Davis — Notes on the Colorado Canyon District. 255 



tainly the surface of the esplanade, as seen in the magnificent 

 prospect from Vulcan's Throne, where the canyon cuts the 

 Toroweap valley, coincides closely with the top of the Red 

 Wall series. Further up the canyon, the topographical maps 

 show the esplanade in Kanab canyon on the north and in Cata- 

 ract canyon on the south, but its distinctness lessens eastward 

 and it disappears before entering the Kaibab. Where we saw 

 the canyon in the Kaibab, its cross section was altogether dif- 

 ferent from that, where the esplanade is well developed in the 

 western Kanab. In the Kaibab there are two structural 

 benches, as shown in fig. 1 : one is determined by the Red 

 Wall limestone, the other by the Tonto sandstone ; and of the 

 two the latter is rather more pronounced than the former. The 

 change in form from the Uinkaret to the Kaibab seems to 

 result from a weakening of the strata that are included between 

 the firm limestones of the Red Wall and the firm sandstones 

 of the Tonto. This change in structure is fully recognized by 

 Dutton, and he explains the difference of form between the 

 Uinkaret and the Kaibab sections by the greater altitude of the 

 Kaibab during the erosion of the canyon (1. c, 257, 258). He 

 does not, however, explain the absence in the Kaibab section of 

 all signs of the pause during the general uplift by which he 

 explained the esplanade further west. To my view, the 

 esplanade, like the Kaibab benches, seemed to be entirely of 

 structural origin ; it does not seem to be necessary to postulate 

 a pause during uplift in order to account for it. 



The Toroweap is a broad valley that has been eroded along 

 the fault that divides the Kanab and the Uinkaret plateaus, 

 north of the canyon. A similar valley, which I have desig- 

 nated in my notes as the South Toroweap, is seen on the 

 southern side of the canyon. Both of these valleys are flat- 

 floored and open on the canyon close to the level of the espla- 

 nade. Dutton concluded that the Toroweap is the broadened 

 valley of an ancient river which disappeared by reason of 

 change from humid to arid climate before the inner canyon was 

 cut. He recognizes that the valley has been somewhat shal- 

 lowed by lava flows from the volcanoes of the Uinkaret on the 

 west, but does not attach much importance to this fact (1. c, 

 92, 99). It does not seem necessary to adopt Dutton's conclu- 

 sion as to climatic change, for the lavas in the floor of the 

 Toroweap appeared to be in sufficient volume to account for 

 its shallowness in contrast to the depth of Kanab and Cataract 

 canyons a little further east. 



Date of faulting. — The displacements by faults and mono- 

 clines, resulting in the division of the plateau region into a 

 number of huge blocks, Shiwits, Uinkaret, Kanab, Kaibab, 

 etc., have been variously dated by different observers. Powell 



