Isle of Wight E-Book

John Downes

• Herausgeber: The Crowood Press
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Veröffentlichungsjahr: 2021

Beschreibung

The Isle of Wight is a geological gem with its 110km (68 mile) long coastline displaying a range of rocks dating from Lower Cretaceous to Oligocene age. Many of the sands and clays yield fossil bivalves and gastropods, and its famous dinosaur footprints attract much attention from geologists and tourists alike. Yet the scenic beauty of the island is the product of its differing strata, former earth movements and the erosive power of the sea and the rivers. The monoclinal fold that crosses the island forms the chalk downland ridge that ends in the splendid cliffs of Culver in the east and The Needles in the west. By contrast, the softer rocks produce low, slumped cliffs often cut by steep-sided chines or alternatively, on the north coast, branching estuaries and salt marsh creeks. With over 120 colour illustrations this book discusses the geological processes that created the island's distinctive landscape; it provides a field guide to the identification of rocks and fossils and includes details of nineteen itineraries to discover the geological examples and fossils discussed.

Leseprobe

The Isle of Wight

LANDSCAPE AND GEOLOGY

John Downes

The outermost sea stacks of The Needles.

The Isle of Wight

LANDSCAPE AND GEOLOGY

John Downes

First published in 2021 by The Crowood Press Ltd Ramsbury, Marlborough Wiltshire SN8 2HR

[email protected]

www.crowood.com

This e-book first published in 2021

© John Downes 2021

All rights reserved. This e-book is copyright material and must not be copied, reproduced, transferred, distributed, leased, licensed or publicly performed or used in any way except as specifically permitted in writing by the publishers, as allowed under the terms and conditions under which it was purchased or as strictly permitted by applicable copyright law. Any unauthorised distribution or use of this text may be a direct infringement of the author’s and publisher’s rights, and those responsible may be liable in law accordingly.

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.

ISBN 978 1 78500 893 1

Acknowledgements

I wish to acknowledge the invaluable contribution of photographic images by Antoinette Pearson of the Open University (pp. 9, 13, 15, 16T,B, 17B, 21, 24, 25all, 26all, 27T, 28all, 30, 35T,B, 36, 37B, 38-39B, 41R, 47T, 52T,B, 53T,B, 55, 77, 80B, 103T, 105T), Stephen Hallett of Cranfield University (pp. 34, 38T, 39T, 45T, 46T,M, 49T, 61, 66, 73, 87, 90, 91T,B, 100, 104, 105B), Tony Cross of the Open University (pp. 37T, 45B, 47B, 82B, 83T,B, 102), Tony Waltham Geophotos (pp. 2, 31, 32T,B, 46B, 79T,B, 81, 96, 103B), Dinosaur Isle Museum (p.19T,B) and John Faulkner (p.94).

Drawings on pp. 40T, 40B and 41L are extracted with permission from British Mesozoic Fossils and British Cenozoic Fossils, British Museum (Natural History), 1975.

Images used under Creative Commons Licence are by Graham Horn (p.74), Peter Trimming (pp. 76, 99), Christine Matthews (p. 88), Ron Saunders (pp. 97T,B) and Mypix (pp. 67, 93, 98).

All other photographs and drawings are by the author.

Front cover: The chalk cliff at the western tip of the Needles headland.

Cover design by Blue Sunflower Creative

Contents

Preface

1Introduction

2Wealden Beds: Land of the Dinosaurs

3Tropical Seas of the Lower Greensand

4The Chalk Seas

5Palaeogene: the Sea Comes and Goes

6Quaternary Events

7Sculpting the Landscape

8How the Coastline is Shaped

9From Prehistoric Burials to Tudor Forts

10Railways and Seaside Towns

11Rural Land Use

12Impact of Tourism on the Countryside

Further Reading

Glossary

Index

Preface

The basic concept underpinning this book is the relationship between geology, the landscape and land use. It is written for the reader with an interest in seeing and understanding the geology and the physical and human geography of the Isle of Wight. Amateur geologists, students, hikers, holidaymakers and even the apocryphal ‘man on the Clapham omnibus’ should find this guide helpful in explaining the development of the island that the Romans called Vectis.

The wonderful coastline, which is some 110km long, exposes rocks that range in age from the Lower Cretaceous to Oligocene. The first six chapters of this book relate the geological story of how the rocks have been formed and what past environments they represent. Each chapter has relevant itineraries for those who want to try ‘hands on’ geology in the field, with essential information regarding directions, access and parking. In addition, there are sections that provide the reader with additional background material.

