The Geological Story of the Isle of Wight Part 5

The limestone is a freshwater formation, and the fossils are mostly freshwater sh.e.l.ls, of the same type as the Headon, Limnaea and Planorbis the most common. There are also land sh.e.l.ls, especially several species of Helix, the genus which includes the common snail,--_H. globosa_, very large,--and great species of _Bulimus_ (_Amphidromus_) and _Achatina_ (_B. Ellipticus_, _A. costellata_).

These interesting sh.e.l.ls were chiefly obtained in the limestone at Sconce near Yarmouth, a locality now inaccessible, being occupied by fortifications. The land sh.e.l.ls have an affinity to species now found in Southern North America. The limestone also abounds in the so-called "seeds" of Chara. The reproductive organs,--the "seeds,"--of this curious water-plant, allied to the lower Algae, are, like the rest of the plant, encased in carbonate of lime, and are very durable. Large numbers are found in the Oligocene strata. Under the microscope they are seen to be beautifully sculptured in various designs, with a delicate spiral running round them. Above the limestone lie the Bembridge marls, varying in thickness in different localities from 70 to 120 feet. North of Whitecliff Bay they stretch on to the Foreland.

They are in the main a freshwater formation, but a few feet above the limestone is a marine band with oysters, _Ostrea Vectensis_. It runs out along the sh.o.r.e, where the oysters may be seen covering the surface. The Lower Marls consist chiefly of variously-coloured clays with many sh.e.l.ls, chiefly _Cyrena pulchra_, _semistriata_, and _obovata_, _Cerithium mutabile_, and _Melania muricata_ (_acuta_); and red and green marls, in which are few sh.e.l.ls, but fragments of turtle occur. A little above the oyster bed is a band of hard-bluish septarian limestone. Sixty years ago Edward Forbes remarked on the resemblance of this band to the harder insect-bearing limestones of the Purbeck beds. In a limestone exactly resembling this, and similarly situated in the lower part of the marls in Gurnard and Thorness Bays, numerous insects were afterwards found,--beetles, flies, locusts, and dragonflies, and spiders. Leaves of plants, including palms, fig, and cinnamon, have also been found in this bed, showing that the climate was still sub-tropical. The upper Marls consist chiefly of grey clays with abundance of _Melania turritissima_ (_Potamaclis_). The chief sh.e.l.ls in the marls are _Cyrena_, _Melania_, _Melanopsis_ and _Paludina_ (_Viviparus_). They are often beautifully preserved; the species of Cyrena often retain their colour-markings.

Bembridge Foreland is formed by a thick bed of flint gravel resting on the marls, which are seen again in Priory Bay, where in winter they flow over the sea-wall in a semi-liquid condition. They lie above the limestone at Gurnard, Thorness, and Hamstead. West of Hamstead Ledge the whole of the beds crop out on the sh.o.r.e, where beautifully preserved fossils may be collected. Large pieces of drift wood occur, also seeds and fruit. Many fragments of turtle plates may be found.

Large crystals of selenite (sulphate of lime) occur in the Marls.

Last of the Oligocene in the Isle of Wight are the Hamstead beds.

These strata are peculiar to the Isle of Wight. The Bembridge beds also are not found on the mainland, except a small outlier at Creechbarrow Hill in Dorset. The Hamstead beds consist of some 250 feet of marls, in which many interesting fossils have been found. They cover a large area of the northern part of the Island, largely overlaid by gravels, and are only seen on the coast at Hamstead, where they form the greater part of the cliff, which reaches a height of 210 ft., the top being capped by gravel. In winter the clays become semi-liquid, in summer the surface may be largely slip and rainwash, baked hard by the sun. The lower part of the strata may be best seen on the sh.o.r.e. The strata consist of 225 ft. of freshwater, estuarine, and lagoon beds, with _Unio_, _Cyrena_, _Cyclas_, _Paludina_, _Hydrobia_, _Melania_, _Planorbis_, _Cerithium_ (rare), and remains of turtles, crocodiles, and mammals, leaves and seeds of plants; and above these beds 31 feet of marine beds with _Corbula_, _Cytherea_, _Ostrea callifera_, _c.u.ma_, _Voluta_, _Natica_, _Cerithium_, and _Melania_.

