JOHN WESLEY POWELL DAVIS 



opment when it is first to be applied, because of the irregu- 

 larity of its form, and because of its slow changes as the con- 

 trolling stream lines are worn down to gentler slopes. Nat- 

 urally enough, the more complicated meaning has been little 

 used. The simpler meaning now prevails, under which base- 

 level may be concisely defined as the "level base" with respect 

 to which river erosion is performed, determined either by sea- 

 level in the most general case or by a rock-sill or lake surface 

 or basin floor in various special cases. The idea thus presented 

 is discoverable, provided the reader is already acquainted with 

 it, as an implied factor of various explicit statements in the 

 writings of certain earlier authors ; but Powell makes it wholly 

 explicit, and indeed sets it forth in a very striking and appealing 

 manner. Moreover, he gave its leading element a handy name, 

 as he did in the case of antecedent rivers, with the result of 

 rapidly promoting a clear and general understanding of a prin- 

 ciple of prime importance in the rational study of land forms. 

 Simple as the principle here involved really is, the explicit an- 

 nouncement marks an era in rational physiography. 



A second step of great importance followed from the first, 

 as already intimated. The massive structures on which ero- 

 sional processes operate having been conceived, the erosional 

 processes themselves having been analyzed, and the baselevel 

 with respect to which they work having been recognized, the 

 successive steps in the progress of their work naturally became 

 the subject of study. Powell clearly saw that mountain forms 

 are not the result of disorderly and individual uplift, but of 

 erosion. "The mountains were not thrust up as peaks, but a 

 great block was slowly lifted, and from this the mountains were 

 carved by the clouds patient artists, who take what time may 

 be necessary for their work. We speak of mountains forming 

 clouds about their tops ; the clouds have formed the mountains'' 

 ( Colorado River, 154). This had been recognized by others.__ 

 but Powell went further. "The first work of rains and rivers 

 is to cut channels and divide the country into hills, and perhaps 

 mountains, by many meandering grooves or watercourses, and 

 when these have reached their local base levels, under the ex- 

 isting conditions, the hills are washed down, but not entirely 

 carried away" (Colorado River, 204) that is, a lowland of 



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