The Story of the Earth and Man

by Sir John William Dawson

At the present day the coasts of Britain and other parts of Western Europe enjoy an exceptionally warm temperature, owing to the warm currents of the Atlantic being thrown on them, and the warm and moist Atlantic air flowing over them, under the influence of the prevailing westerly winds. These advantages are not possessed by the eastern coast of North America, nor by some deep channels in the sea, along which the cold northern currents flow under the warmer water. Hence these last-mentioned localities are inhabited by boreal shells much farther south than such species extend on the coasts and banks of Great Britain. In the Glacial period this exceptional advantage was lost, and while the American seas, as judged by their marine animals, were somewhat colder than at present, the British seas were proportionally much more cooled down. No doubt, however, there were warmer and colder areas, determined by depth and prevailing currents, and as these changed their position in elevation and subsidence of the land, alternations and even mixtures of the inhabitants of cold and warm water resulted, which have often been very puzzling to geologists.

I have taken the series of drift deposits seen in Britain and in Canada as typical, and the previous discussion has had reference to them. But it would be unfair not to inform the reader that this succession of deposits after all belongs to the margins of our continents rather than to their great central areas. This is the case at least in North America, where in the region of the great lakes the oldest glaciated surfaces are overlaid by thick beds of stratified clay, without marine fossils, and often without either stones or boulders, though these sometimes occur, especially toward the north. The clay, however, contains drifted fragments of coniferous trees. Above this clay are sand and gravel, and the principal deposit of travelled stones and boulders rests on these. I cannot affirm that a similar succession occurs on the great inland plains of Europe and Asia: but I think it probable that to some extent it does. The explanation of this inland drift by the advocates of a great continental glacier is as follows: (1) In the Pliocene period the continents were higher than at present, and many deep valleys, since filled up, were cut in them. (2) In the Post-pliocene these elevated continents became covered with ice, by the movement of which the valleys were deepened and the surfaces striated. (3) This ice-period was followed by a depression and submergence, in which the clays were deposited, filling up old channels, and much changing the levels of the land. Lastly, as the land rose again from this submergence, sand and gravel were deposited, and boulders scattered over the surface by floating ice.

The advocates of floating ice as distinguished from a continental glacier, merely dispense with the latter, and affirm that the striation under the clay, as well as that connected with the later boulders, is the effect of floating bergs. The occurrence of so much drift wood in the clay favours their view, as it is more likely that there would be islands clothed with trees in the sea, than that these should exist immediately after the country had been mantled in ice. The want of marine shells is a difficulty in either view, but may be accounted for by the rapid deposition of the clay and the slow spreading of marine animals over a submerged continent under unfavourable conditions of climate.

In any case the reader will please observe that theorists must account for both the interior and marginal forms of these deposits. Let us tabulate the facts and the modes of accounting for them.

FACTS OBSERVED.		THEORETICAL VIEWS.
Inland Plains.	Marginal Areas.	Glacial Theories.	Floating Ice Theories.
Terraces.	Terraces and Raised Beaches.	Emergence of Modern Land.[AJ]
Travelled Boulders and Glaciated Stones and Rocks. Stratified Sand and Gravel.	Sand and Gravel, with Sea Shells and Boulders.	Shallow Sea and Floating Ice.
Stratified Clay with Drift Wood, and a few Stones and Boulders. Striated Rocks.	Stratified Clay with Sea Shells. Boulder Clay with or without Sea Shells. Striated Rocks.	Deep Sea and Floating Ice.
		Submergence of the land. Great continental mantle of Ice.	Much floating Ice and local Glaciers. Submergence of Pliocene Land.
Old channels, indicating a higher level of the land.	Old channels, etc., indicating previous dry land.	Erosion by continental Glacier.	Erosion by atmospheric agencies and accumulation of decomposed rock.

[AJ] The phenomena of this period, with reference to rainfall, melting snows, and valley deposits, must be noticed in the next chapter.

This table will suffice at least to reduce the great glacier controversy to its narrowest limits, when we have added the one further consideration that glaciers are the parents of icebergs, and that the question is not of one or the other exclusively, but of the relative predominance of the one or the other in certain given times and places. Both theories admit a great Post-pliocene subsidence. The abettors of glaciers can urge the elevation of the surface, the supposed powers of glaciers as eroding agents, and the transport of boulders. Those whose theoretical views lean to floating ice, believe that they can equally account for these phenomena, and can urge in support of their theory the occurrence of drift wood in the inland clay and boulder clay, and of sea-shells in the marginal clay and boulder clay, and the atmospheric decomposition of rock in the Pliocene period, as a source of the material of the clays, while to similar causes they can attribute the erosion of the deep valleys piled with the Post-pliocene deposits. They can also maintain that the general direction of striation and drift implies the action of sea currents, while they appeal to local glaciers to account for special cases of glaciated rocks at the higher levels.