V GLACIAL EROSION OF LAKE BASIN8 121 



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 Professor 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. The lake was not 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 hccause 

 the rod's in that dircctio7i vjere easily deimided. 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 com- 

 bination of characters more easily denuded. A number 

 of favourable 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 ; 

 owing 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 low^est level ; and, lastly, all the 

 sub-glacial torrents would accumulate in this lowest valley, 

 and, as erosion went on, would, perhaps under 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 favoured 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. 



1 See Map at p. 128. 



