626 JOHN LYON RICH 



the silts and gravels. The resulting terrace is from twenty to 

 forty feet in height and is clearly marked. At the point where the 

 wash-apron feature is best developed, south of Pretty Water Creek, 

 the minor topography is entirely subsequent to the wash-apron, 

 which forms partly dissected, fiat divides between the streams. 

 This period of erosion and dissection of the gravels is referred to a 

 cessation of the arid conditions and a return to a moister climate. 



4. Second period of aggradation. — The full extent of the dissec- 

 tion of the preceding period cannot be seen, for a second period of 

 aggradation has followed and the bottoms of the stream-valleys 

 have again been silted up to an unknown depth. This is the 

 "valley-fiH" of the present streams. All the streams, large and 

 small, of the Rock Springs region have this filling which varies, 

 according to the size of the stream, from a few feet to more than 

 a mile in width. The material of this "valley-fiir' is a sandy 

 loam with many of the characteristics of loess. It stands up in 

 vertical cliffs along the stream-trenches, sometimes to a height of 

 twenty feet. 



The aggradation responsible for this valley-fill is thought to be 

 due without question to a change to more arid climatic conditions. 

 In one of the narrow valleys cut through the high sandstone scarp 

 west of Salt Wells Creek the filling progressed until the bottom 

 of the valley was changed from a graded stream-bed to a series of 

 fans which entirely destroy the grade along the valley-bottom. 

 It is plain that such an effect could be produced only in an arid 

 climate. In this case the valley was small and evidently, during 

 the arid period, could not support even a temporary stream. 



5. Present period of slight dissection. — In the case of the valley 

 just described the stream is at present cutting through the fans and 

 has almost succeeded in bringing its bed down to grade again. The 

 stream now flows only in times of flood, so that this action is not 

 rapid. All the streams of the region are beginning the dissection 

 of the "valley-fill." Along the larger ones a series of distinct ter- 

 races is forming (see Fig. 10). These are migrating up stream. 

 Three terraces in the lower course of a stream give place to two 

 higher up and to a single trench still higher. In several places the 

 mode of retreat was noted. It is by a fall from one or two to five 



