The Geological Story of the Isle of Wight Part 1

The Geological Story of the Isle of Wight.

by J. Cecil Hughes.

PREFACE

No better district could be chosen to begin the study of Geology than the Isle of Wight. The splendid coast sections all round its sh.o.r.es, the variety of strata within so small an area, the great interest of those strata, the white chalk cliffs and the coloured sands, the abundant and interesting fossils to be found in the rocks, awaken in numbers of those who live in the Island, or visit its sh.o.r.es, a desire to know something of the story written in the rocks. The Isle of Wight is cla.s.sic ground of Geology. From the early days of the science it has been made famous by the work of great students of Nature, such as Mantell, Buckland, Fitton, Sedgwick, Owen, Edward Forbes, and others, who have carried on the study up to the present day. Many of the strata are known to geologists everywhere as typical; several bear the names of the Island localities, where they occur; some--and those not the least interesting--are not found beyond the limits of the Island.

Though studied for so many years, there is no exhausting their interest: new discoveries are constantly made, and new questions arise for solution. To those who have become interested in the rocks of the Island, and the fossils they have found in them, and who wish to learn how to read the story they tell, and to know something of that story, this book is addressed. It is intended to be an introduction to the science of Geology, based on the Geology of the Isle of Wight, yet leading on to some glimpse of the history presented to us, when we take a wider outlook still, and try to trace the whole wondrous path of change from the world's beginning to the present day.

I wish to express my warmest thanks to Miss Maud Neal for the beautiful drawings of fossils which ill.u.s.trate the book, and to Professor Grenville A. J. Cole, F.R.S., for his kindness in reading the ma.n.u.script, and for valuable suggestions received from him. I have also to acknowledge my indebtedness to Mr. H. J. Osborne White's new edition of the _Memoir of the Geological Survey of the Isle of Wight_, 1921; and to thank Mr. J. Milman Brown, of Shanklin, for the three photographs of Island scenery, showing features of marked geological interest, and Mr. C. E. Gilchrist, Librarian of the Sandown Free Library, for kindly reading the proofs of the book.

J. CECIL HUGHES.

Mar., 1922.

Chapter I

THE ROCKS AND THEIR STORY

Walking along the sea sh.o.r.e, with all its varied interest, many must from time to time have had their attention attracted by the sh.e.l.ls to be seen, not lying on the sands, or in the pools, but firmly embedded in the solid rock of the cliffs and of the rock ledges which run out on to the sh.o.r.e, and have, it may be, wondered sometimes how they got there. At almost any point of the coast of the Isle of Wight, in bands of limestone and beds of clay, in cliffs of sandstone or of chalk, we shall have no difficulty in finding numerous sh.e.l.ls. But it is not only in the rocks of the sea coast that sh.e.l.ls are to be found. In quarries for building stone and in the chalk pits of the downs we see sh.e.l.ls in the rock, and may often notice them in the stones of walls and buildings. How did they get there? The sea, we say, must once have been here. It must have flowed over the land at some time. Now let us think. We are going to read a wonderful story, written not in books, but in the rocks. And it will be much more valuable if we learn to read it ourselves, than if we are just told what other people have made out. We know a thing much better if we see the answers to questions for ourselves than if we are told the answers, and take some one else's word for it. And if we learn to ask questions of Nature, and get answers to them, it will be useful in all sorts of ways all through life. Now, look at the sh.e.l.ls in the rock of cliff and quarry.

How are they there? The sea cannot have just flowed over and left them. The rock could not have been hard, as it is now, when they got in. Some of the rocks are sandstone, much like the sand on the sea sh.o.r.e, but they are harder, and their particles are stuck together.

Does sand on a sea sh.o.r.e ever become hard like rock, so that sh.e.l.ls buried in it are found afterwards in hard rock? Now we are getting the key to a secret. We are learning the way to read the story of the rocks. How? In this way. Look around you. See if anything like this is happening to-day. Then you will be able to read the story of what happened long, long ago, of how this world came to be as it is to-day.

We have asked a question about the sandstone. What about the clays and the limestone? As before, what is happening to-day? Is limestone being made anywhere to-day, and are sh.e.l.ls being shut up in it? Are sh.e.l.ls in the sea being covered up with clay,--with mud,--and more sh.e.l.lfish living on the top of that; and then, are they, too, being covered up?

So that in years to come they will be found in layers of clay and stone like those we have been looking at in quarry and sea cliff?

We have asked our questions. Now we must look around, and see if we can find the answers. After it has been raining heavily for two or three days go down to the marshes of the Yar, and stand on one of the bridges over the stream. We have seen it flowing quite clear on some days. Now it is yellow or brown with mud. Where did the mud come from?

