were unable to obliterate the effects of the older and greater powers. The roches moutonnées owe their peculiar form to the grinding of ice certainly, but they were blocked out anterior to the formation of the glaciers. They were, when the ice quitted them, to what they were before the glaciers began to work, very much like what an old worn coin is to one that is newly struck. The hollows were not so much affected, but the eminences were ground down; the depressions of the modelling remained, but the parts in relief were taken away. It requires, therefore, some little effort to imagine what the rock forms were like before the glaciers of the glacial period began to operate upon them, but we cannot be wrong in assuming that the forms were similar to those exhibited by weathered rocks at the present time.
§ 2. Glacier ice is plastic, and can be moulded by pressure to almost any form. Hence, if a glacier could remain perfectly stationary, it would be moulded, by means of its own weight, to the surface upon which it reposed. But glaciers move, and consequently the bottom of one is never completely moulded to its rock-bed. The pressure from the weight of the ice is opposed by the motion of the glacier, and the ice is urged past depressions before it can be moulded to them.
For example, let Fig. 1 of the diagram on p. 144 represent a section of a portion of the bottom of a glacier which is beginning to work upon weathered rocks; g, g, indicating the glacier, and the arrow the direction in which the glacier is moving. The ice, after passing the eminences a, b, c, does not completely fill the hollows d, e, f.[1]
These things can be observed at the sides of most considerable glaciers, and particularly well at several places on each bank of the Gorner glacier. At several places (such as at d in Fig. 1) one can get underneath and see the ice bridging hollows; and notice proof of its motion, and that it is partially moulded to the
- ↑ The outline is a tracing from a photograph of weathered, tinglaciated rocks.
