The Geological Story of the Isle of Wight

by J. Cecil Hughes

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 Palæotherium 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 passed away, while in the waters of a deeper sea the great thickness of the chalk was built up, mainly by the accumulation of microscopic shells. In time the sea bed rose, and new land appeared, and another river bore down fruits to be buried with sea shells 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 passed 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 space, and cooling like any other hot body surrounded by space 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 mass 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 Archæan ("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 masses 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 Archæan 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 Archæan 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 quantities 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 masses 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 pass 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.