CENTRAL ROCKIES 



333 



veneer was removed, and the crystalline rocks were exposed. The thrust 

 sheets dip fairly steeply in this belt both northeasterly and southwesterly. 



Numerous folds in the Paleozoic and Mesozoic strata developed at the 

 same time as the thrusting. The Reaverhead conglomerate was much 

 eroded after the thrusting and folding, and a singular remnant, the Sphinx 

 conglomerate, now holds up the highest peak in the Madison Range, 

 Sphinx Mountain, northwest of the northwest corner of Wyoming. 



Several porphyry stocks were intruded immediately after the thrusting 

 along the Idaho-Montana border in the Nicholai and Cabin thrust sheets 

 (Fig. 22.6), and it is probable that a good deal of the intrusive and min- 

 eralizing activity in the Melrose, Rutte, and Philipsburg areas, immedi- 

 ately to die north, occurred at this time. 



Late Eocene to Early Miocene Phase 



Following the main thrusting in southwestern Montana, a long episode 

 of erosion, with possibly some additional crustal movements, changed the 

 topography to an almost unrecognizable extent. The arches and thrust 

 sheets that had brought Precambrian rock to exposure were irregularly 



: reduced, and perhaps broadly downfolded in places. Instead of concen- 

 trating their attack on the sedimentary rocks, the erosional processes cut 



; great intermontane valleys through the Precambrian crystalline rocks as 

 well, with only local structural control. 



Then, in late Eocene time, volcanism broke out in nearby regions, and 

 focused in Yellowstone Park and the Absaroka Range (Fig. 22.8). Vol- 

 canism of superior magnitude also broke out in the Coast Range region 

 of Oregon and Washington at this time. It resulted in the damming of 

 drainage ways and in abundant ash and dust falls. The regimen of erosion 

 changed to one of alluviation in the great intermontane valleys, and the 

 heavy deposition of the Sage Creek formation (late Eocene) resulted in 

 southwestern Montana. Other formations of equivalent age were laid 

 down in the basins elsewhere over a wide region. 



Local deformation and erosion in early Oligocene time are noted by an 

 unconformity between the Sage Creek beds and those that overlie it. 

 Volcanism continued nearby, and the deposition of the Cook Ranch beds 



i in middle Oligocene time on the Sage Creek beds resulted. 



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Fig. 22.7. Orogeny, sedimentation, and volcanism in late Cenozoic time in the Central Rockies. 

 The Great Basin is brought into existence by block faulting and becomes a region of considerable 

 sedimentation (the Salt Lake group) on the downfaulted blocks. The east end of the Uinta Moun- 

 tains sank along the axial area and the Browns Park formation was deposited in the depression. 

 Volcanism starting in late Eocene and running through the Cenozoic was widespread. Much of the 

 Tertiary volcanic rocks in the Geat Basin are buried by later alluvium. 



