EVOLUTION OF MODERN SURFACE FEATURES 37 



southward through the area of the Bidahochi formation, and so to 

 the sea, but was later diverted westward by upHft of the southern 

 rim of the plateau (Repenning^/a/.). It is believed that filling of the 

 area upstream from the Kaibab Plateau to a depth of about 600 feet 

 would be sufficient to allow the river to drain westward, utilizing 

 the smaller consequent and subsequent stream valleys that had al- 

 ready been established between the Kaibab Plateau and the Grand 

 Wash Cliffs. 



Nevertheless, the problem of the course of the Colorado River in 

 its lower segment across the Colorado Plateau remains one of the 

 riddles of the Cordillera, and will no doubt be debated for years to 

 come. 



Central Part of Cordillera (Great Basin and Sierra Nevada) 



Basin and Range Topography and Structure. During the post- 

 orogenic phase, the Great Basin, or region between the Colorado 

 Plateau and Sierra Nevada, acquired its distinctive Basin and Range 

 topography — a succession of discontinuous, subparallel ranges, sep- 

 arated by desert basins. 



The Great Basin itself is a region of interior drainage; streams 

 that flow into its basins have no outlet to the sea. This is in part a 

 product of the structure, for some of the basins, of which Death 

 Valley is an extreme example, have been depressed lower than any 

 possible outlet. To some extent the interior drainage of the Great 

 Basin is a product of its arid climate and its remoteness from the 

 sea; part of its streams could flow out of the region if they had 

 sufficient volume (Hubbs and Miller, 1948, pp. 94-98), Basin and 

 Range topography extends far south and southeast from the Great 

 Basin into Arizona, New Mexico, and Sonora, which are nearer the 

 sea and are drained by the Colorado, Gila, Rio Grande, and other 

 through-flowing rivers. 



Some geologists have thought that the distinctive quality of Basin 

 and Range topography is primarily a product of erosion of a com- 

 plexly deformed bedrock under arid conditions; clearly, the arid 

 regime has done much to shape the details of the landscape. Most 

 geologists believe, however, that the topography of much of the 

 province is related to a distinctive Basin and Range structure, which 

 was superimposed on the earlier orogenic structures, in part so 

 recently that the forms of basins and ranges are a direct result of 



