Vol. (3 J Be id: The Geomorphogeny of the Sierra Nevada. 143 



down-dropped crust blocks, or graben. This mode of origin, 

 which can hardly be questioned, is rendered the more certain by 

 the presence of prominent fault blocks within the valley limits, 

 quite similar to the blocks present in the floor of north Little 

 Valley. Also, the real similarity of Washoe Lake and valley to 

 that of Marlett Lake, whose origin is so evident, has a bearing 

 on the inquiry. Were Washoe Lake to fill its valley, as it did 

 originally, the three bodies of water, Tahoe, Washoe, and Mar- 

 lett, would differ only in size. 



Lastly, literally as a connecting link, is the presence of the 

 AVashoe Mountains opposite the much faulted Carson topographic 

 area of the Sierra Nevada, traversed by the same main east-west 

 fault lines and composed of the same rocks. 



RELATIVE AGES OF THE FAULTS. 



In the region of intensely complex faulting, which has ex- 

 isted from late Tertiary time to the present, it is impossible from 

 the work already accomplished to do more than outline the his- 

 tory of the orogenic movements. Yet it is possible to establish 

 some order of succession among these various faulting move- 

 ments that have produced the great Sierra fault zone. The 

 physiographic features in and connected with Little Valley are 

 those that furnish the necessary criteria for this determination. 

 Four distinct times or periods of faulting, with principal move- 

 ments in a definite direction, stand out pre-eminently, and sec- 

 ondary or induced faulting becomes apparent upon further in- 

 vestigation. 



Of all the lines of faults, the north-south is the most im- 

 portant, as this determines the main structural lines of the 

 ranges. In only one place is it possible to demonstrate that at 

 least two distinct periods of this motion have occurred. This is 

 in the section across middle Little Valley. As already noted, the 

 east and west fault-scarps here are greatly different in character 

 and appearance. These differences are briefly as follows: (1) 

 The west scarp rises steeply, hardly notched by stream erosion 

 and but little degraded. The east scarp rises more gently, is 

 dissected by the streams, and is considerably degraded. (2) The 

 profile of the stream courses on the west scarp is indicative of ex- 



