122 Robinwn — Tertiary Peneplain of the Plateau District. 



Where crossed, the Salt River flowed in a canyon in the 

 Red "Wall limestone, the northern side of which, being- 

 descended after night-fall, was not examined. It was much 

 lower than the southern side, however, which reached a height 

 of nearly 1,800 feet above the stream. Of this about ten or 

 twelve hundred feet were of Red Wall, capped by some sand- 

 stone, the remainder being basaltic lava. . . The limestone 

 must all be affected by a strong, constant northeast dip, for, 

 passing over the high basalt top southward 2 or 3 miles, the 

 Lower Tonto (vitreous) sandstone is encountered at a height 

 equal to that of the limestone near the river, the whole thick- 

 ness of the latter, together with that of the Tonto shales, being 

 covered with lava." 



The Bradshaw Mountains, and the locality just described, 

 lie about 30 miles beyond the border of the plateau and appear 

 to mark the limit of the peneplain toward the south. That it 

 did not exist in the vicinity of Clifton, Arizona, nor apparently 

 at Globe, is indicated by the following quotation :* 



" The country was now (in the Tertiary) a land area with 

 rough topographic features, to which the faulting contributed 

 important elements. An active erosion following the epoch of 

 faulting has not been able to entirely efface its influence, and, 

 though obscured by the Tertiary lava flows, these fault carps 

 are still dominent features visible in the granite bluffs of the 

 Coronado and Copper mountains, and the down thrown valley 

 between them." 



It would seem, however, from the thoroughness of the plana- 

 tion of the plateau, as though in general erosion must have 

 reduced the relief of the Basin Range country to a mature 

 type, and it would follow that many intermontane valleys were 

 thoroughly graded contemporaneously with the development 

 of the peneplain in the plateau country. 



Conclusion. — The area covered by the remnants of the 

 peneplain in the southwestern part of the plateau is about 5,000 

 square miles. ' Such isolated masses of lava-capped red shale 

 and sandstone as Red Butte and others, at present unmapped, 

 clearly indicate that the peneplain originally extended over a 

 much larger region. The boundaries of this area may roughly 

 be considered on the west as the boundary of the plateau, on 

 the north and northeast the Kaibab Plateau and the high cliffs 

 that limit the Grand Canyon District and San Francisco Plateau 

 in those directions, on the south and southwest a line about 

 30 miles beyond the border of the plateau and parallel to it, 

 comprising in all some 20,000 square miles of country. In the 



* Lindgren, Copper Deposits of the Cliftou-Morenci District, Ariz., P. P., 

 No. 43, U. S. G. S., p. 95, 1905. 



