622 JOHN LYON RICH 



The fact that the succession of events indicated by the gravel- 

 covered peneplain with its associated features, as worked out inde- 

 pendently of these other studies, agrees so closely with their 

 results, strengthens the conclusions reached as a result of our 

 local investigation. 



PHYSIOGRAPHIC CHANGES SUBSEQUENT TO THE GRAVEL- 

 DEPOSITION 



I. Erosion.- — Practically all the erosion which gives rise to the 

 present diverse topography of the Rock Springs region is sub- 

 sequent to the deposition of the gravels. Over wide areas these 

 are entirely removed, and wherever present they are merely 

 remnants standing, in general, over i,ooo feet above the major 

 stream- valleys. The present streams, which over the area once 

 covered by the gravels must have been initiated on the gravel 

 surface, are superposed discordantly on the underlying rocks. The 

 minor tributaries are subsequent on the structure, while the major 

 streams are entirely independent of it. In one instance, that of 

 Salt Wells Creek, the stream three times breaks across a scarp 

 'over i,ooo feet in height. Wide areas in the softer rocks have been 

 reduced to low relief at elevations from 800 to i ,000 feet below the 

 old peneplain level. Except where protected by lava-flows or the 

 gravels, even the tops of the scarps of harder sandstone are several 

 hundred feet below the peneplain level. All of this erosion has 

 taken place since the deposition of the gravels. 



Present denudation is being accomplished by wind and water 

 together. The hardness and position of the rocks are the determin- 

 ing factors in the rate of erosion. In general the harder rocks stand 

 out in the form of scarps. Horizontal rocks have shown greater 

 resistance than the same rocks in an inclined position. 



As already pointed out,' the Uinta Mountains from which the 

 gravels were derived are now lower than the tops of the gravel- 

 deposits fifteen miles and more away and are separated from them 

 by a valley at least 2,500 feet deep (see photograph. Fig. 8). They 

 must therefore have been subjected either to profound denudation 

 since the gravels were spread out over them, or to downthrow by 

 faulting. If their present low elevation is due to denudation this 



