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Sierra Club Bulletin 



and middle regions have not been denuded the one-hundredth 

 part of an inch. Farther down measuring tablets abound bear- 

 ing the signature of the ice. The amount of torrential and av- 

 alanchial denudation is also certainly estimated within narrow 

 limits by measuring down from the unchanged glaciated sur- 

 faces lining their banks. Farther down the range, where the 

 polished surfaces disappear, we may still reach a fair approxi- 

 mation by the height of pot-holes drilled into the walls of 

 gorges, and by the forms of the bottoms of the valleys contain- 

 ing these gorges, and by the shape and condition of the general 

 features. 



Summing up these results, we find that the average quantity 

 of post-glacial denudation in the upper half of the range, em- 

 bracing a zone twenty-five or thirty miles wide, probably does 

 not exceed a depth of three inches. That of the lower half has 

 evidently been much greater — probably several feet — but cer- 

 tainly not so much as radically to alter any of its main fea- 

 tures. In that portion of the range where* the depth of glacial 

 denudation exceeds a mile, that of post-glacial denudation is 

 less than a foot. 



From its warm base to its cold summit, the physiognomy of 

 the Sierra is still strictly glacial. Rivers have only traced shal- 

 low wrinkles, avalanches have made scars, and winds and 

 rains have blurred it, but the change, as a whole, is not greater 

 than that effected on a human countenance by a single year of 

 exposure to common alpine storms. 



*See study No, IV, in Sierra Club Bulletin o£ January, 1918. 



