THE ICE AGE AND ITS WORK. 253 



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 thai direction to ere easily denuded. Object- 

 ors 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 char- 

 acters 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 



* Falsan, La Periode Gladaire, p. 153, Fabre, Origine des Lacs Alpins, p. 4. 



