Lawson.] 



The Upper Kern Basin. 



359 



whereby the material from the face of the cliffs was fed to the ice 

 and so removed. In the absence of such joiutage the tendency 

 of the glacier to sap the cliffs would fail of realization, and merely 

 shallow troughs would be evolved instead of cirques. If this 

 view be correct it would discredit the hypothesis of glacial sapping 

 as an explanation of such massive and little jointed granite walls 

 as those of El Capitan in Yosemite. The writer recognizes fully 

 the glaciation of Yosemite Valley and the glacial sculpture of its 

 upper part, but the main walls of the valley appear to have an 

 origin independent of and antecedent to its occupancy by ice. 



A more important phase of the cirque question, however, as 

 it appears to the writer, and one which is fully appreciated by 

 Johnson, but which has not received the recognition which it 

 merits, as an important morphogenic process, is the efficacy of 

 cirque erosion to reduce mountain summits. "Where cirques 

 abound they usually occur on both sides of a mountain crest and 

 have encroached upon the latter from opposite directions. The 

 walls of opposing cirques eventually intersect, and the reduction 

 of the mountain crest, which is thus inaugurated, proceeds rapidly 

 till it may be lowered a thousand feet or more. The writer can- 

 not do better than quote Johnson's graphic picture of the pro- 

 cess. He says:* 



"By recession of the amphitheatre head — and the glacier 

 makes the amphitheatre rather than merely occupies it — the 

 amphitheatral wall is cai'ried backward, and divides are cut 

 through. A summit region, upon either slope of which glacial 

 streams are extended, will be trenched by streams heading thus 

 in opposition. A first effect of the meeting of an opposing pair 

 will be the arete, or thin comb, the most evanescent of mountain 

 forms, the final effect will be the col, a low, level pass between 

 the walls. The ultimate result of continued glaciation must be 

 truncation of the crest region, close to the lower level of the 

 glacial generation." 



The principles here laid down are splendidly exemplified in the 

 summit region of the Sierra Nevada as seen from the top of Mt. 

 Whitney and in the approaches to it. The sea of sharp crests seen 

 looking south and southwest from this point, (Plate 32 a), are 



*Loc. Cit. p. 113. 



