Geoi-.— Vol. I.] TURNER— ORIGIN OF VOSERIITE VALLEY. 265 



Lake Tahoe to Sierra Valley, and all that portion of this 

 valley from the north end of Lake Tahoe to Sierra Valley is 

 filled with Tertiary lavas, and north of Boca there are Neo- 

 cene lake beds, so that Lake Tahoe may be said to be 

 a dammed-iip canyon or valley. Lindgren also found evi- 

 dence^ that the Tertiary crest-line of the Sierra Nevada was 

 approximately in the same place that it now is in the Lake 

 Tahoe region, and that the Neocene rivers studied by him 

 headed near where the corresponding modern rivers now 

 begin, in a region of lofty peaks and ridges. Moreover, 

 the Tertiary lavas that came out of volcanoes along the 

 crest of the range in this region flowed both east and west, 

 also indicating that the crest-line at the time of these erup- 

 tions was approximately where it now is. These facts, 

 taken in connection with the apparent existence of a fault 

 basin at Lake Tahoe before the time of the lava flows, seem 

 clearly to point to faulting in this region in early Miocene 

 time. 



Where this fault was examined by myself in the vicinity 

 of Mohawk Valley, the evidence tends to show that there 

 the faulting was after the period of the deposition of the 

 Neocene gravels. On the edge of the plateau west of 

 Mohawk Valley there are river gravels of Tertiary age 

 lying on the brink of the fault-scarp. It is evident that the 

 river which deposited these gravels could not have existed 

 after the fault-scarp formed. These gravels are quite the 

 same as other river gravels, to which Professor Knowlton 

 has assigned an upper Miocene age on the basis of the 

 fossil flora; hence, the faulting here would seem to be 

 post-Miocene. Nevertheless, in Mohawk Valley, which 

 was formed by the subsidence of the area to the east 

 of the fault-scarp, there are Neocene lake-beds, showing 

 the existence of a basin there in Tertiary time; and these 

 beds show no signs of disturbance, lying horizontally at all 

 points, so far as noted, except along a recent fault-line, 

 which lies on the east side of the valley, and not on the 

 west, where the older faulting occurred. If these Neocene 



1 Bull. Geol. Soc. Amer. Vol. IV, 1893, pp. 257-298. 