Except for the convenience of dividing so large a ma.s.s of strata, it would not be necessary to divide these from the Bembridge beds, as no break in the character of the life of the period occurs at the junction. The bas.e.m.e.nt bed of the Hamstead strata is known as the Black Band, 2 feet of clay, coloured black with vegetable matter, with _Paludina lenta_ very numerous, _Melanopsis carinata_, _Limnaea_, _Planorbis_, a small _Cyclas_ (_C. Bristovii_), seed vessels, and lumps of lignite. It rests on dark green marls with _Paludina lenta_ and _Melanopsis_, and full of roots. This evidently marks an old land surface. About 65 feet higher is the White Band,--a white and green clay full of sh.e.l.ls, mostly broken. There are bands of tabular ironstone containing _Paludina lenta_. Clay ironstone was formerly collected on the sh.o.r.e between Yarmouth and Hamstead and sent to Swansea to be smelted. The strata consist largely of mottled green and red clays, probably deposited in brackish lagoons. These yield few fossils except remains of turtle and crocodile and drifted plants. The blue clays are much more fossiliferous. Among other plants are leaves of palm and water-lily. The strata gradually become more marine upwards. The marine beds were called by Forbes the Corbula beds, from two small sh.e.l.ls, _C. pisum_ and _C. vectensis_, of which some of the clays are full. Remains of early mammalia are found in the Hamstead beds, the most frequent being a hog-like animal, of supposed aquatic habits, Hyopotamus, of which there are more than one species.

The fauna and flora of the Oligocene strata show that the climate was still sub-tropical, though somewhat cooling down from the Eocene.

Palms grew in what is now the Isle of Wight. Alligators and crocodiles swam in the rivers. Turtle were abundant in river and lagoon.

Specially interesting in the Eocene and Oligocene are the mammalian remains. They show us mammals in an early stage before they branched off into the various families as we know them to-day. The Palaeotherium was an animal like the tapir, now an inhabitant of the warmer regions of Asia and America. Recent discoveries in Eocene strata in Egypt show stages of development between a tapir-like animal and the elephant with long trunk and tusks. There were in those days hog-like animals intermediate between the hogs and the hippopotami. There were ancestors of the horse with three toes on each foot. There were hornless ancestors of the deer and antelopes. Many of the early mammals showed characters now found in the marsupials, the order to which the Kangaroo and Opossum belong, members of which are found in rocks of the Secondary Era, and are the only representatives of the mammalia in that age. Some of the early Eocene mammalia are either marsupials, or closely related to them. In the Oligocene we find the mammalian life becoming more varied, and branching out into the various groups we know to-day; while the succeeding Miocene Period witnesses the culmination of the mammalia--mammals of every family abounding all over the earth's surface, in a profusion and variety not seen before--or since, outside the tropics.

Chapter X

BEFORE AND AFTER.--THE ICE AGE.

We have read the story written in the rocks of the Isle of Wight. What wonderful changes we have seen in the course of the long history!

First we were taken back to the ancient Wealden river, and saw in imagination the great continent through which it flowed, and the strange creatures that lived in the old land. We saw the delta sink beneath the sea, and a great thickness of shallow water deposits laid down, enclosing remains of ammonites and other beautiful forms of life. Then long ages pa.s.sed away, while in the waters of a deeper sea the great thickness of the chalk was built up, mainly by the acc.u.mulation of microscopic sh.e.l.ls. In time the sea bed rose, and new land appeared, and another river bore down fruits to be buried with sea sh.e.l.ls and remains of turtles and crocodiles in the mud deposited near its mouth to form the London clay. We followed the alternations of sea and land, and the changing life of Eocene and Oligocene times.

