146 



Sierra Club Bulletin 



This brings us to that part of the investigation, the aim 

 of which was to determine the amount of erosional work per- 

 formed by the ice. On a previous page, it will be remem- 

 bered, the statement was made that the sculpture of the Yo- 

 semite Valley is erratic and, as an index to ice erosion, most 

 deceiving. The explanation lies, for the Yosemite as it does 

 for the Hetch Hetchy, in the structural characteristics of the 

 country rock. The interpretation of these characteristics given 

 in the recent booklet on the origin of the Yosemite Valley, 

 published by the Department of the Interior, the writer is 

 happy to state, was verified and confirmed by the structural 

 studies of last summer. As was pointed out in that booklet, 

 the granites of the Yosemite region are peculiar in that they 

 are not everywhere traversed by natural partings or "joints." 

 Large masses of them, some half a mile or more in extent, 

 have remained undivided, absolutely solid, although in other 

 places the same rock may be fissured at intervals of only a few 

 feet. That these extreme variations in structure must have 

 greatly affected the eroding efficiency of the ice, will readily 

 be understood. While the glaciers might work to advantage 

 in rocks divided into small joint blocks, they were relatively 

 powerless when dealing with massive granite. The latter 

 they could reduce only by superficial abrasion, a slow process 

 the efficiency of which has been much overestimated. As a 

 result, the ice-carved topography of the Yosemite country is 

 a singularly varied one, well developed, typical glacial forms 

 alternating with others not suggestive of ice work at all. 



On the whole, it may be said that this phase of the Yo- 

 semite problem has remained the most misunderstood. Former 

 observers have not always made sufficient allowance for the 

 exceptional nature of the granites of the Sierra. Perhaps 

 they had in mind the work of glaciers in regions of more 

 normally jointed rocks and thus were misled in assigning too 

 great erosional achievements to the ice streams of the Yo- 

 semite region. The main lessons learned last summer in this 

 regard are that except in certain restricted localities, such as 

 the Yosemite Valley proper, the rock character precluded ex- 

 tensive remodeling by the ice, and that as a consequence 

 the landscape, although bearing the unmistakable stamp of ice 



