6i2 JOHN LYON RICH 



slopes to the south in a direction opposite to that of the portion 

 covered by the Uinta gravels. 



The low area referred to is evidently fundamentally of a con- 

 structional nature lying at the junction of two fans, the one from 

 the Uinta Mountains, the other from Aspen. Where these fans 

 meet there is a distinct interfingering of the two kinds of gravel. 

 In riding along in an east-and-west direction one is first on Uinta 

 gravels, then on those from Aspen. Areas of one extend out into 

 areas of the other. The vertical interlapping of the two was 

 nowhere seen, though it undoubtedly exists. The whole relation 

 of the two indicates that they were spread out at the same time 

 the one from the Uintas, the other from Aspen Mountain. 



INTERPRETATION 



Planation. — From the relations just described it is evident that 

 the Rock Springs region has been subjected to a long period of 

 planation during which the surface of the land was reduced to the 

 condition of a peneplain. The rocks were worn down until dif- 

 ferences in structure produced no corresponding effect in the topog- 

 raphy of the plain. In a few exceptional cases, like those of Aspen 

 Mountain and Antelope Butte, hard rocks remained above the 

 plain as monadnocks. That the plain was of considerable extent 

 is shown by the fact that it covered not only the area south of 

 Rock Springs as far as the Uinta Mountains, but also, as is indi- 

 cated by the relation of the lava-flows of the Leucite Hills, all 

 the area between the Uintas and the Wind River Mountains 

 200 miles to the north. Its limits are unknown. 



Present differences in elevations of portions of the peneplained 

 surface still remaining indicate that the whole region was reduced 

 to the condition of a gently undulating plain of advanced old age 

 with a relief of 700 to 800 feet in distances of 25 or 30 miles. This 

 conception corresponds well with the observed great regularity of 

 the plain surface in details and its broad irregularity when dis- 

 tances of 25 or 30 miles are considered. 



As to the time of planation it is not possible with the informa- 

 tion at hand to make any definite statement further than that it 

 was later than the Green River Eocene. There can be no question 



