EVOLUTION OF MODERN SURFACE FEATURES 23 



whereas others were produced by close folding of sediments which 

 had been deposited in troughs between the earlier uplifts (Burbank 

 and Lovering, 1933, pp. 283-301). 



The foreland ranges of the southern Rocky Mountains are sep- 

 arated from the miogeosynclinal structures in the Great Basin on 

 the west by the Colorado Plateau, which remained as a more stable 

 block during Laramide orogeny. Its rocks were broadly upwarped 

 and downwarped in much the same manner as those of the Rocky 

 Mountains, but they attained less structural relief. A typical uplift, 

 in the Kaibab and Coconino Plateaus of the Grand Canyon district, 

 is still sheeted over by sedimentary rocks, except where trenched by 

 the Colorado River, and it is bounded at the sides by steeply sloping 

 monoclinal flexures. 



Record of the Laramide orogeny may be read in deposits of latest 

 Cretaceous and Paleocene ages which are preserved to great thick- 

 ness in basins between the ranges of the Rocky Mountains, such as 

 the Bighorn and Powder River basins of Wyoming — and in the Great 

 Plains east of the mountains, as in the Denver and Williston basins 

 of Colorado and North Dakota (Fig. 7). The deposits are somber- 

 colored land-laid clays and sands with layers of coal, whose upper 

 parts contain increasing quantities of detritus eroded from the cores 

 of the ranges. 



Much labor has been expended in a search for immense uncon- 

 formities that were supposed to be concealed in these deposits, and 

 which would mark the time of upheaval of the intervening ranges, 

 but it is now clear that the deposits are essentially conformable 

 sequences. Very likely the original areas in which the deposits were 

 laid did not greatly dififer from the present structural basins, and the 

 deposits were thick, local accumulations derived from erosion of 

 ranges uplifted in the immediate vicinity. The latest Cretaceous and 

 Paleocene deposits are therefore unlike the broad sheet of marine 

 sediments of earlier Cretaceous time, which were derived from 

 erosion of deformed areas far to the west. 



Laramide orogeny destroyed the great Cretaceous seaway of the 

 eastern Cordillera (Fig. 5). Except for a brief marine incursion in 

 the northern Great Plains during late Paleocene time, represented by 

 the Cannonball member of the Fort Union formation, seas returned 

 no more to the region, and all succeeding deposits were land-laid. 

 The Upper Cretaceous and Paleocene land-laid deposits evidently 



