A NEW EROSION CYCLE IN THE GRAND CANYON 

 DISTRICT, ARIZONA 



H. H. ROBINSON 



INTRODUCTION 



The Grand Canyon District of Arizona possesses a particular 

 interest for the geologist in that its history, at least from the begin- 

 ning of the Tertiary, must be interpreted almost entirely from 

 physiographic data; that is to say, the region has been subject 

 to erosion for so long a period that stratigraphic evidence is lack- 

 ing. The absence of such evidence, which long usage has fixed 

 as the conventional means of interpreting geologic history, may have 

 caused, perhaps, some doubt to be felt as to the reality of the 

 conclusions originally reached by Dutton in 1882 in regard to the 

 history of the region, for at that time the data of physiography 

 were in a more or less embryonic state and their interpretative 

 value was hardly recognized. Indeed, Dutton's report on the 

 Tertiary history of the Grand Canyon District' was a pioneer 

 work in this branch of geology and its significance becomes increas- 

 ingly apparent with the passage of time. Today conditions are 

 changed, physiography has taken its place as a systematic science, 

 and the relation between topographic forms and the conditions 

 under which they may originate may be considered as resting on 

 a reasonably broad and well-established foundation. Thus con- 

 clusions based upon physiographic evidence are now accepted as 

 of equivalent value to those based on older and more conventional 

 lines of evidence, where twenty-five years ago they were looked 

 upon with skepticism by the majority of geologists who had not 

 had a physiographic training. That there was good reason for 

 this attitude is well illustrated by the new erosion cycle described 

 in this article, for the facts upon which it is based have been 

 known for twenty-five years without their significance being fully 



I Tertiary History of the Grand Canyon District, Arizona, with Atlas (Monogr. II, 

 U.S.G.S., 1882). 



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