EXCEPTIONAL VALLEYS 297 



these seeming exceptions sufficiently to determine whether they render 

 the view above suggested untenable. 



Exceptional Valleys 



It has been incidentally noted that in certain parts of the Cordilleras 

 valleys occur which clearly indicate a different adjustment of the condi- 

 tions of erosion from those long prevailing in the characteristic refilled 

 broad valleys. So far as I have been able to examine these exceptions 

 they appear to be accounted for by peculiarities of situation or by ex- 

 ceptional movements of the underlying earth. The diminution in the 

 amount of infilling after the periods of great erosion on the Pacific coast 

 is explicable from the fact that these valleys near the ocean have never 

 had their streams so far reduced in flow as to bring about any large 

 amount of aggradation. There are, however, two districts where the 

 process of infilling is absent or noticeably less than might be expected 

 if the hypotheses here advanced to account for the broad valleys were 

 true. These are the sections traversed by the canyon of the Colorado 

 and that on the eastern face of the Cordilleras from central New Mexico 

 to the Canadian line. The reasons for these exceptions may perhaps be 

 found in certain geographical accidents that are noted below. 



As for the Colorado river, it is well known that, while it traverses one 

 of the most arid portions of the Cordilleran region, it derives its water 

 from the snows of the high mountains on the eastern side of the Rocky 

 mountains, where for ages the precipitation must have been consider- 

 able. Cutting as it does through a region which clearly has long been 

 characterized by a very scanty rainfall and where there has been an ex- 

 tensive upward movement of the rocks, it has formed and kept open a 

 deep valley. Although the sides of this trench have been much inter- 

 sected by tributaries, the amount of debris which they have brought 

 down has not been greater than the vigorous river has been able to take 

 away. The result is a valley in which the stream has been able in the 

 main to retain its place on the bed rock. As these conditions of- the 

 Colorado river are rather exceptional, and as they appear to account for 

 the lack of aggradation we find there, we may therefore reasonably 

 conclude that the features of this valley do not militate against the 

 hypothesis. 



As elsewhere noted, the eastern face of the Cordilleran district affords 

 evidence that goes to show a recent augmentation of rainfall beyond any 

 increase that is indicated in the central region. This is most evident in 

 the southern portion of this district, as in southern Colorado and New 

 Mexico. In that area we have fairly broad valleys which often exhibit 

 the form of the bed rock cut, which has been described as existing be- 



