EVOLUTION OF MODERN SURFACE FEATURES 35 



faces, but many of them, as they cut downward, were superimposed 

 on hard rocks in the buried mountain ridges beneath, and were 

 there forced to excavate deep canyons (Atwood and Atwood, 1938, 

 pp. 968-976). 



The same sort of canyon cutting took place in the less deformed 

 rocks of the Colorado Plateau, southwest of the Rocky Mountains, 

 where uplift and degradation had been in progress since early in 

 Miocene time (Hunt, 1956, p. 77). 



Principal cause of the accelerated stream erosion of the eastern 

 part of the Cordillera during Quaternary time must have been re- 

 newed regional uplift, but there is uncertainty as to its amount. All 

 graded surfaces of subaerial erosion and deposition possess an orig- 

 inal slope toward the sea ; if the late Tertiary surfaces were produced 

 in an arid or semi-arid regime, this original slope would have been 

 much steeper than that produced in a more humid regime. How 

 much the slopes of the late Tertiary surfaces and the regional relief 

 of the eastern Cordillera were augmented by uplift is thus difficult 

 to evaluate, but it was probably much less than has been supposed 

 by some authors. 



Whatever the magnitude of the uplift, major climatic changes oc- 

 curred also, as the ice ages of Pleistocene time brought about both 

 refrigeration and increase of rainfall. Under this more humid regime, 

 streams that had become adjusted to steep gradients during the 

 arid times of the later Tertiary were able to readjust themselves to 

 new, lower gradients. Such readjustments were most marked, of 

 course, during the glacial periods, and were less marked during the 

 drier interglacial periods; thus, general downcutting has been punc- 

 tuated by many pauses expressed in the landscape by a succession 

 of terraces and intervening steps. 



Origin of Drainage Courses of Eastern Cordillera. All this is very 

 well as a generalization but what, specifically, has been the history 

 of the individual rivers of the eastern Cordillera? The more specific 

 we become, the more the doubts and confusions multiply. 



Streams draining eastward from the Cordillera into the interior 

 region need trouble us least. Streams of some sort no doubt flowed 

 from the Cordillera in this direction since the Laramide orogeny, 

 although their courses must have shifted with time. The greatest 

 shift in later times was a deflection of streams that formerly flowed 

 into Hudson Bay, southward, around the edges of the Pleistocene 