Landscape and scenery are covered in succeeding chapters, and this is followed by the story of human settlement from Neolithic times to the present day. Indeed, the impact of modern economic activities – dominated by the tourism – represents a considerable threat to the physical environment that we need to thoroughly understand in order to be able to protect and preserve the beautiful landscape of this island.

Most of the localities have been researched and recorded in academic publications (seeFurther Reading) but here I have attempted to bring geology alive to the enthusiastic amateur observer by concentrating on key features of the rocks and landscape. Indeed, being able to practise geology in the field, in an area such as the Isle of Wight, is one of the most rewarding and enjoyable experiences of life, providing a glimpse of the treasures of the Earth, often in the company of the ‘salt of the earth’ … your fellow explorers. Details of access to each locality are correct at the time of writing but you should be aware that ownership of land changes, and rights of way may be diverted or restricted from time to time, particularly where coastal erosion is undermining clifftop paths and collapsing the sides of chines. You may find the local bus services useful when planning walks. These are operated by Southern Vectis Omnibus Company, which runs a variety of tourist buses including the Island Coaster and the Needles Breezer during the summer season. Timetables and information on concessionary fares and holiday rover tickets can be found on the website www.islandbuses.info.

You should read the Geological Fieldwork Code, printed at the start of this book. It is designed to ensure both your safety and the preservation of the geological environment. Some localities are Regionally Important Geodiversity Sites (RIGS) and some are Sites of Special Scientific Interest (SSSI). There is also the Isle of Wight Local Geodiversity Action Plan (LGAP), the primary function of which is to create a strategy to promote the island through the conservation and sustainable development of its Earth heritage. Other administrative bodies, such as the National Trust, English Heritage, the Forestry Commission, the Wildlife Trust and the local authority, also play an important role in protecting and conserving sites of geological and biological interest. Furthermore, almost half of the Isle of Wight is designated as an Area of Outstanding Natural beauty (AONB) in order to conserve and enhance its cultural heritage, wildlife and landscape.

For many years I have tutored courses for The Open University, where the contributions of Earth Science students from all walks of life have so often enriched arcane discussions concerning the origin of the rocks. It is in this spirit of enquiry that we need to approach geology in the field. Moreover, it is important to recognize that more often than not, there is no neat and tidy answer to the riddle of the rocks. Even the experts often disagree and rarely commit themselves fully one way or another; some preface their answers with expressions such as ‘on the one hand there is the possibility that … while on the other it may be that…’ In fact, it is often the case that more than one event in the past has produced what we see today.

Perhaps our best guide must still be the principle of uniformitarianism as proposed by the Scottish philosopher James Hutton in the eighteenth century. His thesis was that the present is the key to the past; in other words, such contemporary activity as erosion by rivers or glaciers, the deposition of sands on a beach or the eruption of volcanoes involves processes that have operated for millions of years in the making of our Earth. Hutton succinctly expressed his thoughts on the origins of the rocks when he declared, ‘There is no vestige of a beginning … no prospect of an end’.

As to the subject of this book; we can do no better than read the words of the Reverend J. Cecil Hughes, who spent many years on the island studying its rocks. In 1922 he wrote in the preface to his book The Geological Story of the Isle of Wight (reprinted in 2011):

No better district could be chosen to begin the study of geology than the Isle of Wight. The splendid coastal sections all round its shores, the variety of strata within so small an area, the great interest of those strata, the white chalk cliffs and coloured sands, the abundant and interesting fossils to be found in the rocks, awaken … a desire to know something of the story written in the rocks.

Geological Fieldwork Code

1. Follow the Country Code and observe local bye-laws. Remember to shut gates and to leave no litter.

2. Avoid undue disturbance to farm animals, wildlife or natural vegetation. Keep clear of nesting birds, particularly on coastal cliffs.

3. Always seek permission before entering private land.

4. Observe and record; make field sketches and photograph geological sections, but do not damage the outcrop by hammering indiscriminately.

5. Restrict the collecting of specimens to a minimum and do not remove in situ specimens.

6. Leave the site in as good a condition as possible for those who come after you.

7. Beware of dangerous cliffs and rock faces, taking care not to dislodge loose rocks.

8. In coastal localities, make sure that you consult tide timetables and warning notices. It is always best to work on a falling tide.

9. When visiting isolated places, make sure that someone knows where you are going and your estimated time of return. It is best to go with a companion rather than alone.

10. Always wear suitable footwear and outdoor clothing. Wear a safety helmet where advisable. Carry a mobile phone, but remember that it may fail to get a signal below cliffs or in remote country.

