GLACIAL FEATURES 289 



relation of the first two unequal factors to the third scarcely need 

 be pointed out. 



We have then an inequality in amount of accumulated material 

 to be explained by either an inequality in the extent of the snow 

 and therefore an inequality of snow action, or an inequality due 

 to the presence of ice in one valley and not in the other, or by 

 both. It is at once clear that if ice is absent above (A) and the 

 mountain slopes are recessed that snow action is responsible for 

 it. It is also recognized that whatever rate of denudation be as- 

 signed to the snow-free surfaces this rate must be exceeded by 

 the rate of snow action, else the inequalities of slope would be 

 decreased rather than increased. The accumulated material at 

 (A) is, therefore, partly but not chiefly due to denudation of snow- 

 free surfaces. It is due chiefly to erosion beneath the snow. Nor 

 can it be argued that the hollows now occupied by snow were 

 formed at some past time when ice not snow lay in them. They 

 are not ice-made hollows for they are on a steep spur above the 

 limits of ice action even in the glacial period. Any past action is, 

 therefore, represented here in kind by present action, though there 

 would be differences in degree because the heavier snows of the 

 past were displaced by the lighter snows of today. 



While it appears that the case presents clear proof of degrada- 

 tion by snow it is not so clear how these results were accomplished. 

 Eeal abrasion on a large scale as in bowlder-shod glaciers is 

 ruled out, since glacial striae are wholly absent from nivated sur- 

 faces according to both Matthes' observations and my own. Yet 

 all nivated surfaces have very distinctive qualities, delicately or- 

 ganized slopes which show a marked change from any original 

 condition related to water-carving. In the absence of striae, the 

 general absence of all but a thin coating of waste even in rock hol- 

 lows, and the accumulation of waste up to bowlders in size at the 

 lower edge of the nivated zone, I conclude that compacted snow 

 or neve of sufficient thickness and gradient may actually pluck 

 rock outcrops in the same manner though not at the rate which 

 ice exhibits. That the products of nivation may be bowlders as 

 well as fine mud would seem clearly to follow increase in effective- 



