EVOLUTION OF MODERN SURFACE FEATURES 31 



conditions inferred in the eastern Cordillera in Eocene time with 

 conditions today — in the Eocene, low general altitude, low relief, 

 subdued landscape, and warm, humid climate; today, high general 

 altitude, high relief, rugged mountains and deep canyons, and 

 sharply contrasted climates. How was this change brought about? 



Geologists agree on many aspects of the stor>% and especially that 

 the whole region has been uplifted as a unit many thousands of feet 

 since Eocene time, but they disagree as to the manner in which it 

 was accomplished, and by what stages. One view, perhaps the more 

 customary, is that it came about through a succession of brief up- 

 heavals, of which the last great one was during the Pleistocene, sep- 

 arated by more prolonged periods of stillstand (Atwood and Atwood, 

 1938, p. 978). Another view is that uplift proceeded slowly, with little 

 interruption since the waning of Laramide orogeny; with the up- 

 ward movement greatest in the first half of the Tertiary and di- 

 minishing afterward (Mackin, 1947, pp. 110-111). These divergent 

 views result from differences of interpretation of the middle and 

 late Tertiary deposits and land forms that are preserved in the 

 eastern Cordillera. 



Middle and Upper Tertiary Deposits. During middle and late 

 Tertiary time deposits were laid down in the eastern Cordillera as 

 widely as during Paleocene and Eocene time, but in a different 

 pattern. They are preserved now as erosion remnants of original 

 broad sheets of sediment, rather than as downfolds in original 

 depositional and structural basins. 



Deposits of the Oligocene White River group are extensive in the 

 northern Great Plains of South Dakota, and are preserved in smaller 

 remnants in the Central Rocky Mountains of Wyoming; they are 

 overlain southward, in Nebraska, by the Miocene Arikaree group. 

 Even more extensive in the Great Plains, however, is the Pliocene 

 Ogallala formation, which spreads southward from Nebraska to 

 Texas, and eastward from the mountain front for nearly 400 miles; 

 its caliche-cemented layers ("mortar beds") form the caprock of the 

 High Plains (Johnson, 1901, pp. 643-647). At about the same time 

 another sheet of deposits, the Bidahochi formation was laid down in 

 the south central Colorado Plateau, and is now preserv^ed as rem- 

 nants in northeastern Arizona (Repenning and Irwin, 1954). In 

 New Mexico, between the plateau and the plains, the Santa Fe 



