Page:Popular Science Monthly Volume 45.djvu/267

Rh and then consider the only alternative theory that has obtained the acceptance of modern writers.

One of the first objections made was, that the lake did not lie in the direction of the greatest action of the glacier, which was straight across to the Jura where the highest erratic blocks are found. This was urged by Sir Charles Lyell, immediately after Ramsay's paper was read, and as it has quite recently been put forth by Prof. Bonney, it would appear to be thought to be a real difficulty. Yet a little consideration will show that it has not the slightest weight. No lake was eroded in the line of motion of the central and highest part of the old glacier, because that line was over an elevated and hilly plateau, which is even now from five hundred to a thousand feet above the lake, and was then even higher, since the ice-sheet certainly effected some erosion. The greatest amount of erosion was of course in the broad and nearly level valley of the pre-glacial Rhone, which followed the great curve of the existing lake, and had produced so open a valley because the rocks in that direction to were easily denuded. Objectors invariably forget or overlook the indisputable fact that the existence of a broad, open, flat-bottomed valley in any part of a river's course proves that the rocks were there either softer or more friable, or more soluble, or by some combination of characters more easily denuded. A number of favorable conditions were combined to render ice erosion easy in such a valley. The rock was, as we have shown, more easy to erode; owing to the low level the ice was thicker and had greater weight there than elsewhere; cowing to the flatness and openness of the valley the ice moved more freely there; owing to the long previous course of the glacier its under surface would be heavily loaded with rock and grit, which during its whole course would, by mere gravitation, have been slowly working its way downward to the lowest level; and, lastly, all the subglacial torrents would accumulate in this lowest valley, and, as erosion went on, would, under great hydrostatic pressure, wash away all the ground-out material, and so facilitate erosion. To ask why the lake was formed in the valley, where everything favored erosion, rather than on the plateau, where everything was against it, is to make mere verbal objections which have no relation to the conditions that actually existed.

Another objection almost equally beside the real question is to ask why the deepest part of the lake is near the south or convex side, whereas a stream of water always exerts most erosive force against the concave side. The answer is, that ice is not water, and that it moves so slowly as to act, in many respects, in quite a