We have heard of the early mammalia found in the quarries of Quarr, and have learnt from the leaf beds of Alum Bay that at that time the climate of this part of the world was tropical. Indeed, I think everything goes to prove that through the whole of the times we have been studying,--except perhaps the earliest Eocene, that of the Reading beds,--the climate was considerably warmer than it is at the present day. After all these changes do you not want to know what happened next? Well, at this point we come to a gap in the records of the rocks, not only in the Isle of Wight, but also in the British Isles. The British Isles, or even England and Wales alone, are almost, if not quite unique in the world in that, in their small extent, they contain specimens of nearly every formation from the most ancient times to the present day. In other parts of the world we may find regions many times this area, where we can only study the rocks of some one period. But just at this point in the story comes a period,--a very important one, too,--the Miocene--of which we have no remains in our Islands. We must hear a little of what happened before we come back to the Isle of Wight again in comparatively recent times.

But, first, perhaps, I had better tell,--just in outline,--something of the earlier history of the world, before any of our Isle of Wight rocks were made. For, if I do not, quite a wrong idea may be formed of the world's history. The time of the Wealden river has seemed to us very ancient. We cannot say how many hundreds of thousands, or rather millions of years have pa.s.sed since that ancient Wealden age. And you may have thought that we had got back then very near the world's birthday, and were looking at some of the oldest rocks on the globe.

But no. We are not near the beginning yet. Compared with the vast ages that went before, our Wealden period is almost modern. We cannot tell with any certainty the comparative time; but we may compare the thickness of strata formed to give us some sort of idea. Now to the first strata in which fossil remains of living things are found we have in all a thickness of strata some 12 times that of all the rocks we have been studying from Wealden to Oligocene, together with the later rocks, Miocene and Pliocene, not found in the Isle of Wight. And before that there is, perhaps, an equal thickness of sedimentary deposits; though the fossils they, no doubt, once contained have been destroyed by changes the rocks have undergone.

Now let me try to give you some idea of the world's history up to the point where we began in the Isle of Wight. If we could see back through the ages to the furthest past of geological history, we should see our world,--before any of the stratified rocks were laid down in the seas,--before the seas themselves were made,--a hot globe, molten at least at the surface. How do we know this? Because under the rocks of all the world's surface we find there is granite or some similar rock,--a rock which shows by its composition that it has crystallised from a molten condition. Moreover we have seen that the interior of the earth is intensely hot. And yet all along the earth must be radiating off heat into the cold depths of s.p.a.ce, and cooling like any other hot body surrounded by s.p.a.ce cooler than itself. And this has gone on for untold ages. Far enough back we must come to a time when the earth was red hot,--white hot. In imagination we see it cooling,--the molten ma.s.s solidifies into Igneous rock,--the clouds of steam in which the globe is wrapped condense in oceans upon the surface. The bands of crystalline rock that rise above the primeval seas are gradually worn down by rain and rivers and waves, and the first sedimentary deposits laid down in the waters. And in the waters and on the land life appeared for the first time,--we know not how.

A vast thickness of stratified rocks was formed, which are called Archaean ("ancient"). They represent a time, perhaps, as great as all that has followed. These rocks have undergone great changes since their formation. They have been pressed under ma.s.ses of overlying strata, and have come into the neighbourhood of the heated interior of the earth; they have been burnt and baked and compressed and folded, and acted on by heated water and steam, and their whole structure altered by heat and chemical action. Limestones, _e.g._, have become marble, with a crystalline structure which has obliterated any fossils they may have once contained. Yet it is probable that, like nearly all later limestones, they are of organic origin. These Archaean rocks cover a large extent of country in Canada. We have some of them in our Islands, in the Hebrides, and north-west of Scotland and in Anglesey, and rising from beneath later rocks in the Malvern Hills and Charnwood Forest.[12]

