384 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



N.50*E 



5,000' 



Fig. 24.17. Cross section of the north flank of the Uinta Mountains after Bradley (1936), show- 

 ing remnants of the Gilbert Peak surface projected laterally to the plane of the section. Note the 

 even truncation of both hard and soft strata. pC Uinta Mountain group; Cu, Carboniferous un- 



differentiated; J— "5 Jurassic and Triassic undifferentiated; Ku, Cretaceous undifferentiated; Tgs, 

 Green River fm.; Tb, Bridger fm.; Tbc, Bishop congl. 



stream of the basin apparently flowed eastward to join the ancestral Platte or 

 some similar river that drained into the Gulf of Mexico. 



The Bishop conglomerate, which covers much of the Gilbert Peak surface, 

 is coarse-grained and very poorly sorted and fills the deepest concavity in the 

 profile of the pediment, where it is about 200 feet thick. The same streams 

 that cut the Gilbert Peak pediment deposited the Bishop conglomerate, be- 

 cause their transporting capacity changed in response to a climatic shift toward 

 still greater aridity. This climatic change, though critical, probably was not 

 great. 



No fossils have been found in the Bishop conglomerate, but the Gilbert Peak 

 surface truncates the latest Eocene rocks and yet is distinctly older than the 

 Browns Park formation (late Miocene or early Pliocene). Hence the Gilbert 

 Peak surface and the Bishop conglomerate are either Miocene or Oligocene. I 

 believe that the Gilbert Peak surface is probably correlative with Blackwelder's 

 Wind River peneplain, near the top of the Wind River Range. 



About 400 to 500 feet below the remnants of the Gilbert Peak surface these 

 same streams later cut the less extensive Bear Mountain erosion surface. The 

 characteristics of the Bear Mountain surface are so nearly identical with those 

 of the Gilbert Peak surface that it is regarded as a pediment formed under arid 

 conditions probably closely similar to those which prevailed while the Gilbert 

 Peak surface was being cut. Correlated with the Bear Mountain surface are 

 two large, rather smooth-floored valleys, the Browns Park Vafley and Summit 

 Valley. These valleys are in the eastern part of the Uinta Range and are each 

 roughly parallel to the range axis. The floor of the Browns Park Valley descends 

 eastward and passes beneath the Browns Park formation, which is of upper 

 Miocene or lower Pliocene age. As there is no indication that the deposition of 

 the Browns Park formation did not follow immediately the completion of the 

 Bear Mountain surface, that surface is probably also of essentially this geologic 

 age. 



After the deposition of the Browns Park formation the east end of the Uinta 

 Mountain arch collapsed by block faulting, . . . and . . . apparently lowered 



the stream flowing along the ancient Browns Park Valley (on the depositional 

 surface of the Browns Park formation) enough for one of its tributaries, which 

 has already cut through the divide on the north side of the valley, to be re- 

 juvenated and thus to extend its course headward so far northward in the soft 

 Tertiary rocks that it finally captured the ancient master stream of the Green 

 River Basin. When this river, the new Green River, first entered the Browns 

 Park Valley it flowed on the uppermost beds of the Browns Park formation, 

 following the ancient Browns Park stream eastward beyond the east end of the 

 range. But soon thereafter it was captured by Lodore Branch, a tributary to the 

 ancestral Cascade Creek, which drained Summit Valley, and so came to flow 

 along the present site of Lodore Canyon. 



UINTA MOUNTAINS 



The Uinta Mountains are eroded from a flat- topped anticlinal uplift, 

 the major details in cross section of which are shown in Fig. 24.18. The 

 thrust faulting along the north flank is post-Green River formation and its 

 character suggests horizontal spilling or mass flowage of the margin of the 

 uplift toward the Green River basin as a late or secondary effect of the 

 primary vertical uplift. The vertical uplift of anticlinal crest over basin 

 trough exceeds 32,000 feet. 



ROCK SPRINGS UPLIFT 



Separating the Green River basin on the west and the Washakie basin 

 on the east is the Rock Springs uplift, a 40-mile-long, doubly plunging, 

 north-south-trending anticline. The resistant sandstones hold up hog- 



