6i8 JOHN LYON RICH 



renewed uplift of the Uintas immediately antedating the deposition 

 of the gravels. 



Cause of the invasion of the gravels. — The gravels which form the 

 capping of the peneplain were, in the southern part of the area, 

 unquestionably derived from the Uintas. During the long period 

 of quiet while the peneplain was developing, any considerable eleva- 

 tions in the Uintas must have been considerably reduced and very 

 possibly planed off to correspond with the character of the adjoin- 

 ing country. At any rate they were not then supplying debris to 

 the adjacent low-lying areas. 



In order to bring about the change from these conditions to 

 those which followed, in which great desert fans spread out from 

 the mountains and covered the adjacent plains to distances of 

 from thirty to forty miles or more, there must have been a decided 

 change in the relations of plains and mountains. A change of 

 conditions capable of producing so great results could have been 

 no less than a pronounced renewed uplift of the Uinta mountain- 

 range with the consequent development of extensive debris fans. 



As a test of the preceding, let us postulate conditions as they 

 seem to have been at the end of the period of planation, with the 

 whole country, including the Uintas, reduced to low relief and 

 lying at a comparatively low altitude. What then would be the 

 eifect of a marked uplift of the mountain-range? 



In the first place there would, in all probability, be a marked 

 change in climate. The high mountain-range to the south would 

 cut off some of the moisture-bearing winds; with the result that 

 the adjacent lowlands would become more arid. This effect would 

 be increased if at the same time the Wasatch range, which runs 

 north and south about one hundred miles west of this area, were 

 also elevated so as to cut off the winds from the west. The high 

 lands of the Uintas, attacked by the agencies of denudation and 

 receiving most of the precipitation, would supply to the streams 

 large quantities of rock-waste which, on account of the arid nature 

 of the climate, could not be entirely removed to the sea, but would 

 accumulate at the base of the range in the form of alluvial fans 

 such as are developing at the base of the desert-ranges today- 

 These fans as they grew would gradually spread out over the 