The Archaean rocks are succeeded by the most ancient fossiliferous rocks, the great series called the Cambrian, because found, and first studied, in Wales. They consist of very hard rocks, and contain large quant.i.ties of slate. They are followed by another series called the Ordovician; and that by another the Silurian. These three great systems of rocks measure in all some 30,000 ft. of strata. They form the hills of Wales and the English Lake District. They contain large ma.s.ses of volcanic rocks. We can see where were the necks of old volcanoes, and the sheets of lava which flowed from them. The volcanoes are worn down to their bases now; and the hills of Wales and the Lakes represent the remains of ancient mountain chains, which rose high like the Alps in days of old, long before Alps or Himalayas began to be made. These ancient rocks contain abundant remains of living things, chiefly mollusca, crustaceans, corals, and other marine organisms, showing that the waters of those ages abounded with life.

We must pa.s.s on. Next comes a period called the Devonian, or Old Red Sandstone, when the Old Red rocks of Devon and Scotland were laid down. These contain remains of many varieties of very remarkable fish.

A long period of coral seas succeeded, when coral reefs flourished over what was to be England; and their remains formed the Carboniferous Limestone of Derbyshire and the Mendip Hills. A period followed of immense duration, when over pretty well the whole earth there seem to have been comparatively low lands covered with a luxuriant and very strange vegetation. The remains of these ancient forests have formed the coal measures, which tell of the most widespread and longest enduring growth of vegetation the world has seen. Strange as some of the plants were--gigantic horsetails and club-mosses growing into trees--many were exquisitely beautiful. There were no flowering plants, but the ferns, many of them tree ferns, were of as delicate beauty as those of the present day. Many of the ferns bore seeds, and were not reproduced by spores, such as we see on the fronds of our present ferns. That is a wonderful story of plant history, which has only been read in recent years.

After the long Carboniferous period came to an end followed periods in which great formations of red sandstone were made,--the Permian, and the New Red Sandstone or Trias. During much of this time the condition of the country seems to have resembled that of the Steppes of Central Asia, or even the great desert of Sahara--great dry sandy deserts--hills of bare rock with screes of broken fragments heaped up at their base,--salt inland lakes, depositing, as the effect of intense evaporation, the beds of rock salt we find in Cheshire or elsewhere, in the same manner as is taking place to-day in the Caspian Sea, in the salt lakes of the northern edge of the Sahara, and in the Great Salt Lake of Utah.

At the close of the period the land here sank beneath the sea--again a sea of coral islands like the South Pacific of to-day. There were many oscillations of level, or changes of currents; and bands of clay, when mud from the land was laid down, alternate with beds of limestone formed in the clearer coral seas. These strata form a period known as the Jura.s.sic, from the large development of the rocks in the Jura mountains. In England the period includes the Lia.s.sic and Oolitic epochs. The Lia.s.sic strata stretch across England from Lyme Regis in Dorset to Whitby in Yorkshire. Most of the strata we are describing run across England from south-west to north-east. After they were laid down a movement of elevation, connected with the movement which raised the Alps in Europe, took place along the lines of the Welsh and Scotch mountains and the chain of Scandinavia, which raised the various strata, and left them dipping to the south-east. Worn down by denudation the edges are now exposed in lines running south-west to north-east, while the strata dip south-east under the edges of the more recent strata. The Lias is noted for its ammonites, and especially for its great marine reptiles, Ichthyosaurus and Plesiosaurus. The Oolitic Epoch follows--a long period during which the fine limestone, the Bath freestone, was made; the limestones of the Cotswolds, beds of clay known as the Oxford and Kimmeridge clays; and again coral reefs left the rock known as coral rag. In the later part of the period were formed the Portland and Purbeck beds, marine and freshwater limestones, which contain also an old land surface, which has left silicified trunks of trees and stems of cycads.

