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. 9-1). 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 insuffi- 

 cient. A much more satisfactory explanation is found in the sup- 

 position that the broad esplanade of the canon between the upper 

 palisades was an ancient base-level of erosion" (b, 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 

 vol. xxxviii. — no. 4. 6 