Go into a ploughed field with a ditch by the side. Down the ditch the rain water is pouring from the field away to the stream. It is thick with mud. Off the ploughed field little trickles of water are running into the ditch. Each brings earth from the field with it. Off all the country round the rain is trickling away, carrying earth into the ditches and on into the stream, and the stream is carrying it down into the sea. Now think. After every shower of rain earth is carried off the land into the sea. And this goes on all the year round, and year after year. If it goes on long enough--? Look a long way ahead, a hundred years,--a thousand,--thousands of years. We shall be talking soon of what takes many thousands of years to do. Why, you say, if it goes on long enough, all the land will be carried into the sea. So it will be. So it must be. You see how the world is changing. You will soon see how it has changed already, what wonderful changes there have been. You will see that things have happened in the world which you never guessed till you began to study Geology.

Now, let us go a bit further. What becomes of all the mud the streams and rivers are carrying down into the sea? Look at a stream coming steeply down from the hills. How it rushes along, rolling pebbles against one another, sweeping everything before it, clearing out its channel, polis.h.i.+ng the rocks, and carrying all it rubs off down towards the sea. Now look at a river near its mouth in flat lowland country. It flows now much slower; and so it has not power to bear along all the material it swept down from the hills. And so it drops a great deal; it is always silting up its own channel, and in flood time depositing fresh layers of mud on the flat meadow land,--the alluvial flat,--through which it generally flows in the last part of its course. But a good deal of sediment is carried by the river out to sea. The water of the river, moving slower as it enters the sea, has less and less power to sweep along its burden of sand and mud, and it drops it on the sea bottom,--first the bigger coa.r.s.er particles like the sand, then the mud; farther out, the finer particles of mud drop to the bottom.

During the exploring cruise of the _Challenger_, under the direction of Sir Wyville Thomson, in 1872-6, the most extensive exploration of the depths of the sea that has been made up to the present time, it was found that everything in the nature of gravel or sand was laid down within a very few miles, only the finer muddy sediments being carried as far as 20 to 50 miles from the land, the very finest of all, under most favourable conditions, rarely extending beyond 150, and never exceeding 300 miles from land into the deep ocean. So gradually layer after layer of sand and mud cover the sea bed round our coasts; and sh.e.l.ls of c.o.c.kles and periwinkles, of crabs and sea urchins, and other sea creatures that have lived on the bottom of the sea are buried in the growing layers of sand and mud. As layer forms on layer, the lower layers are pressed together, and become more and more solid. And so we have got a good way towards seeing the making of clay and sandstone with sh.e.l.ls in them, such as we saw in the sea cliffs and the quarries.

But it is not only rain and rivers that are wearing the land away. All round the coasts the sea is doing the same work. We see the waves beating against the sh.o.r.es, was.h.i.+ng out the softer material, hollowing caves into the cliffs, eating away by degrees even the hardest rock, leaving for a while at times isolated rocks like the Needles to mark the former extension of the land. Most people see for themselves the work of the sea, but do not notice so much what the rain and the frost, the streams and the rivers are doing. But these are wearing away the ground over the whole country, while the sea is only eating away at the coast line. So the whole of the land is being worn away, and the sand and mud carried out into the sea, and deposited there, the material of new land beneath the waters.

How do these beds rise up again, so that we find them with their sea sh.e.l.ls in the quarry? Well, we look at the sea heaving up and down with the tides, and we think of the land as firm and fixed. And yet the land also is continually heaving up and down--very slowly,--far too slowly for it to be noticed, but none the less surely. The exact causes of this are not yet well understood, because we know but little about the inside of the earth. The deepest mine goes a very little way. We know that parts of the interior are intensely hot. The temperature in a mine becomes hotter, about 1F. for every 60 ft. we go down on the average. We know that there are great quant.i.ties of molten rock in places, which, in a volcanic eruption is poured out in sheets of lava over the land. There are great quant.i.ties of water turned into steam by the heat, and in an eruption the steam pours out of the crater of the volcano like the clouds of steam out of the funnel of a locomotive. The people who live about a volcano are living, as it were, on the top of the boiler of a steam engine; and their country is sometimes shaken up and down like the lid of a kettle by the escaping steam. In such a country the land is often changing its level. A few miles from Naples at Pozzuoli, the ancient Puteoli, may be seen columns of what appears to be an ancient market hall, though it goes by the name of the Temple of Serapis. About half way up the columns are holes bored by boring sh.e.l.lfish, such as we may find on the sh.o.r.e here at low tide. We see from this that since the building was constructed in Roman times the land has sunk, and carried the columns into the sea, and sh.e.l.lfish have bored into them. Then the land has risen, and lifted the columns out of the sea again.