And now following on these came our Wealden strata, the beginning of the Cretaceous period. You see what ages and ages had gone before, and that when Wealden times came, far back as they are, the world's history was comparatively approaching modern times. We must remember that all these formations, of which we have given a rapid sketch, are of great thickness,--thousands of feet of rock,--and represent vast ages of time. See what we have got to from looking at the sh.e.l.ls in the sea cliff! We have come to learn something of the world's old history. We have been carried back through ages that pa.s.s our imagination to the world's beginning, to the time of the molten globe, before ever it was cool enough to allow life--we know not how--to begin upon its surface. And Astronomy will take us back into an even more distant past, and show us a nebulous mist of vast extent stretching out into s.p.a.ce like the nebulae observed in the heavens to-day, before sun and planets and moons were yet formed. So we are carried into the infinite of time and s.p.a.ce, and questions arise beyond the power of human mind to solve.

Now we have, I hope, a better idea of the position the strata we have been specially studying occupy in the geological history, and shall understand the relation the strata we may find elsewhere bear to those in the Isle of Wight and the neighbouring south of England.

After this sketch of what went before our Island story, we must see what followed at the end of the Oligocene period. We said that there are no strata in the British Isles representing the next period, the Miocene. But it was a period of great importance in the world's history. Great stratified deposits were laid down in France and Switzerland and elsewhere, and it was a great age of mountain building. The Alps and the Himalaya, largely composed of Cretaceous and Eocene rocks, were upheaved into great mountain ranges. It is probable that during much of the period the British Isles were dry land, and that great denudation of the land took place. But in the first part of the period at all events this part of the world must have been under water, and strata have been laid down, which have since been denuded away. For our soft Oligocene strata, if exposed to rain and river action during the long Miocene period and the time which followed, would surely have been entirely swept away. The Miocene was succeeded by the Pliocene, when the strata called the Crag, which cover the surface of Norfolk and Suffolk, were formed.

They are marine deposits with sea sh.e.l.ls, of which a considerable proportion of species still survive.

We have seen that through the ages we have been studying the climate was mostly warmer than at the present day. The climate of the Eocene was tropical. The Miocene was sub-tropical and becoming cooler. Palms become rarer in the Upper strata. Evergreens, which form three-fourths of the flora in the Lower Miocene, divide the flora with deciduous trees in the Upper. And through the Pliocene the climate, though still warmer than now, was steadily becoming cooler; till in the beginning of the next period, the Pleistocene, it had become considerably colder than that of the present day. And then followed a time which is known as the great Ice Age, or the Glacial Period,--a time which has left its traces all over this country, and, indeed all over Northern Europe and America, and even into southern lands. The cold increased, heavy snowfalls piled up snow on the mountains of Wales, the Lake District, and Scotland; and the snow remained, and did not melt, and more fell and pressed the lower snow into ice, which flowed down the valleys in glaciers, as in Switzerland to-day. Gradually all the vegetation of temperate lands disappeared, till only the dwarf Arctic birch and Arctic willows were to be seen. The sea sh.e.l.ls of temperate climates were replaced by northern species. Animals of warm and temperate climates wandered south, and the Arctic fox, and the Norwegian lemming, and the musk ox which now lives in the far north of America took their place; and the mammoth, an extinct elephant fitted by a thick coat of hair and wool for living in cold countries, and a woolly-haired rhinoceros, and other animals of arctic regions occupied the land. When the cold was greatest, the glaciers met and formed an ice-sheet; and Scotland, northern England and the Midlands, Wales, and Ireland were buried in one vast sheet of ice as Greenland is to-day.

How do we know this? To tell how the story has been read would be to tell one of the most interesting stories of geology. Here we can only give the briefest sketch of this wonderful chapter of the world's history. But we must know a little of how the story has been made out.

