120 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Tue Mature VALLEYS OF THE KalIBAB AND Coconino PLaTEAvus. — 
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 