But it is not only in the neighbourhood of volcanoes that the land is moving. Not suddenly and violently, but slowly and gradually great tracts of land rise and sink. Sometimes the land may remain for a long time nearly stationary. The Southern coasts of England seem to stand at much the same level as in the time of the Romans 1,500 or 2,000 years ago. On the other hand there is evidence which seems to show that the coast of Norway has for some time been gradually rising.

It was thought at one time that the interior of the earth was liquid like molten lava, and that the land we see was a comparatively thin crust over this like the crust of a pie. But it is now believed for various mathematical reasons, that the main ma.s.s of the earth is rigid as steel. Still underneath the surface rocks there must be a quant.i.ty of semi-fluid matter, like molten rock, and on this the solid land sways about, as we see the ice on a pond sway with the pressure of the skaters on it. So the solid land, pressed by internal forces, rises and falls like the elastic ice, sometimes sinking and letting the sea flow over, then rising again, and bringing up the land from beneath the sea.

Again, as the heated interior of the earth gradually cools by the radiation of the earth's heat into s.p.a.ce, it will tend to shrink away from the cooler rocks of the crust. This then, sinking in upon the shrinking interior, will be thrown into folds, like the skin on a shrivelled apple. Seeing, as we often do, layers of rock thrown into numerous folds, so as to occupy a horizontal s.p.a.ce far less than that in which they were originally laid down, we can hardly resist the conclusion that shrinkage of the cooling interior of the earth has been a chief cause of the greatest movements of the surface, and of the lateral pressure we so often find the strata to have undergone.

As we study geology we shall find plenty to show that the land does rise and fall, that where now is land the sea has been, that land once stretched where now is sea, though there is still much which is not well understood about the causes of its movements. We have seen how many of the rocks are made in the sea,--the sandstones and the clays,--but there are two other kinds of rocks, about which we must say a little. The first are the Igneous rocks, which means rocks made by fire. These rocks have solidified, most frequently in crystalline forms, from a molten ma.s.s. Lava, which flows hot and fluid, from a volcano, and cooling becomes a sheet of solid rock, is an igneous rock. Some igneous rocks solidify under ground under great pressure, and become crystalline rocks such as granite. We shall not find these rocks in the Isle of Wight. We should find them in Cornwall, Wales, and Scotland; and, if we could go deep enough, we should find some such rock as granite underneath the other rocks all the world over.

The other rocks, such as the sandstones and clays, are called Sedimentary rocks, because they are formed of sediment, material carried by the sea and rivers, and dropped to the bottom. They are also called Stratified rocks, because they are formed of Strata, _i.e._, beds or layers, as we see in cliff and quarry.

But we have seen another kind of rock,--the limestones. In Sandown Bay towards the Culvers, bands of limestone run through the dark clay cliffs, and broken fragments lie on the sh.o.r.e, looking like pieces of paving stone. Examining these we find that they are made up of sh.e.l.ls, one band of small oysters, the others of sh.e.l.ls of other kinds. You see how they have been made. There has been an oyster bed, and the sh.e.l.ls have been pressed together, and somehow stuck together, so that they have formed a layer of rock. They are stuck together in this way. The atmosphere contains a small quant.i.ty of carbonic dioxide, and the soil a larger quant.i.ty, the result of vegetable decomposition.

Rain water absorbs some of it, and carries it into the rocks, as it soaks into the ground. This gas has the property of combining with carbonate of lime,--the material of which sh.e.l.ls and limestone are made. The bicarbonate of lime so formed is soluble in water, which is not the case with the simple carbonate. Water containing carbonic dioxide soaking into a limestone rock or a ma.s.s of sh.e.l.ls dissolves some of the carbonate of lime, and carries it on with it. When it comes to an open s.p.a.ce containing air, some of the carbonic dioxide is given off, leaving the insoluble carbonate of lime again. So by degrees the hollows are filled up, and a solid layer of rock is formed. Even while gathering in the sea the sh.e.l.l-fragments may be cemented by the deposit of carbonate of lime from sea-water containing more of the soluble bicarbonate than it can hold.

These limestones are examples of rocks which are said to be of organic origin, that is to say, they are formed by living things. Organic rocks may be formed by animal or vegetable growth. Rocks of vegetable origin are seen in the coals. A peat bog is composed of a ma.s.s of vegetable matter, chiefly bog moss, which for centuries has been growing and acc.u.mulating on the spot. At the bottom of the bog will frequently be found trunks of oak, or other trees, the remains of a forest of former days. The wood has undergone chemical changes, has lost much of its moisture, and often become very hard, as in bog oak.

