254 Davis — JSTotes on the Colorado Canyon District. 



lower stand of the region ; in other words, that the plateaus 

 are parts of a vast peneplain produced in a cycle of erosion 

 anterior to that in which the canyon has been eroded, the two 

 cycles being separated by an uplift of several thousand feet. 

 It has been pointed out by other observers (although I believe 

 that no statement of their opinion has been published) that the 

 supposed peneplain surface coincides roughly with the resistant 

 Upper Carboniferous strata over large areas, and hence that 

 the plateaus might be explained as a surface of halting or at 

 least of hesitating erosion in a single cycle, after the stripping 

 of the weaker overlying formations. It is not easy to make 

 definite choice between these two explanations, but I believe 

 Dutton's to be the more probable of the two, for such reasons 

 as the following. There is a mesa in the valley of the Little 

 Colorado, southeast of the road from Flagstaff to Tuba, capped 

 by a horizontal sheet of lava which rests unconformably on a 

 bevelled surface of tilted Permian shales ; but the shales are 

 elsewhere denuded to a lower level. Again, there are numer- 

 ous and extensive landslides at the base of the Echo and Ver- 

 milion (Triassic) cliffs south and west of Lee's Ferry; the 

 slides seem to indicate a revival of sapping after a long pause. 

 Further, the considerable length of many streams that flow 

 against the dip of the strata suggests the need of the most 

 favorable conditions for their development, such as would be 

 associated with two cycles of erosion rather than with only 

 one. Finally, the strong contrast between the gigantic young 

 ravines of the steep canyon walls and the shallow mature valleys 

 of the plateaus seems beyond explanation as dependent on the 

 resistance of the Upper Carboniferous strata during a single 

 cycle of erosion. But much more work should be done before 

 this question can be definitely settled. 



The Esplanade. — The canyon, where traversing the Uinkaret 

 plateau and the greater part of the adjacent Kanab, consists of 

 a broad and flat-floored upper valley, beneath which a deep 

 and narrow chasm has been cut. Dutton terms these two parts 

 the upper (or outer) and the inner canyons, and gives the name, 

 esplanade, to the well developed floor of the upper canyon. He 

 explains the esplanade as the result of valley-broadening dur- 

 ing a pause in the general uplift in consequence of which the 

 plateaus have been dissected : in other words, after a first cycle 

 in which the plateaus were broadly denuded, a second cycle 

 was introduced by general uplift of the region ; but this uplift 

 was interrupted by a pause long enough for the erosion of the 

 upper canyon (1. c, 121). Other observers have suggested that 

 the esplanade is a structural bench determined by the resistance 

 of the Red Wall limestone, from which the overlying weak 

 lower Aubrey sandstones and shales have been stripped. Cer- 