We have already seen that the changes in plant and animal life point to a change from a hot climate, through a temperate, at last to arctic cold. Again, over the greater part of Northern England the rocks of the various geological periods are buried under sheets of tough clay, called boulder clay, for it is studded with boulders large and small, like raisins in a plum pudding. No flowing water forms such a deposit, but it is found to be just like the ma.s.s of clay with stones under the great glaciers and ice sheets of arctic regions; and just such a boulder clay may be seen extending from the lower end of glaciers in Spitzbergen, when the glacier has temporarily retreated in a succession of warm summers. The stones in our boulder clay are polished and scratched in a way glaciers are known to polish and scratch the stones they carry along, and rub against the rocks and other stones. The rock over which the glacier moves is similarly scratched and polished, and just such scratching and polishing is found on the rocks in Wales and the Lake District. Again, we find rocks carried over hill and dale and right across valleys, it may be half across England. We can trace for great distances the lines of fragments of some peculiar rock, as the granite of Shap in Westmorland; and even rocks from Norway have been carried across the North Sea, and left in East Anglia. This will just give an idea how we know of this strange chapter in the history of our land. For, by this time it was our land--England--much as we know it to-day; though at times the whole stood higher above sea level, so that the beds of the Channel and the North Sea were dry land. But, apart from variation of level, the geography was in the main as now.

[Ill.u.s.tration: FIG. 9]

SHINGLE AT FORELAND

Bm _Bembridge Marls._ S _Shingle._ b _Brick Earth._ Cf _Old Cliff in Marls._

[Ill.u.s.tration: FIG. 5]

DIAGRAM OF STRATA BETWEEN SOUTHERN DOWNS AND ST. GEORGE'S DOWN.

Dotted Lines _Former extension of Strata._ Broken Line _Former Bed of Valley sloping to St. George's Down._

The ice sheet did not come further south than the Thames valley. What was the country like south of this? Well, you must think of the land just outside the ice sheet in Greenland, or other arctic country. No doubt the winters must have been very severe,--hard frosts and heavy snows,--the ground frozen deep. Some arctic animals would manage to live as they do now just outside the ice sheet in Greenland. Now, have we any deposits formed at that time in the Isle of Wight? I think we have. A large part of the surface of the Island is covered by sheets of flint gravel. The gravels differ in age and mode of formation. We have already considered the angular gravels of the Chalk downs, composed of flints which have acc.u.mulated as the chalk which once contained them was dissolved away. But there are other gravel beds, which consist of flints which, after they were set free by the dissolution of the chalk, have been carried down to a lower level by rivers or other agency, and more or less rounded in the process. Many of these beds occur at a high level; and, as they usually cap flat-topped hills, they are known as Plateau Gravels. Perhaps the most remarkable is the immense sheet of gravel which covers the flat top of St. George's Down between Arreton and Newport. Gravel pits show upwards of 30 feet of gravel, consisting of flints with some chert and ironstone, and the greatest thickness is probably considerably more than this. The southern edge of the sheet is cut off straight like a wall. To the north it runs out on ridges between combes which have cut into it. In places in the ma.s.s of flints occur beds of sand, which have all the appearance of having been laid down by currents of water.

The base of the gravel where it is seen on the steep southern slope of the down has been cemented by water containing iron into a solid conglomerate rock. The flints forming this gravel have not simply sunk down from chalk strata dissolved away; for they lie on the upturned edges of strata from Lower Greensand to Upper Chalk, which have been planed off, and worn into a surface sloping gently to the north; and over this surface the gravel has somehow flowed. The sharp wall in which it ends at the upper part of the slope shows that it once extended to the south over ground since worn away. Clearly, the gravel was formed before denudation had cut out the great gap between the central and southern downs of the Island. The down where the gravel lies is 363 ft. above sea level, 313 ft. above the bottom of the valley below. So that, though the gravel sheet is much newer than the strata we have been studying, it must nevertheless be of great antiquity.

It seems that at the top of St. George's Down we are standing on what was once the floor of an old valley. In the course of denudation the bottom of a river valley often becomes the highest part of a district.

For the bed of the valley is covered by flint gravel, and flint is excessively hard, and the bed of flints protects the underlying rock; so that, while the rocks on each side are worn away, what was the river bed is eventually left high above them. Thus the highest points of a district are often capped by flint gravel marking the beds of old streams. Tracing up this old valley to the southward, at a few miles distance it will have reached the chalk region on the south of the anticline: and the flints carried down the valley may have come from beds of angular flints already dissolved out of the chalk such as we find on St. Boniface Down.

