CERTAIN PHASES OF GLACIAL EROSION 207 



continued sapping and quarrying by glaciers eating their way 

 backward much as they have in the Alpine cases cited. 



Fig. 9, from the San Juan Mountains of southwestern Colorado, 

 is a striking example of a much more capacious summit cirque. 

 The basin here is very broad, with a nearly level floor, while the 

 cirque rim has been undercut till only a narrow horseshoe-shaped 

 serrate ridge remains. Its jagged crest varies in altitude from 

 13,000 to 13,200 feet, while the mean elevation of the broad floor 

 is about 12,800 feet. The breadth of the basin and the steepness 

 and thinness of the amphitheater walls that skirt it show that 

 this type of action has here gone about as far as it well could as a 

 single stoping operation. The whole constitutes a signal case at 

 its very climax. 



In the light of these illustrations, particularly Figs. 8 and 

 9, there seems no ground to doubt that the erosion suffered by 

 the non-glaciated parts in such situations at least falls greatly 

 behind that suffered by the parts covered by ice. 



III. THE DISTINCTIVE WORKING FACTOR 



The decided superiority of moving ice over moving water as 

 an erosive agent lies chiefly, we think, in the rigid hold of the ice 

 on matter set in its base or sides. This is sharply contrasted 

 with the adhesion of water which is so feeble as to scarcely warrant 

 the term "hold" at all. Water action finds some compensation, 

 indeed, in the higher velocity it usually gives to the matter it 

 carries, but near the point of origin of action — and this is the 

 location chiefly under discussion here — the water is so distributed 

 as not to be able to acquire much efficiency from concentration. 

 The ice mass, on the contrary, is rigidly unified; its velocity is 

 indeed low, but its mass and fixed coherence are high. It is in 

 respect to this coherence that exaggerated views of the viscousness 

 of ice are perhaps most misleading. Rigidity of grasp and mechani- 

 cal firmness of action are specifically implied in the groovings, 

 gougings, and crushings that distinguish the action of glaciers. 

 Effectiveness of corrasive action is further implied in the chemical 

 and physical nature of the rock-flour and fragments which these 

 groovings, grindings, and crushings contribute to the glacial till 



