CENTRAL ROCKIES 



.3-31 



made up in large part from Paleozoic limestones and quartzitic sand- 

 stones derived from the arch, but in places Beltian boulders are present. 

 These may have come from the west, or from a pre-existing conglomerate 

 not yet found in place. Another uplift west of Yellowstone Park may have 

 appeared at this time, but no conglomerate around it is noted, so the time 

 of the appearance of the uplift and the exposure there of the Precambrian 

 rocks is not yet clear. 



The steeply upturned beds and the overriding Precambrian sheet of the 

 southeast flank of the main arch from Lima to Virginia City now stand as 

 the Snowcrest and Green Horn ranges. Part of the northwest flank may 

 be seen in the Blacktail Range southeast of Dillon ( Scholten et al, 1955 ) . 



The Beaverhead conglomerate is believed to be Paleocene ( Lowell and 

 Klepper, 1953). Soon after it was deposited, it was upturned along the 

 Snowcrest Range, and perhaps gently folded in other places. 



Early Eocene (?) Phase 



We find in southwestern Montana two systems of compressional struc- 

 tures nearly at right angles to each other. The northwesterly trending one 

 is clearly the later (see Fig. 22.6). It is characterized by numerous 

 thrust sheets, some of which override the Beaverhead conglomerate or 

 have carried the conglomerate on their backs in the horizontal movement. 

 See cross section, Fig. 21.8, which runs nearly north-south, just south of 

 Lima. Two folds of the earlier northeasterly trending disturbance are 

 impressed as sharp cross folds in the frontal thrust sheet. 



The belt of thrusting of southwestern Montana is undoubtedly a con- 

 tinuation of the one of western Wyoming and eastern Idaho under the 

 Snake River volcanic field, as illustrated in Fig. 22.1. As far as known, 

 all thrusts moved toward the northeast. One or two brought the Pre- 

 cambrian crystalline rocks to exposure, but now are dismembered by 

 erosion into klippen and fensters. They involved the Paleozoic rocks of 

 geosynclinal character, and along the eastern front of the thrust belt the 

 Mesozoic rocks occur and are deformed. 



The belt of thrusting on the northeast, between Virginia City and 

 Bozeman, involved the thin-shelf sediments, and in the uplifting that 

 accompanied each thrust sheet, much of the Paleozoic and Mesozoic 



Fig. 22.4. 



over Adavil 



Orogeny and sedimentation in Montana time in the Central Rockies. Absaroka is thrust 



