WYOMING ROCKIES 





backs on the west side with dips of 30 degrees, and cuestas on the east 

 side where dips do not exceed 10 degrees. The principal ridge-making 

 sandstone is the Mesaverde, which stands 1000 feet high in places and 

 surrounds the elliptical Baxter basin in the center. 



The evolution of die Rock Springs uplift is shown in Figs. 22.3 to 22.6 

 and 24.16. 



The Leucite Hills at the nordi end of the Rock Springs uplift are 

 remnants of cinder cones and lava sheets. The lavas cap hills of sedi- 

 mentary rock that now stand 800 to 1200 feet above the plains and pre- 

 serve remnants of an old, subdued erosion surface (Rich, 1910). This may 

 be equivalent to the Gilbert Peak surface of the Uintas. 



The Rock Springs uplift was formed mainly after the Upper Eocene 

 sediments had accumulated. This seems evident because the sediments 

 do not coarsen appreciably toward the uplift. It may have started to rise 

 . in late Washakie time incident to the bold arching of the Uintas in mod- 

 ern form, because the upper part of the Washakie formation in the 

 'Washakie basin is not present in the Green River basin; but the main 

 ^elevation of the Rock Springs uplift was post- Washakie. 



In the Gilbert Peak erosion cycle of the Uinta Mountains the Rock 

 Springs uplift was beveled, so the folding predated the erosion cycle 

 which terminated in Miocene time. 



EAST END. UINTA MTS 



LARAMIE RANGE AND BASIN AND MEDICINE BOW RANGE 



The Laramie basin is, in general, a northward-plunging syncline be- 

 tween the Laramie Range on the east and the Medicine Bow Range on 

 the west. The ranges on either side are formed of Precambrian crystalline 

 rocks, and about 8000 feet of Carboniferous and Mesozoic beds overlie the 

 crystallines in the basin. The sediments are preponderantly shaly. Along 

 the west side of the basin are four anticlines in en echelon arrangement, 

 with exposed Precambrian cores; and they are known from south to 

 nordi as Bull, Ring, Jelm, and Sheep mountains. High-angle thrust faults 

 occur on one or both sides of these anticlines. 



Thrust faulting was the chief activity in Laramide times, with the 

 sides of the basin generally bounded by thrusts dipping under the moun- 



Fig. 24.18. Cross sections of the eastern and central parts of the Uinta uplift. After Ritzma, 1959. 



tains. The Medicine Bow Mountains are thrust moderately eastward over 

 the western margin of the Laramie basin (Beckwith, 1938, 1942), and 

 the Front Range crystallines at the southern end of the basin are thrust 

 westward over the sedimentaries. See cross sections of Fig. 24.19. Tear 

 faults and faults that turn into stratification faults at depth without pass- 

 ing into the Precambrian have been described. Younger beds have been 

 thrust over older in places, and at the very south end of the valley, where 

 the thrust sheet from the west is opposed to the thrust sheet from the 

 east, the basin is not as wide as the amount of movement on the thrust 

 surface. From this, Beckwith (1942) concludes that the fronts of the 

 thrust sheets were eroded back sufficiently fast as they advanced so that 

 they did not meet head-on. 



The date or phases of deformation cannot be directly determined in 

 the Laramie basin except that they occurred in the interval post-Mesa- 

 verde and pre-Oligocene. From reference to the orogenies in nearby 

 Hanna basin and North Park, Beckwith infers that arching of the Medi- 

 cine Bow and Park ranges started in Late Cretaceous time, while the Cre- 

 taceous seas still persisted a short distance from the present mountains. 

 Folding and thrusting occurred during early Eocene and then again some 

 compression shortly afterward cast the lower Eocene beds into folds. The 

 evidence is set forth as follows by Beckwith: 



The folded sediments in the upper Laramie River Valley are about 8000 feet 

 thick. Farther north the Mesaverde is succeeded conformably bv 3000 feet of 



