630 JOHN LYON RICH 



Seminole Mountains. Darton^ describes fiat-topped deposits of 

 coarse conglomerate capping some of the higher divides of the. 

 Bighorn Mountains of Wyoming. As already stated, Woodruff* 

 finds similar gravels in the Wind River basin, and Ball along the 

 flanks of the Sierra Madre. It is highly probable that further 

 work will demonstrate that all these deposits are genetically 

 related and are the results of the same set of widespread physical 

 conditions. 



The later stream-terraces developed in the valleys of the Rock 

 Springs region are thought to be susceptible of correlation with the 

 fluctuations in size of the lakes of the Great Basin, particularly 

 of Lake Bonneville as worked out by Gilbert.^ Gilbert's succession 

 follows : 



1. Pre-Bonneville low-water epoch. This was of long duration. 



2. First epoch of high water. 



3. Interval of low water, probably with complete desiccation. 



4. Second epoch of high water. This was only about one-fifth as long as 

 the first of the high-water epochs. 



5. Post-Bpnneville epoch of low water, continuing until recent times. 



When the succession worked out from the Rock Springs stream- 

 terraces is compared with this it is found that, beginning with the 

 present and working backward, the long period of erosion following 

 the deposition of the high-level gravels on the plateau includes 

 Gilbert's first epoch of high water. From that to the present the 

 two agree closely, the periods of aggradation corresponding to the 

 epochs of desiccation of the lakes, and periods of erosion correspond- 

 ing with the epochs of high water. 



It is believed that, as in the case of the Great Basin lakes, the 

 climatic changes recorded by the stream-terraces are to be cor- 

 related with those of the Pleistocene glacial epochs. No glaciers 

 invaded any part of the drainage-area of the Rock Springs region, 

 consequently the evidence furnished by stream-terraces is entirely 

 free from complications arising from the presence of outwash glacial 

 gravels. 



1 N. H. Darton, U.S. Geol. Survey, Professional Paper No. 51, 67-70. 



2 Lac. cit. 



3 G. K. Gilbert, "Lake Bonneville," U.S. Geol. Survey, Monograph i. 



