290 THE ICE AGE AND ITS WORK. 



2. The conditions that favor the production of lal-es by ice erosion. — 

 Those who oppose the production of lake basins by ice erosion often 

 argue as if the size of the glacier was the only factor, and urge that 

 because there are no lake basins in one valley where large glaciers 

 have been at work, those which exist in another valley where the 

 glaciers were no larger, could not have been produced by them. But 

 this by no ineans follows, because the production of a lake basin depends 

 on a combination of favorable conditions. In the tirst place it is evi 

 dent that ice erosion to some extent must have taken pla<'e along the 

 whole length of the glacier's course, and that in many cases the result 

 might be simply to deepen the valley all along, not quite equally, perhaps, 

 but with no such extreme differences as to produce a lake basin. This 

 would especially be the case if a valley had a considerable downward 

 slope, and was not very unequal in width or in the nature of the rocks 

 forming its floor. The first essential to lake erosion is, therefore, a 

 differential action, caused locally either by increased thickness of the 

 ice, a more o])en and level valley floor, or a more easily eroded rock, or 

 by any combination of these. - - - 



It must always be remembered that glacial erosion is produced by 

 the tremendous vertical pressure of the ice, by its lower strata.being 

 tliickly loaded with hard rocks frozen into its mass, and by its slow 

 but continuous motion. In the lower part of its course a glacier would 

 be most charged with rocky debris in its under strata, since not only 

 would it have been continually breaking ofl' and absorbing', as it were, 

 fresh material during every mile of its onward course, but more and 

 more of its superficial moraines would be engulfed by crevasses or 

 moulins, and be added to the grinding material below\ That this was 

 so is proved by the great quantity of stones and grit in the "till," 

 which is thought by Prof. James Geikie to consist, on the average, of 

 as much stony matter as clay, sometimes one material i)re])onderating, 

 sometimes the other. The same thing is indicated by the enormous 

 amount of debris often found on tlie lower parts of large glaciers. 

 The end of the great Tasman glacier in New Zealand is thus com- 

 pletely hidden for 5 miles, and most of the other glaciers descending 

 from Mount Cook have their extremities similarly buried in debris. 

 Dr. Diener found the Milam glacier in the Central Himalayas com- 

 l)letely covered with moraine rubbish; and Mr. W. M. Conway states 

 that the lowest 20 miles of the Hispar glacier (40 miles long) are 

 "entirely covered with a mantle of moraine." If these glaciers 

 extended to over 100 miles long, as did the Rhone glacier when it 

 reached the lake of Geneva, much of this debris would probably ha\e 

 found its way to the bottom, and thus sup])ly the necessary grinding 

 material and the abundant stones of the "till" found everywhere in 

 the trades of the old glaciers. 



Again, althongh ice is viscous and can slowly change its sliape to 

 almost any extent, yet it takes a considerable time to adapt itself to 



