EVOLUTION OF MODERN SURFACE FEATURES 27 



Eastern Part of Cordillera (Central and Southern Rocky Mountains, 

 Colorado Plateau, and Great Plains) 



Eocene Environments. We have arbitrarily chosen the end of 

 Paleocene time as the close of the orogenic, or Laramide, phase in 

 the eastern part of the Cordillera, but environmental changes from 

 Paleocene into Eocene were evolutionary rather than revolutionary. 

 It is true that in some of the intermontane basins initial coarse 

 Eocene deposits overstep the edges of Paleocene and earlier rocks 

 with marked unconformity, and that this has sometimes been 

 thought to have followed immediately on the climax of the Laramide 

 orogeny. We have seen, though, that Laramide structures had been 

 in process of growth long before the end of Paleocene time; the 

 Eocene deposits thus record merely a resumption of sedimentation 

 after the crust had reverted to relative stability. 



Within the mountain belt, subsidence of the basins continued 

 through Eocene time; basins which had received Upper Cretaceous 

 and Paleocene deposits also received Eocene deposits (Fig. 7). 

 Significantly, however, basins east of the mountains became quies- 

 cent. The Denver, Williston, and other basins in the Great Plains 

 received Upper Cretaceous and Paleocene deposits, but few or no 

 Eocene deposits. Here, the surfaces of the earlier basins are over- 

 spread by thin sheets of Upper Tertiary sands and gravels. During 

 Eocene time the regime of the Great Plains changed from one of 

 sedimentation to one of erosion. 



Initial Eocene deposits of the intermontane basins (Wasatch 

 formation and equivalents) are red-banded sands and silts of stream 

 origin, which pass marginally into coarse piedmont deposits. The 

 fluviatile deposits contain fossils of large terrestrial mammals which 

 probably lived in savannas and open country; forest-dwelling types, 

 such as those of the preceding Paleocene, are less common. Evi- 

 dently the passage from Paleocene to Eocene time was marked by a 

 restriction of forests and expansion of grasslands (Van Houten, 

 1948, p. 2106). Nevertheless, there is no evidence of marked change 

 in climate or general altitude ; the environment has been compared 

 with that along the present Gulf Coast of the United States (Brad- 

 ley, 1948, p. 641). 



In some of the basins stream deposition continued through the 

 Eocene (as in the Powder River, Wind River, and San Juan basins), 



