116 Robinson — Tertiary Peneplain of ' tJa Plateau District. 



region, consists largely of areas of smoothly beveled strata of 

 widely different degrees of hardness, which have been pro- 

 tected by basalt. A typical case is that of Bellevue Ridge. 1 

 miles south of Dry Canyon (figure 3), concerning which the 

 authors say : 



" Here the level surface on which lies the uplifted lava is 

 composed of Moencopie shale, Shinarnmp conglomerate, 

 Painted Desert shale and sandstone, and Kanab sandstone, all 

 dipping strongly to the east. Where the strata are exposed to 

 erosion the hard Shinarnmp and Kanab form strong cuesta- 

 like ridges separated by valleys excavated in the soft Moencopie 



Fig. 8. Bellevue Ridge. 

 1 Upper Aubrey limestone, 2 Moencopie shales, 3 Shinarump congl. 

 4 Painted Desert shale and sandst., o Kanab sandst., 6 Basalt. 



and Painted Desert, Only under conditions approaching 

 closely to base-levelling would it be possible for a level surface 

 such as that beneath the lava to truncate smoothly strata of 

 such varying hardness." 



The surface of the peneplain has been displaced, as shown 

 in the figure, from 1,000 to 1,500 feet by the recent Hurricane 

 fault, which belongs to the second period of faulting. 



The Mohave peneplain, as described, did not extend beyond 

 the present western boundary of the plateau, as the Basin 

 Range country consisted of u ancient mountains well dissected 

 and mature, and presenting nearly the same appearance as to- 

 day." To the east, in the vicinity of Toqnerville, the High 

 Plateaus marked the boundary, but farther south the peneplain 

 presumably extended eastward to the Kaibab plateau. 



Black Mesa. — Black Mesa is the name applied to the elevated 

 belt of country on the San Francisco Plateau, which extends 

 from the San Francisco Mountains as far as Clear Creek Canyon, 

 a distance of 50 miles. The eastern boundary of the mesa is an 

 erosion cliff that has an average height of 500 feet ; the western, 

 except for the part along Oak Creek Canyon, is more strikingly 

 marked by the Aubrey Cliff — the limit of the plateau — which 

 rises fully 1,000 feet above the Yerde Valley. The surface of 

 the mesa is entirely covered by basaltic lavas, both of the older 

 and more recent periods of eruption, and diversified by many 



