LAND SCULPTURE BY MOUNTAIN GLACIERS 371 



the base of the marginal crevasse. The same process does not go 

 on as rapidly above the surface of the neVe" for the reason that the 

 necessary wetting of the rock surface does not there so generally result 

 from the daily summer thaw. 

 At the bottom of the marginal 

 crevasse alone is this condition 

 fully realized. Intensive frost 

 action where the rock is wet with 

 thaw water daily is thus a 

 fundamental cause, both of the 

 hollowing of the early drift site 

 to form the niche, and of the 

 later enlargement of this niche 

 into an amphitheater or cirque 

 when the drift has been trans- 

 formed into the ne"ve of a 



Scale. 



FIG. 393. Niches and cirques in the same 

 vicinity in the Bighorn Mountains of 

 Wyoming. A, A, unmodified valleys; 

 B, B, niches on drift sites ; C, C, cirques 

 on small glacier sites (after map by 

 F. E. Mathes, U. S. G. S.). 



glacier. Inasmuch as the cre- 

 vasse forms where the snow and 

 ice pull away from the rock 

 toward the middle of the depression, the cirque wall in its early 

 stage has the outline of a semicircle. In the Bighorn Mountains 

 of Wyoming, all stages, from the unmodified valley heads to the 



full-formed cirque, may be seen near 

 one another (Fig. 393). It will be 

 noted that wherever a glacier has 

 formed, as indicated by the cirque, 

 there is a series of lakes which have 

 developed in the valley below (see 

 p. 412). 



Life history of the cirque. In its 

 earliest stage the cirque is more or 

 less uniformly supplied with snow 

 from all sides, and so it enlarges by 

 recession in a manner to retain its 

 early semicircular outline. In a later 

 FIG. 394. Subordinate small cir- stage a larger proportion of the snow 



ques in the amphitheater on the rea ches the cirque at its sides SO that 

 west face of the Wannehorn ../.., 



above the Great Aletsch Glacier ** f urther enlargement causes it to 

 of Switzerland. broaden and to flatten somewhat that 



