22. 



CENTRAL ROCKIES 



SPATIAL RELATIONS 



The system of Laramide mountains referred to here under the heading 

 "Central Rockies" includes the ranges that formed from the geosynclinal 

 sediments of southwestern Montana, eastern Idaho, western Wyoming, 

 central and western Utah, and eastern and southern Nevada. The belt 

 starts at the Idaho batholith and extends southeastward to the Snake River 

 lava plains where it is covered by late Tertiary and Pleistocene lavas and 

 alluvium. See the Geologic Map of the United States and Fig 22.1. Emerg- 

 ing from beneath the lavas, it continues southeastward to the Snake River 



and Hoback ranges of western Wyoming, where it turns southward and 

 extends into northern Utah and to the junction of the east-west-trending 

 Uinta Range. In Utah and Nevada, the Middle and Late Tertiary block 

 faulting has modified somewhat the topographic features resulting from 

 the Laramide orogeny; but it is clear that a belt of complex Laramide 

 thrusting and folding continues on south of the Uinta junction into south- 

 western Utah and southern Nevada. 



The eastern border of the Central Rockies system is sharply defined, 

 whereas the western is indefinite. The eastern margin is made up in part 

 of the Paleozoic strata, in part of the Mesozoic strata and the orogenic de- 

 posits of the Cretaceous and the Early Tertiary; but westward only the 

 Paleozoic and some Triassic rocks of the Cordilleran geanticline are 

 involved. Examine the paleotectonic maps of the late Paleozoic and the 

 Mesozoic. Recause rocks younger than Paleozoic are almost entirely absent 

 in the western part of the Central Rockies, it is generally impossible to 

 date accurately the phases there or to distinguish the Laramide structures 

 from those of the Cedar Hills, Antler, and Nevadan orogenies. Most 

 probably, the Laramide structures were superposed on the Antler and 

 Nevadan in a medial zone, but details are not known. The map, Fig. 21.1, 

 shows the relation of the orogenic belts to the Laramide as well as possible 

 with existing data. 



The Uinta Mountains are a great flat-crested anticlinal uplift and, as 

 far as Paleozoic and Mesozoic strata are concerned, are part of the shelf 

 province. Physiographically, they separate the Colorado Plateau from 

 the great ranges and intermontane basins of Wyoming, and are more 

 closely related to the shelf ranges of Wyoming than to the Colorado 

 Plateau. They are definitely not similar in structure to the Central Rockies, 

 and generally they have thinner formations. Therefore, they are not in- 

 cluded in them. 



Aside from the Uinta re-entrant in the eastern margin of the central 

 Rockies, the great mountain system is one of approximate arcuate pattern 

 with a radius of curvature of about 450 miles. In it, probably all major 

 overriding thrust sheets have moved continentalward, or toward the con- 

 vex side of the arc, viz., northeastward and eastward. 



327 



