624 JOHN LYON RICH 



places along the fault-line. This fault is described as displacing 

 beds of the Brown's Park Tertiary, but since these beds have been 

 considered to be of earlier age than the Bishop Conglomerate, there 

 is no evidence to indicate the exact time of the displacement; 

 whether before or after the development of the conglomerates. 



It is therefore impossible to determine from the data at hand 

 whether the present low elevation of the eastern end of the Uintas, 

 relative to the gravel plateaus, is the result of down-faulting or 

 of simple denudation, with a more rapid lowering of the tilted rocks 

 close to the deep canyon of Green River than of the gently sloping 

 gravel-capped plateaus farther away. Whatever may be the final 

 decision of this point, there can be no question as to the profound 

 denudation subsequent to the gravel-deposition. Red Creek basin, 

 over 2,500 feet deep, and lying outside of the faulted zone, gives 

 a fair measure of this denudation. 



The period of erosion continued until all the valleys of the region 

 were deeper than they are at present, though just how much deeper 

 is not known. This erosion evidently took place under conditions 

 of a moderately moist climate, for great amounts of material have 

 been removed by the rivers. A desert climate favors accumulation 

 rather than removal. 



2. First period of aggradation. — After this period of degradation 

 there came one of aggradation, during which the valleys were 

 partly filled by deposits of silt and gravel washed down from the 

 valley-sides. On the west side of Salt Wells Creek, along and 

 south of Pretty Water Creek, there is an extensive wash-apron 

 derived from the high, gravel-capped Miller Mountain plateau. 

 This wash-apron mantles the slopes down to the valley-bottom, 

 where it grades into a flat-topped valley-filling which now remains 

 as a distinct terrace about forty feet above the present stream (Fig. 

 9). This valley-filling must have had a depth of considerably over 

 forty feet, for bed-rock is nowhere exposed in the stream-bottom. 

 At least one hundred feet is thought to be a moderate estimate. 

 There are no wells to indicate its exact depth. 



This period of aggradation is referred to a change to a more arid 

 climate than that under which the major erosion was accomplished. 

 Under arid conditions the occasional torrential rains, acting on 



