We can realize, in our imagination, the glacier of the mountain-valley crushing and marking the bed in which it moves, or even the plain on which it discharges itself; but it is impossible to conceive of a glacier upon the bare top of a mountain, without walls to restrain it or direct its flow, or higher ice accumulations to feed it.
“If glaciers descended, as they did, on both sides of the great Alpine ranges, then we would expect to find the same results on the plains of Northern Italy that present themselves on the low grounds of Switzerland. But this is not the case. On the plains of Italy there are no traces of the stony clay found in Switzerland and all over Europe. Neither are any of the stones of the drift of Italy scratched or striated."
[1. “The Great Ice Age,” p. 73.
2. Ibid., pp. 491, 492.]
But, strange to say, while, as Geikie admits, no true “till” or Drift is now being formed by or under the glaciers of Switzerland, nevertheless “till” is found in that country disassociated from the glaciers. Geikie says:
“In the low grounds of Switzerland we get a dark, tough clay, packed with scratched and well-rubbed stones, and containing here and there some admixture of sand and irregular beds and patches of earthy gravel. This clay is quite unstratified, and the strata upon which it rests frequently exhibit much confusion, being turned up on end and bent over, exactly as in this country the rocks are sometimes broken and disturbed below till. The whole deposit has experienced much denudation, but even yet it covers considerable areas, and attains a thickness varying from a few feet up to not less than thirty feet in thickness."
Here, then, are the objections to this theory of the glacier-origin of the Drift:
I. The glaciers do not produce striated stones.
II. The glaciers do not produce drift-clay.
III. The glaciers could not have formed continental sheets of “till.”
IV. The glaciers could not have existed upon, and consequently could not have striated, the mountain-tops.
V. The glaciers could not have reached to the great plains of the continents far remote from valleys, where we still find the Drift and drift-markings.
VI. The glaciers are limited in number and confined in their operations, and were utterly inadequate to have produced the thousands of square miles of drift-débris which we find enfolding the world.
[1. “The Great Ice Age,” p. 373.]
WAS IT CAUSED BY CONTINENTAL ICE-SHEETS?
WE, come now to the theory which is at present most generally accepted: