358 



University of California . 



[Vol.. 3. 



and as recently discussed by Matthes* in his account of the 

 Glacial Sulpture of the Bighorn Mountains. This discussion 

 of glacial sculpture in the high mountains has been concerned 

 chiefly with the modus operandi of the glaciers in effecting the 

 recession of cirque cliffs. That question is not yet completely 

 settled, and its status can be improved by more extended obser- 

 vations, and particularly by comparison of the petrographic and 

 structural control of this erosional process. Cirques appear to 

 be best known in granitic mountains, and it has been suggested 

 that the homogeniety of these rocks favors their development. 

 The peculiar jointage which usually characterizes these granitic 

 rocks seems to the writer not to have been sufficiently emphasized 

 as an important condition favoring cirque erosion; and he is per- 

 suaded from his observations on the rim of the Upper Kern Basin 

 that this is one of the chief factors in the problem. As has been 

 shown in another part of this paper, the intensity of the jointage in 

 these high granitic mountains appears to be in part a function of 

 unequal relief of load by degradation. The jointage, as developed 

 by relief of elastic stress, is always in advance of the actual ero- 

 sional surface, but is in part related to that surface as regards 

 orientation. The jointage is much more abundantly developed 

 in the higher levels of the mountain masses than in the lower. 



Since the cirques and canons of the region have been vacated 

 by the ice there has been a measurable recession of their walls, 

 chiefly by the shedding of blocks bounded by joint planes. The 

 evidence of this consists in the talus at the base of the walls. If 

 an agency were present competent to remove this talus as it fell, 

 and so prevent its accumulation, the recession would be practi- 

 cally as rapid at the base of the cliffs as in their upper part, above 

 the talus slope; and we would have just such a recession of cliffs 

 as has given us these remarkable theatre-like indentations in the 

 mountain slopes. Now the blocks which make up these talus 

 accumulations have been dislodged by the freezing of water in 

 the joint cracks. This process was undoubtedly an active one in 

 the depths of the bergschrund, probably many times more active 

 than under present condition, and seems to the writer to be an 

 adequate explanation of a very large part, at least, of the method 



*U.S.Gt.S. 21st Aim. Rpt. 1899-1900, Pt. II, p. 1<!7 et seq. 



