, cut by storm waves during times of high sea level, have been claimed to be present around the Dartmoor and Bodmin granite areas, although they are not as distinctive as those discussed on the Land’s End and Carnmenellis granites of Area 1. The Area 2 platforms are at heights of between 200 and 300 m above sea level, but in the absence of dated deposits similar to the St Erth beds of Area 1, their relevance to sea-level changes is open to doubt. Indeed, as mentioned above, terraces have been recognised around the Dartmoor granite that are thought to be the result of down-slope mass movement under freeze-thaw conditions, rather than due to sea-level changes.
Landscape B: Killas and other Devonian bedrock
Apart from the granites, Devonian sediments make up the bedrock of the southern and central part of Area 2. They consist largely of slates and mudstones with some sandstones, and are known generally as killas to distinguish them from the granites and other younger, less altered sediments. In a few localities around Plymouth (b1) there are Devonian limestones, similar to the well-known limestones around Torquay (d1) and Chudleigh (d9). The settings in which these Devonian sediments may have formed are illustrated in Figure 38, in the general section of this chapter.
The youth of the coastal scenery of this Landscape combines with the vigour of many of the processes operating to make it much more distinctive and dramatic than the inland scenery. In the west of Area 2, cliffs characterise the Cornish section of the south coast and often intersect deeply incised valleys that are clearly older features (Fig. 71).
Cornwall and Devon are separated from each other in this Area by the River Tamar, and this meets the south coast in a large drowned valley system around which Plymouth (b1) has grown (Fig. 72). Plymouth Sound is one of the best natural harbours in the Southwest and the historical naval importance of this city is the result. Similar, but smaller, valley systems (sometimes called rias) are common all along this stretch of coast, as they are further west in Area 1. Flooded valleys form the estuaries of the River Fowey, east of St Austell, and farther east still at Salcombe (b3) and Dartmouth (b8).
FIG 71. Polperro, on the south Cornwall coast. (Copyright Dae Sasitorn & Adrian Warren/ www.lastrefuge.co.uk)
FIG 72. The Tamar and Brunel Bridges, between Plymouth (Devon), to the right, and Saltash (Cornwall). (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
The headlands from Bolt Tail (b2) to Start Point (b5) are made of some of the most highly altered and probably oldest bedrock in Devon, although the age of their deposition as sediments is not known. They have been changed locally to mica-rich and hornblende-rich schists that must have been altered (metamorphosed) several kilometres below the surface, before being pushed upwards during the Variscan mountain-building event. The local resistance of these schists to erosion has led to a particularly intricate pattern of small but sharp headlands and tight small bays. The slope map (Fig. 78) reveals a strong east-west orientation of slopes in this area that must be a reflection of folding in the bedrock. Three separate coast platforms, the highest at about 7 m above present sea level, are very clear at Sharpers Head (b4). Each platform represents an episode in the retreat and relative lowering of the sea before the latest Flandrian rise.
FIG 73. Slapton Sands (Fig. 66, b7). (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
Just north of Start Point (b5) lies the ruined village of Hallsands (b6), which vividly illustrates the damage people can unwittingly do in changing features of the coastal zone. From 1897 to 1902 over half a million tonnes of gravel were removed from the bay off Hallsands to construct an extension to the dockyard at Plymouth. Engineers believed that natural storm currents offshore would replenish the material they had taken, but this did not happen. Instead, the removal of the shingle left the beach open to intense storm erosion, and in January 1917, some 15 years later, the lower part of the village and a sizeable section of coastline were removed by a combination of storm and tide conditions.
Further north, the 3.5 km long barrier beach of Slapton Sands (b7) is another shingle barrier kept active by storm waves from the southeast (Fig. 73). The freshwater lagoon behind it, Slapton Ley, is a nature reserve, home to many rare species of plants and animals. It is under threat from erosion and breaching of the shingle barrier, causing flooding by salt water, and from silting up because of ploughing and deforestation of the inland landscape.
This Landscape of Area 2 also includes a short section of the north Cornish coast around Tintagel (b9), which was an important trading centre on this difficult coast and became the site of a twelfth-century Norman castle, linked to the legends of King Arthur (Fig. 74). The coastline here is often sheer and rugged, and the bedrock contains sharp folds, fracture surfaces and multiple surfaces of mica-rich cleavage, providing evidence of extreme compression during the Variscan mountain building. Many of the cliffs are flat-topped, because erosion has been controlled by relatively flat-lying surfaces in the bedrock, which contrasts sharply with the hog’s-back or whaleback cliffs of other coastal stretches.
Landscape C: The Carboniferous Culm of Devon
The northern landscape of Area 2 is underlain by Carboniferous bedrock (locally known as the Culm) which occupies a complex downfold in this part of the eroded Variscan mountain belt. The coastal bedrock here contains many beautiful examples of folding, for example at Boscastle (c3), famous for the flash flood that did so much damage in August 2004. Spectacular folding is also clearly visible at Millook Haven (c2; Fig. 75), and at Crackington Haven (c1; Fig. 76). In both cases, the convergence directions represented by the folds are near vertical, suggesting that the Variscan folding may have involved a later tilting of an earlier fold set.
FIG 74. Tintagel Head (Fig. 66, b9). (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
FIG 75. Chevron folding of Carboniferous sandstones and mudstones, Millook Haven (Fig. 66, c2). (Copyright Landform Slides – Ken Gardner)
FIG 76. Overturned fold in Crackington Formation, Culm Measures, Crackington Haven (Fig. 66, c1). (Copyright Landform Slides – Ken Gardner)
Landscape D: New Red Sandstone and younger bedrock
Along the eastern edge of Area 2, relatively unfolded New Red Sandstone of Permian and Triassic age rests on the folded Devonian and Carboniferous sediments of Landscapes B and C. The junction of the younger material with the older was formed when the younger sediment was deposited on the eroded margins of the Variscan hills. The New Red Sandstone occurs in a wide, north-south trending belt, extending southwards as far as Torquay (d1) and Paignton, and with fingers extending westwards to the north of Dartmoor (Fig. 37). Along the coast, from Exmouth (d4) southwards via Dawlish (d3) and Teignmouth (d2), the New Red Sandstone has been quarried and penetrated by the tunnels of the main coastal railway to the Southwest. The sandstone forms dramatic red cliffs, and marks the western edge of the World Heritage Site that extends to the east along the coast of Dorset.
The New Red Sandstone consists of sandstones, gravels and mudstones that formed as alluvial fans, desert dunes and in short-lived lakes along the edge of an irregular hilly landscape of older bedrock. The characteristic red colour so typical of many Devon soils has largely been derived from these New Red Sandstone rocks. In the Exeter area (d5), roads have been spectacularly cut through the New Red sediments, and also through some scattered deposits of volcanic rock, mainly lava. These lavas have been highly altered and have not resisted weathering at the surface any more than the sediments of the succession, so they have not had much influence upon the scenery.
An intriguing feature of the New Red Sandstone is the way the original landscape on which it formed is reappearing as the present landscape erodes. For example, the Crediton Basin (d6), north of Dartmoor, is now obvious as a remarkably finger-like strip of sediment, only 2–3 km across north to south, but extending almost 40 km west to east (Fig. 37). Detailed examination of the New Red sediment in this basin shows that it was deposited as the fill of a long, narrow valley, with material being derived from north and south as well as along its length from the west. The valley formed parallel to the folds and faults of the earlier Carboniferous bedrock on each side of it, and must have been cut first by river erosion in Permian times, controlled by the earlier folds and faults that were formed during the Variscan mountain convergence. A few kilometres further north, the Tiverton Basin in Area 3 has a similar west-to-east trend, though it is more open and less elongate.
From Exeter (d5) to Torquay (d1), the base of the New Red Sandstone reveals topography of hollows eroded westwards into a higher ground of Devonian and Carboniferous bedrock. The New Red Sandstone pattern is of alluvial fans radiating downstream, but generally draining towards the east, and it bears a striking general similarity to the present drainage and scenery of the area (Fig. 77). During Variscan convergence, the Devonian limestones were moved into their present pattern by flat-lying faults, and this was then followed by the intrusion of the Dartmoor granite, which may help to explain why the New Red valleys here were shorter than those preserved as the Crediton (d6) and Tiverton basins.
FIG 77. Reconstruction of the Permian topography and drainage, looking southwards towards the location of present-day Torquay (Fig. 66, d1). The current coastline is indicated purely for reference; there is no evidence for a sea in Permian times where the sea now is.
I have already described the importance of the Devonian limestones in creating topography in the Torquay area (d1) and around Torbay generally. This material is an important part of the bedrock in its own right, and has been quarried widely as a building stone. It was a popular stone for ornaments and furniture, particularly in Victorian times, when cut and polished fossil corals featured in many of the washstands that were produced in the period.
In Torquay the Kent’s Cavern complex of caves, formed by solution of Devonian limestones, is an important archeological site, preserving evidence of Heidelberg and Neanderthal man from deposits about 450,000 years old. These were deposited during the Anglian cold phase of the Ice Age, when ice sheets spread across East Anglia and into the Thames valley, though not into the Southwest. The remains of cave bears, hyenas and sabre-tooth cats have also been found in the cave complex, as well as those of modern humans.
Конец ознакомительного фрагмента.
Текст предоставлен ООО «ЛитРес».
Прочитайте эту книгу целиком, купив полную легальную версию (https://www.litres.ru/peter-friend/southern-england/) на ЛитРес.
Безопасно оплатить книгу можно банковской картой Visa, MasterCard, Maestro, со счета мобильного телефона, с платежного терминала, в салоне МТС или Связной, через PayPal, WebMoney, Яндекс.Деньги, QIWI Кошелек, бонусными картами или другим удобным Вам способом.
Peter Friend
Тип: электронная книга
Жанр: Природа и животные
Язык: на английском языке
Издательство: HarperCollins
Дата публикации: 17.04.2024
Отзывы: Пока нет Добавить отзыв
О книге: Illustrated with beautifully detailed photographs throughout, New Naturalist Southern England comprehensively explores the formation of these wonderful landscapes that are so universally admired.Most people share an enthusiasm for beautiful and breathtaking scenery, explored variously through the physical challenge of climbing to the top of the tallest mountains or the joy of viewing the work of a painter; but while easy to admire from a distance, such landscapes are usually difficult to explain in words. Harnessing recent developments in computer technology, the latest New Naturalist volume uses the most up-to-date and accurate maps, diagrams and photographs to analyse the diverse landscapes of Southern England.Peter Friend highlights the many famous and much loved natural landscapes of the southern half of England, ranging from the Chalk Downs to the bays of Cornwall, Devon and Dorset, and provides detailed explanations for the wide variety of natural events and processes that have caused such an exciting range of surroundings.Setting apart the topography that has resulted from natural rather than man-made occurrences, Friend focuses on each region individually, from East Anglia to London and the Thames Valley, and explains the history and development of their land structures through detailed descriptions and colourful diagrams.