A more complete code is published by The Geologists’ Association.

Safety Notice

It is the responsibility of the user of this field guide to take all necessary precautions, particularly when on exposed cliff sections where falling rocks and adverse tidal conditions can create dangerous situations. Check tide timetables before beginning a coastal excursion to avoid being trapped by the incoming tide (see Time and Tide on the Foreshore in Chapter 8).

CHAPTER 1

Introduction

The Isle of Wight has been described as the ‘diamond in Britain’s geological crown’, and it does indeed provide a wonderful example of how geology influences the development of the landscape and the imprint of human activity on the land. Moreover, the island has been studied and written about in detail for several centuries. The scientific philosopher Robert Hooke (1635–1703) was born in Freshwater on the island and was a leading scientist of his day, using the newly invented reflecting microscope to study fossils, plant and insects, which he used as illustrations in his book Micrographia.

By Victorian times, scientific interest in the island had increased, as fossil bones and other evidence of past life emerged. Gideon Mantell (1790–1852) wrote several papers on the dinosaur remains found in the Wealden Beds of Compton Bay. Charles Lyell, the famous Scottish geologist, made use of his work on the island’s strata in writing his influential Principles of Geology (1830), which Darwin took with him on his scientific expedition in the Beagle. Well-known literary figures including Tennyson, Keats and Walter Scott were also attracted to the picturesque beauty of the Isle of Wight, and when Queen Victoria established her summer palace at Osborne in 1851, the island’s popularity was assured. Iconic views of the chalk cliffs of the Needles, the coloured cliffs of Alum Bay and the holiday beaches of Shanklin, so beloved of the railway artists of the 1930s, helped fuel the tourist industry that today provides the lifeblood of the island’s economy.

A Geological Timescale

To explain the geology of the Isle of Wight, we need to consider how it fits into the framework of geological time. The oldest rocks exposed at the surface are of Lower Cretaceous age (140–100 million years, or Ma); they include the Wealden Beds at the base, followed by the Lower Greensand, Gault Clay and Upper Greensand. Compared to the Cambrian rocks of Wales that are over 500 million years old, the strata on the Isle of Wight are relatively young! The chalk is of Upper Cretaceous age (100–66 Ma) at the top of which is a break in the sequence. This is because at the end of the Cretaceous the whole area was uplifted and eroded before the succeeding Paleogene rocks were deposited. We refer to this hiatus in the succession as an unconformity, and in some areas the time interval may be measured in tens of millions of years. The youngest ‘solid’ rocks on the island belong to the Oligocene period (34–23 Ma); after this there is no material evidence on the ground except for scattered superficial deposits such as plateau gravels, solifluction deposits, river alluvium and clay-with-flints covering the chalk downlands. However, although there are no rocks of Miocene age (23–5 Ma), this was a time of great upheaval in southern England and the Isle of Wight when the Alpine earth movements uplifted, folded and faulted the pre-existing rocks.

Time chart showing the succession of bedrocks exposed at the surface across the Isle of Wight.

The most significant Alpine structure is the east–west trending monocline forming the chalk ridge extending from the Needles in the west to Culver Cliff in the east. This fold has a steep northerly dip and an almost horizontal southern flank as a result of pressure from the south. As you will see from the structural map of the Isle of Wight, the fold axis is in two sections: from the Needles, the western section runs ESE towards Shorwell, and the eastern section extends roughly west to east from Newport to Culver Cliff. On the east side of Freshwater Bay, you can see not only monoclinal folding in the chalk but also numerous shear planes cutting across the bedding. These are caused by the intense compressional forces applied to the rocks during folding. The axial trend of the monocline can be traced westwards across to the Isle of Purbeck in Dorset. The fold was continuous until breached by the sea when the Isle of Wight was separated from the mainland by the Flandrian rise in sea level, some 12,000 years ago.

Simplified geological map of the Isle of Wight.

The Ferruginous Sands at Red Cliff looking towards the white chalk exposed along the cliffs of Culver Down.

Miocene fold structures in southern England produced by the Alpine orogeny.

There are several other important folds on the island. The Brighstone anticline runs WNW–ESE through the Wealden Beds and Lower Greensand around Brighstone Bay but much of its southwestern flank has now been eroded by the sea. On the opposite side of the island, the axis of the Sandown anticline trends NW–SE and exposes Wealden Beds in its core. North of the chalk ridge, the Bembridge syncline extends from Foreland Point ESE–WNW across the island. The Bembridge Limestone forms the flanks of the syncline while the younger Hamstead Beds occupy the central axial zone. This is an asymmetrical fold since the beds on the southern side are much steeper than those to the north. In the northwest of the island, the Bouldnor syncline and the complementary Porchfield anticline and Thorness syncline extend across the Solent to the mainland.

