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STRUCTURAL GEOLOGY OF NORTH AMERICA 



Range but narrows toward the Rlack Hills. The east-bounding monocline 

 has a structural relief of about 9000 feet within a few miles, and the 

 one on the northwest drops the strata as abruptly and about an equal 

 amount. 



The top of the Hartville uplift is about 2000 feet below that of the 

 Laramie uplift. Its sedimentary veneer has been stripped off only in a 

 narrow zone along the axis; from this it is deduced that the uplift was 

 never as high as the Laramie or Sweetwater uplifts, and that its relatively 

 lower position today is not due principally to late Laramide subsidence 

 like that of the Sweetwater, but to the fact that it was never elevated 

 high enough in the face of much higher elevations nearby to have suf- 

 fered as much erosion. It was largely buried by the White River beds in 

 lower Oligocene time, and in the later cycles of erosion it has been partly 

 exhumed. 



REGIONAL UPLIFT IN LATE CENOZOIC 



The Eocene deposits of Wyoming and adjacent areas accumulated in 

 swamps, on flood plains, and in fresh-water lakes. The flora and fauna 

 indicate a warm, rain-forest climate, and the elevation above sea level 

 at which the sediments were laid down is generally considered to have 

 been not in excess of 1000 feet. Today they occur at about 7000 feet, 

 especially in the Green River, Wind River, and Big Horn basins. The 

 Great Plains adjacent to the Rockies contain Early Tertiary sediments 

 deposited at low levels, but which now stand in places as high as 5000 

 feet. It is patent that uplift on a very broad scale has occurred. We must 

 be aware of the history of subsidence and sedimentation in central 

 Wyoming, the eastern end of the Uintas, and elsewhere in mid and late 

 Eocene, Oligocene, and early Miocene time, and post-Miocene normal 

 faulting resulting in further subsidence. However, in the broad picture 

 from the Northwest Territories of Canada, southward through Alberta, 



and the outer Rockies of Montana, Wyoming, Colorado, and New Mexico, 

 the dominant late Cenozoic activity was uplift, and in an amount from 

 2000 to 6000 feet. The southern part of the Colorado Plateau was up- 

 lifted perhaps 8000 feet. 



Most of the literature concerning the erosional and depositional activity 

 during the building of the Laramide Rockies and the later regional uplift 

 depicts a history as follows. Immediately after the Laramide ranges were 

 uplifted, extensive erosion surfaces were developed — in places several, 

 one below the other — indicating times of crustal stability separated by 

 uplift and dissection. The erosion surfaces in places can be traced out 

 and are said to level the basin fill deposits. Then, with the regional uplift, 

 just mentioned, the erosion surfaces were greatly dissected and produced 

 our present topography. 



Mackin, Van Houten, and others more recently view the history as fol- 

 lows. Erosion affected the uplifts immediately as they appeared from the 

 Cretaceous seas and removed sediments to the intervening basins. The end 

 result of the erosional and transportational processes was a vast graded 

 surface, in part erosional and in part depositional. With the building of 

 great volcanic piles in the Yellowstone and Absaroka region in mid- 

 and late Eocene and Oligocene time the streams draining eastward, per- 

 haps fanlike from the volcanic field, were overloaded with fine debris, and 

 according to Love (1956b), the intermontane basins of Wyoming were 

 so filled that the large bordering ranges were almost submerged. Volcanic 

 activity broke out in other areas, and as pointed out some areas subsided, 

 so that sedimentation was of irregular thickness in places and continued 

 where subsidence continued. But in late Miocene or early Pliocene time 

 most of the graded surface was uplifted, an arid climate resulted, and a 

 regimen of erosion started. This has continued in most places until 

 today. With the coming of the arid and semiarid climate the grassy plains 

 came into existence, and many animals evolved and adapted to a life on 

 the open prairie. 



