376 Noble — Geology of the Grand Canyon, Arizona. 



western part of the area the Tonto platform fades out,fthe 

 river closes in a deep inner gorge in the Esplanade, andjthe 

 profile of the canyon becomes that of the Kanab division, — 

 the view of the canyon westward from Bass Camp discloses a 

 broad expanse of Esplanade for fifteen miles, through which 

 the inner gorge sweeps in two great meanders almost from wall 

 to wall. In the center of the Shinumo area the Esplanade is 

 rather more dissected than in the Kanab division farther west, 

 but its identity as a topographic feature is already well 

 established. 



The floors of the Esplanade and Tonto platforms, like the 

 surfaces of the plateaus, are structural surfaces developed 

 everywhere at the same horizon on the underlying formation. 

 In conformity with the dip of the rock system they likewise 

 slope to the southwest at the rate of about 200 feet to the 

 mile. 



Another topographic contrast is presented in the difference 

 in dissection of the two sides of the Grand Canyon, a feature 

 which is characteristic of the entire pathway through the 

 Kaibab. The north rim lies three times as far back from the 

 river as the south rim ; the great amphitheaters with their 

 limiting promonotories extending far into the canyon, the 

 buttes and temples, and the deep lateral gorges all belong to 

 the north side of the canyon. The south wall presents a 

 simple aspect : the side gorges rarely extend back into the rim 

 of the canyon, there are few buttes and outliers, and the great 

 amphitheaters are wholly lacking. Compared with the fan- 

 tastic topography of the north side, the scenic effect of the 

 south wall is precipitous and somber. 



The Colorado River enters the area in the southeast corner, 

 flows northwestwardly to the center of the area, and turns 

 sharply west at the point where it is joined by the Shinumo. 

 The Shinumo is the only living tributary in the area, and is 

 the master stream that drains the Shinumo amphitheater of the 

 north wall. It is a stream of clear water of the same order of 

 magnitude as Bright Angel Creek twenty miles east, and is in 

 striking contrast to the muddy Colorado. 



The topography of the Shinumo amphitheater, besides pre- 

 senting an equal development of both the Esplanade and Tonto 

 platforms, is remarkable in another way. In the other great 

 amphitheaters of the Kaibab division the master gorges trend 

 to the southwest ; the tributary gorges trend in the same general 

 direction and lateral gorges perpendicular to the main axes of 

 the amphitheaters are of minor development. In the Shinumo 

 amphitheater the lateral gorges have become the dominant 

 feature, so that the main axis of the amphitheater trends to the 

 northwest at right angles to the course of the master stream 