Landscape and Geological Structure

The topography of the Isle of Wight is closely related to the underlying geological outcrops. It is the east–west chalk ridge that forms the long axis of the lozenge-shaped island, which it divides into two areas of very different character. The central chalk outcrop in both its eastern and western extremities is relatively narrow, reflecting its steep dip on the north side of the monoclinal fold. Marine erosion has created a pointed chalk headland at the Needles, from which extends a series of sea stacks to the west, and on the opposite side of the island is the headland of Culver Cliff.

Geological section across the Isle of Wight monocline.

Geological structure of the Isle of Wight.

However, in the middle of the island, southwest of Carisbrooke, the dip of the chalk is lower and the outcrop wider, and here the chalk upland reaches a height of 214m on Brighstone Down. You will also notice that the summits of the chalk hills in both the central and southern outcrops are bevelled, forming a dissected plateau surface. This is thought to be the result of erosion by the Pliocene sea, when these hills became low islands. Subsequently the chalk surface has been deeply dissected by rivers, and since the end of Pleistocene times, the water table has fallen considerably, leaving dry valleys and little surface drainage.

Looking at the geological map, you will see that there is a chalk outlier in the far south of the island where St Boniface Down rises to 266m. The chalk dips gently south and presents a steep, scarped edge to the north, where it overlooks the undulating lowlands of the Lower Greensand, which have been exhumed by the erosion and removal of the chalk that once formed a continuous cover across the southern half of the island. On the south coast near Ventnor, the permeable Chalk and Upper Greensand overlie the impermeable ‘blue slipper’ or Gault Clay, producing an undercliff. Masses of chalk have slipped down over the wet, lubricated clay to create an undercliff at sea level and produce an uneven bench below the upper cliffs.

Another area of landslipping is the southwest coast from Compton Bay to Blackgang Chine, where the less resistant Wealden sands and clays and the Lower Greensand are being actively undermined by the sea, and frequent slippage occurs on the cliff faces.

The northern part of the Isle of Wight is relatively low-lying and formed of soft Palaeogene sands and marls, except for the resistant Bembridge Limestone that forms the Foreland, the most easterly extremity of the island. This limestone has been used extensively for building in the past, for example in Yarmouth Castle. A glance at the topographical map will show you that the northern shoreline is interrupted by several river estuaries, including the Western Yar, Newtown River, the Medina, Wootton Creek, and the Eastern Yar. These are all classic examples of drowned river mouths, formed when sea levels rose during the Flandrian transgression some 10,000 years ago. At this time the Isle of Wight became separated from the mainland. However, before this happened, these rivers would have extended northwards to join the eastward-flowing ‘Solent river’, which was fed by the Frome, the Stour and the Avon draining the Hampshire basin.

Why should the Isle of Wight rivers all be flowing north? The answer lies in the fact that they were initiated on the chalk cover (now much eroded), which would have had a gentle northward gradient. Newport and Brading now occupy gaps in the chalk ridge that were cut as the land surface was lowered by erosion. Also look at the Western Yar on the map; it is only about 4km long, rising near Freshwater Bay; clearly it was formerly much longer and its source would have been some distance south of the present coastline before the sea removed its headwaters. Not all drainage is to the north; on the south and west coasts, small streams locally known as chines descend rapidly from the higher ground to the sea, cutting deep ravines in the Greensand.

Plate Tectonics, Folds and Faults

The Earth’s surface has evolved over millions of years, and the driving force behind its changing continents is the concept of plate tectonics. It states that the surface of the Earth is composed of large plates that are constantly in motion in respect to one another. Magma continually rises up at the mid-oceanic ridges, cools and solidifies and slowly forces the oceanic plates apart. As these plates move outwards, they eventually collide with adjacent continental plates, where they are forced down in a process known as subduction. This plate movement has been going on throughout the Earth’s history, causing oceans and continents to drift across the surface of the globe, so that the present configuration of land and sea is vastly different to that in Precambrian times. When converging tectonic plates collide, one may be subducted below the other, as, for example, where denser oceanic crust is dragged down deep below the lighter continental crust. As the two plates grind together, seismic waves are produced, resulting in earthquakes, and the high pressures and temperatures in the subduction zone generate melts that give rise to volcanic activity in the overlying continental crust. Plate convergence also leads to mountain building, when the rocks may become intensely folded and faulted. This classic scenario can be seen today along the Pacific coast of Peru and Chile, where the Nazca oceanic plate is subducted beneath the continental plate of South America and volcanoes and earthquakes occur within the Andes mountain range.

