Geol.-Vol. I.] TURNER-ORIGIN OF YOSEMITE VALLEY. 315 



that east of Royal Arch Lake. The Royal Arches them- 

 selves in Yosemite Valley are perhaps due to the same pro- 

 cess, but here the scales are several feet in thickness. 



Half Dome has been called one of the most remarkable 

 rocks in the range from a scenic aspect. It is popularly 

 supposed to represent the half of an enormous dome, the 

 other half of which fell off at some past time. According 

 to Muir, along the base of the vertical portion the stumps of 

 vertical slabs may be seen, which have been successively 

 split off of the vertical face while the rock was being 

 shaped into its present form. As already noted under the 

 section on joints, the shoulders that appear to represent por- 

 tions of this sheeted zone described by Muir can be seen 

 to be cut by highly inchned or vertical joints. The more 

 simple hypothesis, that the vertical north face of Half 

 Dome has resulted from the scaHng off of the vertically 

 jointed granite, and that the residual portion has a dome- 

 form as a result of the exfoliation of an unjointed mass, 

 seems to satisfactorily explain its origin. 



I J. For7natio7i of the Valley Floor. 



If faulting should occur across a canyon and the mass 

 on the down-stream side of the fault be heaved up, it is 

 clear that a barrier would be formed, resulting in the clog- 

 ging of the drainage and ultimate silting up of the basin, 

 up-stream from the barrier. A valley with flat bottom 

 would thus result. Such a theory might be applied to the 

 Yosemite to account for the formation of the valley floor. 

 However, we have already seen that the small terminal mo- 

 raines near the west end of the valley probably constitute 

 sufficient barriers to cause the deposit of the sand and 

 gravel forming the valley bottom, providing this deposit is 

 not of great depth. 



At the west end of the valley the rock is in place on the 

 side of the gorge and this suggests that the rock-bottom 

 of the valley may not be far below the top of the allu- 

 vium; unless we suppose that there was faulting as above 



