NEW EROSION CYCLE IN GRAND CANYON DISTRICT 751 



In the vicinity of Flagstaff, close to the boundary between the 

 upper Aubrey limestone and Moencopie (Permian) formation, the 

 difference between the two planes of erosion is not over 200 feet. 

 Along the eastern side of the Black Mesa, farther south, it ranges 

 from 400 to 600 feet. At Cedar Ranch, 15 miles northerly from 

 San Francisco Mountain, it has increased to 700 feet, while at Red 

 Butte, nearer the canyon, it reaches a maximum of about 1,000 

 feet. Corresponding differences between the two planes of erosion 



Fig. 2 



occur on the north side of the canyon. In the southwest section 

 of the plateau the difference is practically nothing, since the pene- 

 plain was developed on the upper Aubrey limestone and the forces 

 of erosion, which were powerful enough to remove the in general 

 weak Permian and Triassic strata elsewhere, had little effect on 

 the resistant basalt capping the peneplain and on the under- 

 lying limestone. The only locality in the district which appears 

 to have surely risen above the peneplain at the close of the period 

 of the great denudation is the Kaibab Plateau. The surface 



