BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



Vol. 12, pp. 271-300 June 17, i9oi 



BROAD VALLEYS OF THP] CORDILLERAS 



BY N. S. SHALER 



{Presented before the Society December 29, 1900) 

 CONTENTS 



Page 



Introduction .... 271 



State of the valleys 272 



Structure of the valley deposits , 274 



Method of infilling of the broad valleys 282 



Effects of deposition 294 



Exceptional valleys 297 



Possible cause of increased rainfall in eastern Cordilleras 299 



Conclusions 300 



Introduction 



In the following pages I propose to set forth the results of observations 

 made in that part of the Cordilleran system which lies within the limits 

 of the United States, excluding Alaska. The main end in view is to ex- 

 hibit the history of the erosive processes in that region with reference to 

 successive changes in climate during the later geological periods. Inci- 

 dentally, reference will be made to the general conditions of erosion in 

 mountainous countries and to the effects of alternating plentiful and 

 scanty rainfall on the forms of valleys as well as the influence of the 

 volcanic ejections which have occurred in the Cordilleras since the ex- 

 isting topography began to be developed. 



Although the results of the inquiries which are recorded below repre- 

 sent a considerable amount of labor, including journeys aggregating a 

 distance of more than 10,000 miles within the area above noted, they 

 must be regarded as in a great measure tentative. The field of the in- 

 quiry is large and, while the facts set forth are tolerably clear, the con- 

 clusions to be drawn from them are open to debate. It is, moreover, 

 evident that the matter deserves a more extended study. This paper 

 undertakes little more than to indicate in general the value of the 



XXXIX— Bull. Geol. Soc. Am., Vol. 12, 1900 (271) 



