148 



University of California Publications. 



[Geology 



Evidence of like import is found associated with the growth 

 of Slide Mountain. The main fault underlying Little Valley 

 extends northward to the limits of the map, but has been dis- 

 placed to the east on the east flank of Slide Mountain. The east 

 ridge here has been badly broken and much displaced, yet is 

 plainly present. The main outline of Washoe Valley must have 

 been established at this time, for the preceding characteristics 

 must surely have been effected by such a great movement, and 

 there exist no traces of later movements sufficiently large to have 

 been able to form so important an element of the physiography. 



The last and slight motion along the east-west fault-planes 

 already discussed has probably caused the warping in Washoe 

 Valley, and the low divide east of Slide Mountain, by movements 

 along the same line and resultant compression of the alluvium 

 of the valley. The contemporary movement along the east-west 

 fault bounding Washoe Valley on the south is probably respon- 

 sible in part for the tilting of the valley to the eastward. Later 

 movements up to the present, the resultant of a complex series of 

 faults, have continued the tilting of Washoe Valley, as well as 

 Eagle and Carson valleys. This establishes four distinct periods 

 of faulting, not counting the later and lesser movements that 

 have produced and are almost certainly still producing slight 

 physiographic change. But these do not cover all the important 

 and visible effects due to movements within the rocks. In the 

 Franktown topographic area there are two groups of such move- 

 ments not yet placed in time. One of these groups is connected 

 with the structure and genesis of Little Valley, and will be dis- 

 cussed under that head ; the other group is concerned with the 

 boundaries of Washoe Valley. 



Washoe Valley is roughly rectangular, and the southwest cor- 

 ner is sharply a right angle ; the southeast corner is more rounded, 

 due in part to rhyolite Hows. The main valley projects in a tri- 

 angular shaped area south to Lakeview, separating the Washoe 

 Mountains of the Virginia Range from the Sierra Nevada. The 

 south boundary of the main valley is therefore a rampart of hills 

 broken near the middle. The two portions of this rampart are 

 hypsometric-ally discordant, the western group standing roughly 

 500 feet higher, using the plateau remnant as a datum plane. 



