Page:Popular Science Monthly Volume 45.djvu/57

Rh favorable conditions. In the first place it is evident that ice erosion to some extent must have taken place along the whole length of the glacier's course, and that in many cases the result might be simply to deepen the valley all along, not quite equally, perhaps, but with no such extreme differences as to produce a lake basin. This would especially be the case if a valley had a considerable downward slope, and was not very unequal in width or in the nature of the rocks forming its floor. The first essential to lake erosion is, therefore, a differential action, caused locally either by increased thickness of the ice, a more open and level valley floor, or a more easily eroded rock, or by any combination of these.

If we look at the valley lakes of our own country and of Switzerland, the first thing that strikes us is their great length and their situation, usually at the lower end of the valley where it emerges from the higher mountains into comparatively low country. Windermere is over ten miles long, Ullswater nearly eight miles, and the larger lakes of Switzerland and North Italy are very much longer. The first essential condition, therefore, was a valley the lower part of which was already nearly level for several miles, and with a considerable width to the base of the mountain slopes. In the non-glaciated districts of our own country, the Dart and the Tamar are examples of rivers which have cut their valleys down nearly to sea-level while still among the hills; and in South Wales the Wye, the Usk, and the Severn have a similar character.

It must always be remembered that glacial erosion is produced by the tremendous vertical pressure of the ice, by its lower strata being thickly loaded with hard rocks frozen into its mass, and by its slow but continuous motion. In the lower part of its course a glacier would be most charged with rocky débris in its under strata, since not only would it have been continually breaking off and absorbing, as it were, fresh material during every mile of its onward course, but more and more of its superficial moraines would be ingulfed by crevasses or moulins, and be added to the grinding material below. That this was so is proved by the great quantity of stones and grit in the "till," which is thought by Prof. James Geikie to consist, on the average, of as much stony matter as clay, sometimes one material preponderating, sometimes the other. The same thing is indicated by the enormous amount of débris often found on the lower parts of large glaciers. The end of the great Tasman Glacier in New Zealand is thus completely hidden for five miles and most of the other glaciers descending from Mount Cook have their extremities similarly buried in débris. Dr. Diener found the Milam Glacier in the central Himalayas completely covered with moraine rubbish; and Mr. W. M. Conway states that the lowest twenty miles of the