RANGES OF SOUTHERN NEVADA 237 



there are no simple fault-scarps. On the contrar}^ the faults indicated 

 either have no expression at the surface or have determined gulches, 

 showing in both cases mastery of dislocation by erosion. 



Nevertheless, Mr Gilbert is inclined to consider that the range in gen- 

 eral is due to dislocation and uplift and not to erosion.* He writes : 



" The valley in the range, due to the occurrence of soft shales between harder 

 beds, opens to the south, and is deepening very slowly, because it is little elevated 

 above the plain. If the depression occupied by the gravels of the plains had itself 

 been not only emptied, but excavated, it is inconceivable that the shale in the 

 mountain should have escaped deep erosion." 



The detritus, in places several thousand feet thick,t which fills the 

 broad valleys between the ranges indicates of itself the long erosion 

 which has proved sufficient to overcome all primary deformation in the 

 interior of the mountains. However, this erosion has been very largely 

 under arid conditions, which differ from those of moister climates. 



In the desert erosion may be divided into two kinds. The first 

 kind is active where there are local permanent water bodies, usually 

 derived from some source outside the district. Such is the Colorado 

 river, which runs through an arid plateau and derives its supplies chiefly 

 from the distant Rocky mountains. These exogenous water bodies pro- 

 duce topographic forms whose importance is greatly increased by the 

 relative inactivity of erosion on other parts of the surface. Thus by the 

 Colorado river the stupendous Grand canyon has been cut, and in many 

 of the Basin ranges springs of moderate volume have excavated deep 

 and picturesque gorges. This kind of erosion may be called special or 

 basal. 



The second kind operates where the extraneous factor is absent. Here 

 the moisture is confined to occasional cloudbursts, and its effect as an 

 agency of erosion is equaled or exceeded by disintegration, gravity, and 

 eolian action. The result is that in the lower valleys leveling, instead 

 of dissection, is brought about, and in the higher ones dissection is much 

 less marked than in moister regions. So the shallow valley which lies 

 within the Timpahute range, and was carved during a period of moister 

 climate, is probably not being deepened, but indeed is being made rela- 

 tively shallower by the degradation of the higher mountains which form 

 its sides. 



The writer, therefore, sees as yet no evidence that simple deformation 

 has played a part in creating these mountains, and believes that by far 

 the most important agency in producing them was erosion. 



*0p. eit., p. 42. 



fSee Davis, Physical Geography, p. 309. 



