Robinson — Tertiary Peneplain of the Plateau District. 115 



impression that the peneplain as developed on the resistant 

 Upper Aubrey chert y limestone was of extremely low relief. 

 The fact, too, that so far as known no monadnocks rise above 

 the surface of the basalt covering the peneplain points to a 

 very small amount of relief, as the maximum thickness of the 

 lava, where shown in the walls of canyons and about the edge 

 of the field, is not over 100 feet. 



The peneplain may be actually traced over very considerable 

 areas by means of the basalt which caps its surface. A general 

 acquaintance with the region leads to the belief that the basalt 

 may be safely used to estimate the present and former extent 

 of the planation, since wherever the peneplain is unmistakably 

 present the basalt rests upon its surface. 



The peneplain may now be traced on the San Francisco 

 Plateau over the western side of the Little Colorado Valley 

 between the Atchison, Topeka and Santa Fe Ry. and Coconino 

 Point. The eastern boundary is everywhere one of erosion, 

 while westward the recent basalts cover its surface, the bound- 

 ary being an irregular line ranging from 8 to 15 miles distant 

 from the river. The exposures at Cedar Ranch and Anderson 

 Mesa are at the summit of easterly-facing mesa cliffs now 400 

 to 700 feet above the surface of the plateau. This means, of 

 course, that the peneplain once had a greater extension eastward, 

 though it cannot be stated positively that the surface was 

 formerly continuous between these localities and the Little Col- 

 orado Valley. From the section at Anderson Mesa it is possible 

 to trace the peneplain westward to the Aubrey Cliff, especially 

 as there is evidence of its existence under the old lava on the 

 down-faulted block of Moencopie strata at Sycamore Canyon. 

 The peneplain is definitely known to cover between 1,000 to 

 2,000 square miles and originally had a much greater exten- 

 sion in all directions, except the northeastern. In that direc- 

 tion, on the east side of the Little Colorado, higher land is 

 encountered within 10 to 15 miles, which must have marked 

 the limit of the peneplain. This is on the assumption that 

 there has been no marked later faulting or warping which 

 would raise the region east of the Little Colorado above that 

 on the west. This remains to be proved, but it is believed, 

 as the result of a study of the region west of the river, that 

 no large faults are present -and that the tilting has tended to 

 lower the region east of the river rather than raise it. 



Mohave Peneplain. — