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Sierra Club Bulletin 



reached. Fig. 3 is a rock about two miles west of Lake Tenaya, 

 with a train of boulders derived from it. The boulders are 

 scattered along a level ridge, where they have not been dis- 

 turbed in any appreciable degree since they came to rest toward 

 the close of the glacial period. An examination of the rock 

 proves conclusively that not only were these blocks — many of 

 which are twelve feet in diameter — derived from it, but that 

 they were torn off its side by the direct mechanical action of 

 the glacier that swept over and past it. For had they simply 

 fallen upon the surface of the glacier from above, then the rock 

 would present a crumbling, ruinous condition — which it does 

 not — and a talus of similar blocks would have accumulated at 

 its base after there was no glacier to remove them as they fell; 

 but no such talus exists, the rock remaining compact, as if it 

 had scarcely felt the touch of a single storm. Yet, what count- 

 less seasons of weathering, combined with earthquake violence, 

 could not accomplish, was done by the Tenaya Glacier, as it 

 swept past on its way to Yosemite. 



A still more striking and instructive example of side-rock 

 erosion may be found about a mile north of Lake Tenaya. 

 Here the glaciated pavements are more perfectly preserved 

 than elsewhere in the Merced basin. Upon them I found a 

 train of granite blocks, which attracted my attention from their 

 isolated position, and the uniformity of their mechanical char- 

 acters. Their angles were unworn, indicating that their source 

 could not be far off. It proved to be on the side of one of the 

 lofty elongated ridges stretching toward the Big Tuolumne 

 Meadows. They had been quarried from the base of the ridge, 

 which is ice-polished and undecayed to the summit. The rea- 

 son that only this particular portion of the ridge afforded 

 blocks of this kind, and so abundantly as to be readily trace- 

 able, is that the cleavage planes here separated the rock into 

 parallelopipeds which sloped forward obHquely into the side of 

 the glacier, which was thus enabled to grasp them and strip 

 them off, just as the spikelets of an ear of wheat are stripped 

 off by running the fingers down from the top toward the base. 

 An instance where the structure has an exactly opposite effect 

 upon the erodibility of the side of a rock is given in Fig. 4, 

 where the cleavage planes separate it into slabs which overlap 



