36 H. L. FAIRCHILD — ICE EROSION THEORY A FALLACY 



" Ice seems to have played a considerable part in clearing the canyons of frag- 

 ments and in excavating shattered and decomposed patches, so that in a sense one 

 must ascribe a large erosive effect to the glaciers; but the ice seems, nevertheless, 

 to have been incapable of cutting into solid masses to any extent, or even into 

 much fissured rock where little decomposition had preceded and where the blocks 

 were tightly wedged together. 



" In many cases the glaciers have polished rock surfaces, the contours of which 

 are so thoroughly characteristic of surface exfoliation, due to weathering, that no 

 observer could doubt their character, and some of the surfaces are such that ice 

 could not possibly have modeled them. Such evidence, together with that derived 

 from the occurrence of glaciated lavas near the bottoms of the present canyons, in- 

 dicates very clearly that the present system of canyons was established long before 

 glaciation began, and probably during the warm and no doubt very wet Pliocene 

 epoch." (Page 68.) 



From study of the canyons in the region about lake Mono, Russell * 

 concluded that they were not the product of ice erosion, and Gilbert f 

 agrees with all the other experts that the present drainage features of the 

 Sierras were far advanced before the ice occupation. 



The important theoretical paper on glacier mechanics, already quoted 

 (page 29) as adverse to extreme erosion, published by McGee in 1894, was 

 an outcome of his work in the Sierras. 



In 1900 a critical and important paper was published by Turner J on 

 the south central Sierras. In this paper he gives an excellent review 

 and discussion of the problem of ice erosion in application to the Sierras, 

 and Yosemite in particular, and proves conclusively that the canyons, 

 including Yosemite, were not made by ice, although he recognizes slight 

 glacial action in valleys where it had not previously been noted. 



Regarding rock basins, Turner states that such are abundant in the 

 glaciated Sierras, and ordinarily in granite, and says : 



" Many of these rock basins are at the base of steep slopes, and it is possible in 

 such cases that the great downward pressure of the ice excavated such basins, 

 especially where the rock was much jointed. The location of other rock basins, 

 however, although always where the ice mass appears to have been very thick at 

 one time, is such as not to suggest the probability of there having been sufficient 

 w r eight of ice to scoop out a basin in hard rock. . . . It is more than likely that 

 in many such cases the basin was originally produced by unequal disintegration of 

 the granite in preglacial time, this decomposed material being subsequently ex- 

 cavated by the ice." (Pages 286, 287. ) 



Some of the rock basin lakes are more than a mile long and toward 

 100 feet deep. Lakes Washburn, Tenaya, and Johnson are mentioned. 



*I. C. Russell: "Quaternary history of Mono valley." Eighth Ann. Rep. U. S. Geol. Survey, 

 pt. i, 1889, p. 352. 



|G. K. Gilbert: "Lake Bonneville." Monograph i, U. S. GqoI. Survey, 1890. 



JH. W. Turner: "The Pleistocene geology of the south central Sierra Nevada, with especial 

 reference to the origin of Yosemite valley." Proc. Calif. Acad. Sci., iii ser., vol. i, no. 9, 1900, pp. 

 261-321. 



