750 H. H. ROBINSON 



as present on the Kanab Plateau. Apparently they are best 

 preserved in those localities where they were developed on the 

 upper Aubrey cherty limestone. 



The valleys of the Kaibab (and also the Coconino) Plateau 

 have been described by Davis as follows: 



The limestone [upper Aubrey] capping these plateaus is maturely dis- 

 sected. Broad-floored, well-graded valleys with gently sloping sides ramify 

 through the uplands in a 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 accom- 

 plished only during the rains and thaws of winter and occasional showers of 

 summer [a, p. 120]. 



The above description embraces the valleys of the Coconino 

 Plateau and also applies without alteration to the similar valleys 

 farther south in the vicinity of the San Francisco Mountains. 

 There is, therefore, little doubt, as will appear from later consider- 

 ations, that all these valleys were developed in the same cycle 

 of erosion. It may be noted, moreover, that, as these valleys had 

 maturely developed form even to their heads, their lower courses 

 must have originally possessed at least equal maturity of form, 

 so that there is no doubt as to the thoroughly mature development 

 of the topography throughout the Grand Canyon District during 

 this cycle. 



The post-peneplain cycle of erosion is separated from the pre- 

 ceding cycle of the great denudation, which closed with the wide- 

 spread development of a peneplain, not only because of differences 

 in topographic expression, but primarily because the mature 

 topography occurs at a level distinctly below that at which the 

 peneplain is found. This is a fact that is perfectly evident at 

 many localities where structural complexities are absent. As, 

 however, the peneplain involves a considerable thickness of strata 

 ranging from the upper Aubrey (upper Carboniferous) limestone 

 through Triassic formations, while the mature topography, so far 

 as known, is developed, or at least preserved, only on the upper 

 Aubrey limestone, the actual difference between the two planes 

 of erosion is a variable one depending upon the thickness of Permian 

 and Triassic strata that may be present at any locality. 



