CERTAIN PHASES OF GLACIAL EROSION 209 



Now the regional precipitation is much the same for like areas 

 and like situations in the glaciated and in the non-glaciated val- 

 leys. Such differences as there may be appear to favor a greater 

 run-off in the glaciated than in the non-glaciated basins, for the 

 former are likely to be cooler and hence better condensers and the 

 concentration of snow by wind action is there more effective. If so, 

 the advantage in absolute carrying power lies with the waters of 

 the glaciated valleys. If therefore the passing of a part of the 

 precipitation through the glacial form and the moving of this 

 part, so far as it moves as ice, is protective, the debris should tend 

 to remain in the upper glaciated sections thus protected; while 

 the waters of the valley below the glacier having some excess in 

 volume and having less burden to carry should tend to degrade the 

 lower reaches of the valley more effectively than if the glacier were 

 absent. That the facts are precisely the opposite seems good 

 additional evidence that the glacial form of water compared with 

 the aqueous form increases notably the corrasion and the trans- 

 porting power. And so it seems to us that the fringing outwash 

 aprons and the thick-headed valley trains of the Pleistocene 

 join with the aggraded states of the lower stretches of present 

 glacial valleys and with their turbid glacial streams, their mouton- 

 need walls, and their glacial scorings to testify to the superior 

 erosive efficiency of glaciers. 



Traced back analytically to the properties that gave rise to 

 them, these corrasive products point to a glacier's power to take 

 firm hold on rock fragments imbedded in its base and sides and 

 move them on while it uses them as graving tools. A special 

 feature that is of interest here is the ice's habit of freezing to 

 fragments in contact with it, especially when moisture is present. 

 Ice also strengthens its adhesions by a tendency to freeze and thus 

 to attach additional ice at points where tension is developed. 

 This amounts to an inherent tendency to strengthen its hold when 

 threatened with severance. 



IV. THE EVOLUTION OF THE CIRQUE 



In a young glacier-head just coming into action as the result 

 of the growth of its stresses as a snow mass, it is inevitable that 