But how have these great ma.s.ses of flints been swept along? Can the land have been down under the sea; and have sea waves washed the stones along? But these flints, though water-worn, are not rounded as we find beach shingle. What immense rush of water can have spread these flints 30 feet deep along a river valley? We must go to mountain regions for torrents of this character. And then, mountain torrents round the stones in their bed while these are mostly angular. The history of these gravels is a difficult one. I can only give what seems to me the most probable explanation. It appears to me probable that in the Ice Age, south of the ice sheet, the ground must have been both broken up by frosts, and also held together by being frozen hard to some depth. Then when thaws came in the short but warm summers, or when an intermission of the severe cold took place, great floods would flow down the valleys in the country south of the ice sheet, and ma.s.ses of ice with frozen earth and stones would be borne along in a sort of semi-liquid flow. In this way Mr. Clement Reid explains the ma.s.s of broken-up chalk with large stones found on the heads of cliffs on the South coast, and known by the name of "combe-rock" or "head."

The Ice Age was not one simple period, and it is still difficult to fit together the history we read in different places, and in particular to correlate the gravels of the south of England with the boulder clays of the glaciated area. There were certainly breaks in the period, during which the climate became much milder, or even warm; and these were long enough for southern species of animals and plants to migrate northward, and occupy the lands where an arctic climate had prevailed. There were moreover considerable variations in the relative level of land and sea. So that we have a very complex history, which is gradually coming into clearer light.

That the gravels of the south of England belong largely to the age of ice, is shown by remains of the mammoth contained in many. These, however, are found in later gravels than those we have considered so far, gravels laid down after the land had been cut down to much lower levels. These lower gravels are known as Valley gravels, because they lie along the course of existing valleys, the Plateau gravels having been laid down before the present valleys came into existence. Teeth of the mammoth are found in the Thames valley, and on the sh.o.r.es of Southampton Water, in gravels about 50 to 70 feet above sea level, and have been found also in the Isle of Wight at Freshwater Gate, at the top of the cliffs near Brook, and in other places. The gravels near Brook with the clays on which they rest have been contorted, and the gravel forced into pockets in the clay, in a manner that suggests the action of grounding ice ploughing into the soil.

The high level gravels must belong to an early stage of the Glacial Epoch. We get some idea of the great length of time this age must have lasted, as we look from St. George's Down over the lower country of the centre of the Island. After the formation of the St. George's Down gravel the vast ma.s.s of strata between this and the opposite downs of St. Boniface and St. Catherine's was removed by denudation; and gravels were then laid down on the lower land, along Blake Down, at Arreton, over Hale common, and along the course of the Yar. Patches of gravel occur on the Sandown and Shanklin cliffs. At Little Stairs a gravel, largely of angular chert, reaches a thickness of 12 feet, and in parts are several feet of loam above gravel.

At the west of the Island a great sheet of gravel covers the top of Headon Hill, reaching a height of 390 feet. It appears sometimes to measure 30 feet in thickness. Like that on St. George's Down it slopes towards the Solent, resting on an eroded surface, in this case of Tertiary strata; and here too the upper part of the sheet has been removed by the wearing out of the deep valley between the Hill and the Freshwater Downs. The sheet lies on an old valley bottom, which sloped from the chalk downs on the south, then much higher and more extensive than now. Here too we may see something of the length of the Glacial Period. For at Freshwater Gate is a much later gravel, in which teeth of the mammoth have been found. It was probably derived from older gravels that once lay to the south, as the flints are rounded by transport. But the formation of all these gravels appears to belong to the Glacial Period; and as we stand in Freshwater Gate, and look at this great gap in the downs worn out by the Western Yar, and think of the time when a river valley pa.s.sed over the tops of the High Downs and Headon Hill, we receive a strong impression of the length of the great Ice Age.

