EVOLUTION OF MODERN SURFACE FEATURES 33 



withering streams flowing eastward from tlie mountain are of such a 

 texture that, under semi-arid conditions, they could hardly have 

 moved down a slope much less than that of the present (Johnson, 

 1901, p. 628). 



Erosion Surfaces in the Ranges. So much for middle and late 

 Tertiary conditions in the plains and lowlands around the mountain 

 ranges. What were conditions in the ranges, which were the sources of 

 the streams and of much of the detritus deposited roundabout? 



Wide areas in the ranges are beveled by a subsummit surface, 

 marked by accordant crests which extend across the deformed bed- 

 rock structures, above which chains and clusters of peaks project on 

 the divides, and below which modern valleys and canyons have been 

 cut to depths of thousands of feet. The surface has been given local 

 names in different ranges, and has been variously dated as Miocene 

 and Pliocene. Precise age does not matter greatly, as the surface may 

 not have been completed simultaneously everyw^here ; it expresses a 

 general late Tertiary erosional condition, hence deserves the general 

 title of Rocky Mountain peneplain (Atwood and Atwood, 1938, 

 pp. 964-965). 



Analysis of this surface on the north slope of the Uinta Moun- 

 tains, where it is unusually well preserv^ed, indicates that it has a 

 gradient of 400 feet per mile near the high peaks along the mountain 

 axis, flattening to 55 feet per mile toward the plains, where it is 

 largely mantled by the coarse gravels of the Bishop conglomerate 

 (Bradley, 1936, pp. 170-176). This outward flattening of gradient is 

 believed not to have resulted from late differential uplift of the 

 range, but to have been inherent in the nature of the surface itself. 

 The surface must have been cut under conditions of considerable 

 aridity; its graded profile, much steeper and more concave than 

 those of humid regions, was just sufficient in an arid climate for the 

 transport of materials across it. 



Regional studies indicate that the Rocky Mountain peneplain in 

 other ranges is like that in the Uinta Mountains, and that it proba- 

 bly formed under similar conditions. They show, as well, that the 

 peneplain in the mountains was originally confluent with deposi- 

 tional surfaces in the Great Plains and other lowlands where, as we 

 have seen, the nature of the deposits suggests deposition on a slope 

 nearly as steep as the present slope of the plains. 



Environments of Middle and Later Tertiary times. Between 



