146 



PHYSICAL GEOLOGY 



The development of cirques is apparently not necessarily limited to the heads of 

 former stream valleys, although this is generally the case, but they may have their origin 

 in a somewhat different way. If the drifts on a mountain slope last year after year until 



late in the spring, it will be 

 found that their edges are 

 usually bordered by fine 

 soil which is slowly being 

 removed by water and 

 deposited in deltas at the 

 lower margins of the drifts. 

 This fine soil is the result 

 of the alternate freezings 

 and thawings of the water 

 in the cracks and pores 

 of the rock, which is thus 

 finally broken up into 

 small fragments. In this 

 way, by nivation, a niche, 

 the beginning of a cirque, 

 may be formed on a moun- 

 tain slope. 



Development of 

 Cirques. — A high re- 

 gion which has been 

 partly cut into cirques 

 " resembles nothing 

 so much as a layer 

 of dough from which 

 biscuit have been 

 cut." As the amphi- 

 theaters or cirques on 

 the two sides of a 

 mountain ridge en- 

 large, they finally en- 

 croach on each other, 

 first forming a nar- 



Fig. 129. — A shows mountain valleys formed by 

 stream erosion. Ii shows the same valleys after they 

 have been occupied and strongly eroded by glaciers. The 



main valley has become U-shaped, and the side valleys row > ra gg e u> COmb- 

 have become hanging valleys with strongly developed like ridge (Fig. 129 

 cirques. An attempt has been made to show the prob- a d\ a „j somewhat 

 able approximate deepening, in feet, by glacial erosion. ' . 



later, as the separat- 

 ing walls of tin- cirques are partially quarried away, producing tooth- 

 like peaks. 



Fate of Cirques. — If changes in climate cause a glacier gradually 



