122 STUDIES, SCIENTIFIC AND SOCIAL chap. 



Another objection almost equally beside the real ques- 

 tion 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 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 forcibly at the side than in the 

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

 old river, where the valley Avas 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 complete non seqtiitur. 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 vertically, both on account of the smaller 

 pressure because it would mould 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 therefore 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 stationar}^ 



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

 that the thickness of the ice has been usually underestim- 

 ated. 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 



^ Falsan, La Periode Glaciaire, p. 153. Fabre, Oriyine des Lacs 

 Aljjhiis, p. 4. 



