368 EARTH FEATURES AND THEIR MEANING 



tively smooth, this will be found in most cases upon the lee of the 

 mountain crest. 



In normal cases at least the inherited irregularities of the higher 

 zones of mountain upland are the gentle depressions which develop 

 at the heads of streams. These become, then, the sites of snow- 

 drifts that are augmented in size from year to year, though at 

 first they melt away in the late summer. 



The niches which form on snowdrift sites. Wherever a drift 

 is formed, a process is set in operation, the effect of which is to 

 hollow out and lower the ground beneath it, a process which has 

 been called nivation. The drift shown in Fig. 390 was photo- 

 graphed in late summer at an elevation of some 9000 feet in the 

 Yellowstone National Park. The very gently sloping surface 



FIG. 390. Snowdrift hollowing its bed by nivation and building a delta (at the 

 left). Quadrant Mountain, Yellowstone National Park. 



surrounding the drift is covered with grass, but within a zone a 

 few feet in width on the borders of the drift no grass is growing, 

 and in its place is found a fine brown soil which is fast becoming 

 the prey of the moving water derived by melting of the drift. 

 This is explained by the water permeating the crevices of the rock 

 and being rent by the nightly freezing. Farther from the drift 

 the ground is dry, and no such action is possible. With each suc- 

 ceeding spring the augmented drift as it melts carries all finely 

 comminuted rock material down slopes beneath the snow to emerge 

 at the lowest margin and be there deposited in the form of a delta. 

 By the operation of this process of nivation the higher parts of the 



