NEW EROSION CYCLE IN GRAND CANYON DISTRICT 745 



Davis/ as the result of two trips through the region, advanced 

 several pertinent arguments supporting the broad conclusion of 

 Button that the Grand Canyon District had experienced two 

 cycles of erosion separated by a period of pronounced uplift. In 

 the absence of local fossiliferous evidence he designated the 

 earlier period of the great denudation as the plateau cycle and the 

 later one, in which the canyon cutting occurred, as the canyon 

 cycle. Especially suggestive was his argument in favor of two 

 cycles based on the relatively great retreat of the cliffs from the 

 canyon on its north side as compared with the slight retreat of 

 the upper walls of the canyon from the river, the correctness of 

 which was later confirmed by independent evidence {a, p. 118). 

 He pointed out that the esplanade of the Kanab and Uinkaret 

 sections of the canyon was best explained as a structural feature 

 and from a variety of considerations concluded "that while many 

 partial cycles of erosion may have preceded the long pause during 

 which the broad denudation of the plateaus was completed, only 

 a single uplift and a single downcutting are recorded in the canyon" 

 {a, p. 185). This conclusion has since been confirmed by Dr. 

 L. F. Noble as the result of detailed field work in the canyon in 

 the vicinity of Bass's camp,^ 



Of critical significance was the recognition by Davis of an 

 important period of faulting during the plateau cycle of erosion 

 distinctly separated from the later faulting at the beginning of the 

 canyon cycle. Definite evidence was obtained that relief of at 

 least 1,000 feet, due to this faulting, was obliterated during the 

 plateau cycle and that by the end of that cycle the region in gen- 

 eral had been reduced to a peneplain. 



In speaking of the reduction of the region to a peneplain at the 

 close of the period of great denudation, Davis says: 



It therefore seems legitimate to say that the peneplain, so far as one was 

 developed at the close of the first cycle, lay in the Permian formation at some 

 unknown height above the present plateau surface in the Kanab district; and 

 that the Carboniferous platform as now exposed in the Kanab Plateau is a 



1 a) "An Excursion to the Grand Canyon of the Colorado," Bull. Mus. Comp. 

 Zoology, Harvard College, XXXVIII, May, 1901; b) "An Excursion to the Plateau 

 Province of Utah and Arizona," ibid., XLII, June, 1903. 



2 Unpublished manuscript. 



