Geol.— Vol. I.] TURNER— ORIGIN OF YOSEMITE VALLEY. 299 



shallower river valleys, and found in the topography what 

 he considered good evidence that there was more than one 

 period of the ice extension. 



RusselP has compared the steps formed in some of the 

 glaciated canyons to a cyclopean stairway. These steps 

 have formed by erosion along horizontal joint planes, but 

 Professor Van Hise does not regard them as the result of 

 river erosion, giving as an example the fine steps at the 

 Nevada and Vernal falls, and Russell and Johnson think 

 that the steps are allied to cirques in origin. However, as 

 heretofore stated, I have seen exactly such steps, on a 

 smaller scale, formed in stream beds with a high grade, 

 where the rocks are intersected by horizontal joints, far 

 below the limit of glaciation; for example, in some of the 

 ravines on the west slope of the Chowchilla Mountains, 

 where they are cut out of Carboniferous slates and quartz- 

 ites, and in the bed of Avalanche Creek where it tumbles 

 down into the Merced, the steps here being cut out of a 

 granitic rock. 



The presumption that some of the benches and gentle 

 slopes to be noted at the present time in the higher Sierra 

 Nevada are remnants of a gentle surface of Tertiary time 

 does not rest altogether on hypothetical grounds. One very 

 marked bench, indeed, is certainl}^ a portion of the bottom 

 of the valley of the Neocene Tuolumne River. This bench 

 lies on the north side of the Grand Canyon of the Tuol- 

 umne, east of Rodger's Canyon, at an elevation of about 

 8,000 feet. Where this old river valley is cut by Rodger's 

 Canyon, portions of the old surface are perfectly preserved 

 underneath volcanic material (andesite-breccia) of Tertiary 

 age. Farther west, the old valley lies more to the north of 

 the Tuolumne Canyon and hence no longer forms a bench. 

 Where cut through by Piute Creek, this Neocene valley is 

 filled with river gravel and volcanic material, and it next 

 appears exposed to view in Deep Canyon, as a certain por- 

 tion of Rancheria Creek basin is called. This canyon is 

 practically the Neocene valley of the Tuolumne, from which 



1 Eighth Ann. Rep. U. S. Geol. Surv., Part I, 1889, p. 348. 



