36 P- B. KING 



continental glaciers, to form the Missouri River (Howard, 1958, 

 pp. 585-587). 



Greater problems attend the streams that flow southward and 

 southwestward from the Rocky Mountains and find their way 

 through long reaches of mountain, plateau, and desert country, 

 especially the Rio Grande and the Colorado rivers. 



The Rio Grande flows southward from its source in the Rocky 

 Mountains for 500 miles through a succession of desert basins before 

 it breaks through the eastern ridges of the Cordillera in the Big 

 Bend country of Texas and enters the slope toward the Gulf Coast. 

 Deposits in the basins of northern New Mexico contain channels of 

 foreign stream-worn gravels that indicate existence there since early 

 Pliocene time of a river or rivers ancestral to the Rio Grande (Bryan, 

 1938, pp. 205-208), but such gravels are unreported in basin deposits 

 of southern New Mexico and Texas. Perhaps the Rio Grande drained 

 at first into the lake region of northwestern Chihuahua (Lee, 1907, 

 p. 22) , and later found its way across the ridges to the east by filling 

 a succession of basins, until it overflowed each in turn at the lowest 

 point on its rim (King, 1935, p. 260). 



Very likely the Colorado has drained southwestward from the 

 Rocky Mountains for a long span of Tertiary time, during which it 

 may have persisted in its present position across much of the north- 

 eastern half of the plateau. Its lower course across the plateau is 

 more puzzling. It has there cut the Grand Canyon through the south 

 end of the Kaibab Plateau, which is one of the highest uplifts of the 

 region. Moreover, below the lower end of the canyon, the desert 

 basins traversed by the river are filled by the late Tertiary Muddy 

 Creek formation, which is made up of locally derived detritus, 

 without deposits of any large, through-going river (Longwell, 1946, 

 pp. 821-826); the river could not have entered these basins until 

 after Muddy Creek time. 



It has been suggested that the river coursed across such uplifts as 

 that of the Kaibab Plateau when they were in an early state of 

 growth, that renewed uplift ponded the drainage on their upstream 

 sides, until the river overflowed through its original valley and cut 

 this to its present depth (Hunt, 1956, pp. 65-67). Such a sequence of 

 events is possible, but field relations suggest otherwise; so far as 

 known the Kaibab uplift was folded entirely by Laramide orogeny. 

 It has also been suggested that the Colorado River formerly flowed 