Beds of coal have been formed by a similar process, on a much vaster scale, and continued much longer. The remains of ancient forests have been buried under sand stones and other rocks, have undergone chemical change, and been compressed into the hard solid ma.s.s we call coal.

Fossil wood, which has not reached the stage of hard coal, but forms a soft brown substance, is called lignite. This is of frequent occurrence in various strata in the Isle of Wight.

Of organic rocks of animal origin the most remarkable are the chalk, of which we shall speak later, and the coral-reefs, which are found in the warm waters of tropical seas. Sailing over the South Pacific you will see a line of trees--coconut trees chiefly--looking as if they rose up from the sea. Coming nearer you see that they grow on a low island, which rises only a few feet above the water. These islands are often in the form of a ring, and look "like garlands thrown upon the waters." Inside the ring is a lagoon of calm water. Outside the heavy swell of the Southern Ocean thunders on the coral sh.o.r.e. If a sounding line be let down from the outer edge of the reef, it will be found that the wall of coral goes down hundreds of feet like a precipice. On an island in the Southern Sea, Funafuti, a deep boring has been made 1,114 ft. deep. As far as the boring went all was coral. All this ma.s.s of coral is formed by living things,--polyps they are called. They are like tiny sea anemones, only they grow attached to one another, forming a compound animal, like a tree with stem and branches, and little sea anemones for flowers. The whole organism has a sort of sh.e.l.l or skeleton, which is the coral. Blocks are broken off by the waves, and ground to a coral mud, which fills up the interstices of the coral; and as more coral grows above, the lower part of the reef becomes, by pressure and cementing, a solid coral limestone. Once upon a time there were coral islands forming in a sea, where now is England. These old coral reefs form beds of limestone in Devon, Derbys.h.i.+re, and other parts of England. In the Isle of Wight we have no old coral reefs, but we shall easily find fossil corals in the rocks. They helped to make up the rocks, but there were not enough here to make reefs or islands all of coral.

The great branching corals that form the reefs can only live in warm waters. So we see that when corals were forming reefs where now is England the climate must have been warm like the tropics. That is a story we shall often read as we come to hear more about the rocks. We shall find that the climate has often been quite warm as the tropics are now: and we shall also read another wonderful story of a time when the climate was cold like the Arctic regions.

Chapter II.

THE STRUCTURE OF THE ISLAND.

The best place to begin the study of the Geology of the Isle of Wight is in Sandown Bay. North of Sandown, beyond the flat of the marshes, are low cliffs of reddish clay, which has slipped in places, and is much covered by gra.s.s. At low tide we shall see the coloured clays on the sh.o.r.e, unless the sand has covered them up. Variegated marls they are called--_marl_ means a limy clay, _loam_ a sandy clay; and very fine are the colours of these marls, rich reds and purples and browns.

Beyond the little sea wall below Yaverland battery we come to a different kind of clay forming the cliff. It is in thin layers. Clay in thin layers like this is called _shale_. Some of these shales are known as paper shales, for the layers are thin almost like the leaves of a book. The junction of the shales with the marls is quite sharp, and we see that the shales rest on the coloured marls, not horizontally, but sloping down towards the North. Bands of limestone and sandstone running through the shales, and a hard band of brown rock which runs out on the sh.o.r.e as a reef, slope in the same direction. As we pa.s.s on by the Red Cliff to the White Cliffs we notice that the strata slope more steeply the further North we go. We have seen that these strata were laid down layer by layer at the bottom of the sea. If we find a lot of things lying one on top of another, we may generally conclude that the ones at the bottom were put there first, then the next, and so on to the top. And this will generally be true with regard to the rocks. The lowest rocks must have been laid down first, then the next, and so on. But these layers of shale with sh.e.l.ls in them, and layers of limestone made of sh.e.l.ls, must have been laid down at first fairly flat on the sea floor; but as they were upheaved out of the sea they have been tilted, so that we now see them in an inclined position. And when we come to the chalk, we should see, if we looked at the end of the Culver Cliffs from a boat, that the lines of black flints that run through the chalk are nearly vertical. The strata there have been tilted up on end.

[Ill.u.s.tration: FIG. 1.]

DIAGRAM OF COAST, SANDOWN BAY, DUNNOSE TO CULVER CLIFF.