Now surely the question will be asked, what caused these changes of climate in the world's past history--so that at times a tropical vegetation spread over this land, and vegetation flourished sufficient to leave beds of coal within the Arctic circle, and in the Antarctic continent, and at another the climate of Greenland came down to England, and an ice sheet covered nearly the whole country? This still remains one of the difficult problems of Geology. An explanation has been attempted by Astronomical Theory, according to which the varying eccentricity of the earth's...o...b..t--that is to say a slight change in the elliptic orbit of the Earth, by which at times it becomes less nearly circular--a change which is known to take place--may have had the effect of producing these variations of climatic conditions. The theory is very alluring, for if this be the cause, we can calculate mathematically the date and duration of the Glacial Period. But, unfortunately, supposing the astronomical phenomena to have the effect required, the course of events given by the astronomical theory would be entirely different to that revealed by geological research.

Geographical explanations have usually failed through being of too local a character to explain a phenomenon which affected the whole northern hemisphere, and the effects of which reached at least as far south as the Equator,[13] and are seen again in the southern hemisphere in Australia, New Zealand, and South America. It is now believed that great world-movements take place, due to the contraction by cooling of the Earth's interior, and the adjustment of the crust to the shrinkage.[14] Possibly some explanation might be found in these world-wide movements; but their effect seems to last through too long periods of time to suit our Ice Ages. Again, while the geographical distribution of animals and plants in the present and past seems to imply very great changes in the land ma.s.ses and oceanic areas,[15]

these changes appear to bear no relation to glacial epochs. The cause of the Ice Ages remains at present an unsolved problem. More than one Ice Age has occurred during the long geological history. The marks of such a period are found in Archaean rocks, in the Cambrian, when glaciers flowed down to the sea level in China and South Australia within a few degrees of the tropics, and above all in early Permian times. The Dwyka conglomerate of the Karroo formation of South Africa (deposits of Permo-Carboniferous age) show evidence of extensive glaciation; deposits of the same age in Northern and Central India, even within the tropics, a glacial series of great thickness in Australia, and deposits in Brazil, appear to show a glaciation greater than that of the recent glacial period. Yet these epochs formed only episodes in the great geological eras. On the whole the climate throughout geological time would seem to have been warmer than at the present day. It may, perhaps, be doubted whether the earth has yet recovered what we may call its _normal_ temperature since the Glacial Epoch.

Note on Astronomical Theory.--If the Ice Age be due to the increased eccentricity of the Earth's...o...b..t, the theory shows that a long duration of normal temperature will be followed by a group of Glacial Periods alternating between the northern and southern hemispheres, the time elapsing between the culmination of such a period in one hemisphere and in the other being about 10,500 years. While one hemisphere is in a glacial period, the other will be enjoying a specially mild,--a "genial" period. Now, according to the record of the rocks, the "genial" periods were far from being those breaks in the Glacial which we know as Inter-glacial periods. We have the immensely long warm period of the Eocene and Oligocene, the Miocene with a still warm but reduced temperature, and then the gradual cooling during the Pliocene, till the drop in temperature culminates in the Ice Age. Moreover, the duration of each glaciation during this Ice Age is usually considered to have been much longer than the 10,000 years or so given by the Astronomical Theory. Add to this that the periods of high eccentricity of the Earth's...o...b..t, though occurring at irregular intervals, are, on the scale of geological time, pretty frequent; so that several of such periods would have occurred during the Eocene alone. Yet the geological evidence shows unbroken sub-tropical conditions in this part of the world throughout the Eocene.

[Footnote 12: The older division of the Archaean rocks--the Lewisian gneisse--consists entirely of metamorphic and igneous rocks; a later division--the Torridonian sandstones--is comparatively little altered, but still unfossiliferous.]

[Footnote 13: The great equatorial mountains Kilimanjaro and Ruwenzori show signs of a former extension of glaciers.]

[Footnote 14: For an account of such movements, see Prof.