THE ICE AGE AND ITS WORK. 297 



Tlierock 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 roi'k 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 sub-glacial tor- 

 rents would accumulate in this lowest valley, and as erosion went on 

 would, under great hydrostatic pressure, wash away all tlie ground-out 

 material, and so facilitate erosion. 



Another objection almost equally beside the real question is to ask 

 why the deepest i^art 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 different manner. Its 

 greatest action is where it is deepest, in the middle of the ice stream, 

 while water acts least where it is deepest, and more forcibl^^ at the side 

 than in the middle. The lake is no doubt deepest in the line of the 

 old river, where tlie valley was lowest; and that may well have been 

 nearer the southern than the northern side of the lake. 



Another frequently-urged objection is, that as the glacier has not 

 widened the narrow valley from Martigny to Bex it could not have 

 eroded a lake nearly a thousand feet deep. This seems to me a com- 

 l)lete non sequitur. As a glacier erodes mainly by its vertical pressure 

 and by the completeness of its grinding armature of rock, it is clear that 

 its grinding power laterally must have been very much less than verti- 

 cally, both on account of the smaller pressure because it would mold 

 itself less closely to the ever-varying rocky protuberances, and mainly, 

 perhaps, because at the almost vertical sides of the valley it would 

 have a very small stony armature, the blocks continually working their 

 way downward to the bottom. Thus, much of the ice in contact with 

 the sides of narrow ravines might be free of stones, and would there 

 fore exert hardly any grinding power. It is also quite certain that the 

 ice in this narrow valley rose to an enormous height, and that the 

 chief motion and also the chief erosion would be on the lateral slopes? 

 while the lower strata, wedged in the gorge, would be almost station- 

 ary. 



The most recent researches, according to M. Falsan, show that the 

 thickness of the ice has been usually under-estimated. A terminal 

 moraine on the Jura at Chasseron is 4,000 feet above the sea, or 2,770 

 feet above Geneva. In order that the upper surface of the ice should 

 have had suthcient incline to How onward as it did it was probably 

 5,000 or 6,000 feet thick below Martigny and 4,000 or 5,000 feet over 

 the middle of the lake. It is certain, at all events, that whatever 



* Falsan, La Piriodc Glaciaire, p. 153. Fabie, Orujine dts Lacs Alpius, p. 4. 