Some of the more common fold and fault structures.

This process of mountain building, referred to by geologists as orogenesis, has taken place in three major episodes during the last 500 Ma and has had considerable influence on the physical development of Britain. The Caledonian orogeny occurred when the Iapetus Ocean closed as plates converged towards the end of Silurian times, and the subsequent uplift and folding of sediments produced the mountain ranges of Scotland and Wales. Later, at the end of the Carboniferous, the Rheic Ocean closed, initiating the Variscan orogeny that produced intense folding and faulting in southwest England.

The third great earth building movement, known as the Alpine orogeny, occurred from late Cretaceous to Miocene times as a result of the northward movement of the African and Arabian plates. They were subducted beneath the Eurasian plate as the Tethys Ocean closed, leaving the proto-Mediterranean Sea. This plate convergence squeezed the marine sediments then uplifted and folded them to produce a complex mountain zone including the Pyrenees, the Alps and the Atlas Mountains.

While major uplift took place in these areas, the outer ripples of this orogeny were felt in southern Britain and northeast France. The Weald-Artois anticline and the Purbeck-Wight monocline are impressive testimony to the power of the compressive tectonic forces during the Miocene period, when the Alpine earth movements reached their zenith. The Cretaceous and Palaeogene sediments that occupy the Hampshire Basin (including the Isle of Wight) were all subject to Alpine tectonic disturbance, with pressure being predominantly from the south.

When sediments are deposited, they are usually formed in sub-horizontal layers and, according to the Law of Superposition, the oldest rocks are found at the base of a sequence and successively younger rocks lie above. However, if the layers are later subjected to earth movements, they may be folded and faulted and, in some cases, completely inverted. Anticlines (upfolds) and synclines (downfolds) are the simplest type of fold, but if the lateral pressure is greater on one side of the fold then it will become asymmetric where one limb is steeper than the other. A recumbent fold is one that has been pushed over to such an extent that its lower limb is upside down. Another type of fold is the monocline, where one limb is almost vertical and the other more or less horizontal.

A walk along the forshore southeast from Hanover Point. The cliffs are formed by marls of the Wessex Formation, capped by Pleistocene sands and gravels.

You can simulate the formation of folds simply by moving a carpet on a smooth surface. If you apply equal pressure from both sides you can make symmetrical folds, and by applying more pressure on one side, asymmetrical ones. Compression will not only produce folds, but it will also fracture rock layers to create reverse faults; when rocks are pulled apart under tension, normal faults occur. In both these cases the movement is up or down the dip of the fault plane, but where the movement along the fault plane is horizontal, a wrench or tear fault is produced. This is also called a strike slip fault because the horizontal displacement is parallel to the strike of the fault plane.

CHAPTER 2

Wealden Beds: Land of the Dinosaurs

The oldest beds exposed on the Isle of Wight belong to the Wealden Group (145–126 Ma) of Lower Cretaceous age. They are known as Wealden Beds (in modern terminology, the Wealden Group) because the same strata outcrop in the Weald of Kent and Sussex. They consist mainly of clays and shales with sandstone layers such as are found along the coast of Brighstone Bay in the southwest and behind Sandown Bay on the east side of the island. During Wealden times, rivers draining a land area in the west transported silt, sand and grit to form extensive delta flats with braided distributary channels over much of southern England.

These delta plains were covered with subtropical vegetation, as witnessed by the numerous plant remains found in the sediments. Pieces of wood and pine logs accumulated near the mouth of the delta rivers (distributaries) and are now preserved within the Wealden sediments. Later, as the volume of sand and mud brought down by the rivers decreased due to the erosion of surrounding land masses, large freshwater lagoons emerged, in which finely laminated shales and thin limestones were deposited.

The Wessex Formation

On the Isle of Wight, the Wealden Group is divided into two distinct divisions. The lower beds are known as the Wessex Formation. These consist of red, purple and green marls with some sandstone beds laid down in freshwater conditions on swampy delta tops and wide floodplains crossed by sluggish, meandering rivers. The sandstones were deposited in sinuous river channels while the marls are overbank deposits on the floodplains. The massive Sudmore Point Sandstone that forms the cliffs to the northwest of Chilton Chine is a good example of a channel sandstone; it divides into several sandy horizons separated by marls beyond Sudmore Point, suggesting the possibility that the meandering river became silted up and changed channels over time.