RECENT FAULTS. 275 



The scarps differ from the steep slopes bounding water-built terraces and 

 embankments — that neither their upper or lower limits are horizontal for 

 any considerable distance ; they are characterized by irregularity, and do 

 not define the boundaries between deposits of different character Tney 

 occur botli above and below the highest beaches of the Quaternary lakes 

 of the region where they are found, and exist in valleys that have a free 

 drainage as well as in those that are inclosed and once held lakes. It is, 

 therefore, evident that their origin is totall}- independent of the action of 

 waves and currents, and it is equally clear that they cannot be the result 

 of erosion. 



Scarps of this nature were first observed in the Great Basin by Mr. 

 Gilbert, while examining the western base of the Wasatch Mountains, and 

 were recognized as the result of I'ecent orographic movements.*' In other 

 words, they are fault scarps of very late origin. Their recency is shown 

 by the fact that they commonly occur in Quaternary lacustral sediments and 

 recent alluvial slopes, and form steep slopes of earth and gravel that are but 

 little modified by erosion, and in many instances are bare of vegetation. 



In many cases, it is evident that they could not have existed in their 

 present condition for moi-e than a few years. Sometimes they are more 

 than a hundred miles in length, and vary from a few feet to more than a 

 hundred feet in height. 



Recent faults of this nature have been observed along the western base 

 of the Wasatch Mountaius, at the eastern base of the Sierra Nevada, and 

 on the foot-slopes of many of the intermediate Basin ranges. In the La- 

 hontan area recent fault scarps are a common feature in the topography of 

 the valleys, and furnish one of the many interesting jjroblems in the physi- 

 cal geology of the region. 



All of the lines of post-Lahontan displacement that are actually known 

 to exist in the Lahontan basin are sketched on Plate XLIV, with as much 

 accuracy ?.s the topography of the maj? admits. It is evident that our 

 knowledge of this phenomenon is incomplete, as only the more recent dis- 

 placements are apt to attract attention, for the reason that when erosion has 

 modified the scarps it is frequently impossible to determine whether post- 

 s' Second Annual Report U. S. Geological Survey, p. 192. 