W _Wealden._ P _Perna Bed._ LG _Lower Greensand._ Cb _Clay Bands._ S _Sandrock and Carstone._ g _Gault._ UG _Upper Greensand._ C _Chalk._ Sc _Shanklin Chine._ Lc _Luccombe Chine._

In describing how strata lie, we call the inclination of the strata from the horizontal the _dip_. The direction of a horizontal line at right angles to that of the dip is called the _strike_. If we compare the sloping strata to the roof of a house, a line down the slope of the roof will mark the direction of the dip, the ridge of the roof that of the strike. The strata we are considering dip towards the North; the line of strike is East and West.

Returning towards Sandown we see the strata dipping less and less steeply, till near the Granite Fort the rocks on the sh.o.r.e are horizontal. Continuing our walk past Sandown to Shanklin we pa.s.s the same succession of rocks we have been looking at, but in reverse order, and sloping the other way. It is not very easy to see this at first, for so much is covered by building; but beyond Sandown we see Sandstone Cliffs like the Red Cliff again, the strata dipping gently now to the south, and in the downs above Shanklin we see the chalk again. So we have the same strata north and south of Sandown, forming a sort of arch. But the centre of the arch is missing. It must have been cut away. We saw that the land was all being eaten away by rain and rivers. Now we see what they have done here. Go up on to the Downs, and look over the central part of the Island. We see two ranges of downs running from east to west,--the Central Downs of the Island, a long line of chalk down 24 miles from the Culver Cliff on the east to the Needles on the west; and the Southern Downs along the South Coast from Shanklin to Chale. In the Central Downs the chalk rises nearly vertically, and turns over in the beginning of an arch towards the South. Then comes a big gap, and the chalk appears again in the Southern Downs nearly horizontal, sloping gently to the south. The chalk was once joined right across the central hollow, where now we see the villages of Newchurch, G.o.ds.h.i.+ll, and Arreton. All that enormous ma.s.s of rock that once filled the s.p.a.ce between the downs has been cut away by running water.

An arch of strata like this [Inverted-U], such as the one we are looking at, is called an _anticline_. When the arch is reversed, like this [U], it is called a _syncline_. Looking north from the Central Downs over the Solent we are looking at a syncline. The chalk, which dips down at the Culvers and along the line of the Central Downs, runs like a trough under the Solent, and rises again, as we see it on the other side, in the Portsdown Hills.

We might suppose the top of an anticlinal arch would be the highest part of the country; that, even if rain and running water have worn the country down, that would still stand highest, and be worn down least. But there are reasons why this need not be so. For one thing, when the horizontal strata are curved over into an arch, they naturally crack just at the top of the curve, so and into the cracks the rain gets, and so a stream is started there, which cuts down and widens its channel, and so eats the land away. Again, the rising land only emerges gradually from the sea, and the sea may cut off the top of the arch before it has risen out of its reach. Moreover on the higher land the fall of rain and snow is greater, and the frosts are more severe; so that it is just there that the forces wearing down the land are most effective.

[Ill.u.s.tration: curve with two v-shaped marks at center]

We must notice another thing which happens when rocks are being upheaved and bent into curves. The strain is very great, and sometimes the strata crack and one side is pushed up more than the other. These cracks are called _faults_. At Little Stairs, about half way between Sandown and Shanklin, two or three faults may be seen in the cliff.

The effect of two of the faults may be easily seen by noticing the displacement of a band of rock stained orange by water containing iron. The strata are thrown down towards the north about 8 ft. A third fault, the effect of which is not so evident at first sight, throws the strata down roughly 50 ft. to the south. These are only small faults, but sometimes faults occur, in which the strata have been moved on opposite sides of the fault thousands of feet away from one another. We might think we should see a wall of rock rising up on the surface of the ground where a fault occurs; but the faults have mostly taken place ages ago; and, when they do happen, the rocks are generally moved only a little way at a time. Then after a while another push comes on the rocks, and they s.h.i.+ft again at the same place, and go a bit further. All this time frost and rain and rivers are working at the surface, and planing it down; so that the unevenness of the surface caused by faults is smoothed away; and so even a great fault does not show at the surface.

As we follow the Sandown anticline westward it gradually dies away, the upheaved area being actually a long oval--what we may call a turtle-back. As the Sandown anticline dies out, it is succeeded by another a little further south, the Brook anticline. There are in fact a series of these east and west anticlines in the Island and on the adjacent mainland, caused by the same earth movement. As a consequence of the arching of the strata we find the lowest beds we saw in Sandown Bay running out again on the west of the Island in Brook Bay, and a general correspondence of the strata on the east and west of the Island; while, as we travel from Sandown or Brook northward to the Solent, we come to continually more recent beds overlying those which appear to the south of them.

When, as in the south side of our central downs, the strata are sharply cut away by denudation, we call this an _escarpment_. The figure shows the structure of the Sandown anticline we have described.