RECENT FAULTS. 277 



borders of the ancient lake, exhibit scarps in lacustral beds and gravel de- 

 posits, and are therefore more recent than the last rise of the lake. 



The appearance of the fault at the western base of the West Humboldt 

 range is shown on Plate XLV ; the point of view being near the southern 

 end of Humboldt Lake. The precipitous mountain face shown in the pic- 

 ture is in reality an ancient fault scarp of grand proportions, which was 

 somewhat eroded before the existence of Lake Lahontan. During the time 

 the lake occupied Humboldt Valley its waves carved a number of terraces 

 along the base of the mountains, which are represented in the sketch, and 

 are familiar to many who have traveled over the Central Pacific Railroad. 

 Between the highest terrace and the shore of the present lake there is an 

 irregular line of clifiFs — in part obscured by talus slopes — which has been 

 produced by recent orographic movement This fault scarp is composed 

 principally of cemented gravels of Lahontan age, bitt in places the rock 

 forming the mountains may be seen beneath the clastic beds. The charac- 

 ter of the section exposed at many localities along this fault is represented in 

 diagram c, Fig. 36. This fault scarp may be traced continuously from the 

 Mopung Hills northward, along the bases of the West Humboldt and Star 

 Peak ranges, to the neighborhood of Winnemucca, a distance of over a hun- 

 dred miles ; its full extent, however, remains to be determined. Throughout 

 the greater part of its course it crosses alluvial slopes, with a fresh scarp 

 from ten to twenty feet, or more, in height, its greatest magnitude being- 

 near its southern end. Along the eastern shore of Humboldt Lake it forms 

 a nearly vertical escarpment, fully fifty feet high. At the Mopung Hills it 

 divides into several branches, which may be traced to the border of the 

 Carson Desert, and then become obscured. 



In describing the shore phenomena on the Niter Buttes, a spur of the 

 main range, at the southern end of Humboldt Lake (see ante, page 112), 

 some account was given of sloping terraces, which indicate that orographic 

 movement must have taken place during inter-Lahontan time. We have 

 evidence, therefore, that the fault along the west base of the West Humboldt 

 range attained a great magnitude previous to the existence of Lake Lahon- 

 tan, that it underwent some disturbance during inter-Lahontan time, and 



