120 bulletin: museum of comparative zoology.. 



The Mature Valleys of the Kaibab and Coconino Plateaus. — 

 The upper Aubrey limestone caps the Kaibab plateau and the high 

 ground opposite to it south of the canyon, to which the name, Coconino 

 plateau, has been given. Powell refers to the southern highland as a 

 " companion, or twin plateau " of the Kaibab, but a separate name is 

 needed for it not only because the Grand canyon is cut down between 

 the two plateaus, but also because they are separated by a strong mono- 

 clinal flexure with dip to the northeast, of which some account will be 

 given in a later section. 



The limestone capping of these plateaus is maturely dissected. Broad- 

 floored, well-graded valleys with gently sloping sides ramify through the 

 uplands in the most perfect manner, presenting a maturely developed 

 form even to their heads ; and this in spite of the fact that they are 

 nearly always dry, for the wash of waste down their sides and along 

 their floors is accomplished only during the rains and thaws of winter 

 and the occasional showers of summer. It was of the southern or 

 Coconino plateau that Newberry wrote : " The surface has been con- 

 siderably modified by erosion, and now presents many broad and shallow 

 excavated valleys " (p. 58). Dutton describes the Kaibab plateau as 

 undulating " with rolling hills and gently depressed vales." The valleys 

 are open-floored with gentle descent ; they are innumerable, covering 

 " the whole broad surface of the plateau with an infinite network of 

 ramifications" (c, pp. 131, 134). Our party saw these well-established 

 drainage ways both north and south of the main canyon, and we were 

 much impressed with the maturity of their graded sides and floors, in 

 contrast to the youthful expression of the precocious canyon. Plate 2 

 shows one of these mature valley floors in the Coconino plateau : a hill 

 and farm of the farmer ant are in the foreground. If the upper Aubrey 

 were so resistant as to have prevented the widening of the canyon 

 while the heavy Triassic sandstones were stripped away for scores of 

 miles on the north and south, no such mature valley system should be 

 carved in the stripped Aubrey surface. 



Furthermore, the contrast between the rapid wasting of the cliff in 

 the canyon walls and the slow change of the mature valleys on the 

 plateaus strongly suggests that the two processes represent different 

 cycles of erosion. Dutton notes that, on the Kaibab rim of the canyon, 

 " we often find an old ravine suddenly cut off on the brink of an abyss, 

 and the continuation of the same ravine is seen upon the other side of the 

 amphitheatre " (c, p. 170). A similar encroachment of the canyon upon 

 the mature valleys of the Coconino plateau may be seen in the neighbor- 

 hood of Hance's camp. "We could also see, when looking up the canyon 



