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 mass 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 passed 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 orbit—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 masses 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 Archæan 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 orbit, 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 orbit, 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.
[12] The older division of the Archæan rocks—the Lewisian gneisse—consists entirely of metamorphic and igneous rocks; a later division—the Torridonian sandstones—is comparatively little altered, but still unfossiliferous.
[13] The great equatorial mountains Kilimanjaro and Ruwenzori show signs of a former extension of glaciers.
[14] For an account of such movements, see Prof. Gregory's Making of the Earth in the Home University Library.
[15] See The Wanderings of Animals. By H. Gadow, F.R.S., Cambridge Manuals.
Chapter XI