360 THE GREAT ICE AGE 



glaciers great power of cutting out and deepening their valleys. 

 But this is evidently an error, just as it would be an error to 

 suppose the flour of a grist mill ground out of the mill stones. 

 Glaciers, it is true, groove and striate and polish the rocks over 

 which they move, and especially those of projecting points and 

 slight elevations in their beds ; but the material which they 

 grind up is principally derived from the. exposed frost-bitten 

 rocks above them, and the rocky floor under the glacier is 

 merely the nether mill stone against which those loose stones 

 are crushed. The glaciers, in short, can scarcely be regarded 

 as cutting agents at all, in so far as the sides and bottoms of 

 their beds are concerned, and in the valleys which the old 

 glaciers have abandoned, it is evident that the torrents which 

 have succeeded them have far greater cutting power. 



The glaciers have their'periods of advance and of recession. 

 A series of wet and cool summers causes them to advance and 

 encroach on the plains, pushing before them their moraines, 

 and even forests and human habitations. In dry and warm 

 summers they shrink and recede. Such changes seem to have 

 occurred in bygone times on a gigantic scale. All the valleys 

 below the present glaciers present traces of former glacier 

 action. Even the Jura mountains seem at one time to have 

 had glaciers. Large blocks from the Alps have been carried 

 across the intervening valley and lodged at great heights on the 

 slopes of the Jura, leading the majority of the Swiss and 

 Italian geologists to believe that even this great valley and the 

 basin of Lake Leman were once filled with glacier ice. But, 

 unless we can suppose that the Alps were then vastly higher 

 than at present, this seems scarcely to be physically possible, 

 and it seems more likely that the conditions were just the 

 reverse of those supposed, namely, that the low land was sub- 

 merged, and that the valley of Lake Leman was a strait like 

 Belle-Isle, traversed by powerful currents and receiving ice- 

 bergs from both Jurassic and Alpine glaciers, and probably 



