“DAVIS: THE GRAND CANYON OF THE COLORADO. 185 
very late date for the fault, as Dutton concluded when he wrote: “ It 
seems very plain that the outer chasm had been formed and attained 
very nearly its present condition before the [Toroweap] fault started ” 
(c, p. 94). So recent a date of faulting seems inconsistent with the 
evidence presented in the section on the Pipe spring fault. 
Conclusion as to the Origin of the Esplanade. — In view of these various 
considerations, it seems necessary to conclude 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 down-cutting are recorded in the canyon. It should be 
noted that Dutton considered this supposition, but rejected it. Speak- 
ing of the esplanade, he said: “ We might explain it by assuming the 
rocks of the inner gorge to be much more obdurate than those above. 
This is true in part, but still the difference in this respect is insuff- 
cient. A much more satisfactory explanation is found in the sup- 
position that the broad esplanade of the caiion between the upper 
palisades was an ancient base-level of erosion” (6, p. 121). In an- 
other place he explains the difference between the Kaibab and Kanab 
sections of the canyon in the following manner: ‘The causes which 
have produced in the Kaibab a topography differing so widely from 
that which is seen in the other divisions of the chasm may be readily 
explained. The Kaibab is now, and throughout the period of evolution 
of the chasm it always has been, higher than the other plateaus. Cor- 
rasion has, therefore, penetrated there more deeply than elsewhere. It 
has, moreover, laid bare the edges of the softer beds underlying the Red 
Wall, and the rapid decay of these lower beds has undermined and wasted 
the Red Wall to a great extent. In the other divisions of the chasm 
corrasion has only at a very recent period cut below this great series of 
hard limestones. . . . Besides the greater altitude leading to deeper 
corrasion, the climate of the Kaibab is moister, and the degrading forces 
are, therefore, more efficient ”’ (c, pp. 257, 258). From the other point 
of view, it seems as if these several considerations might legitimately 
be adduced to account for the esplanade as of structural origin ; for if 
differences of structure, altitude, and climate all lead to differences of 
form between the Kaibab and Kanab sections, there seems to be all the 
less need for explaining the esplanade in the Kanab plateau by a pause 
during the uplift. On the other hand, the earlier date of the Kaibab 
arch must have led to the erosion of the Aubrey limestones on its crest 
much earlier than the same strata were attacked in the erosion of the 
canyon further west. The Kaibab must have been trenched, as has 
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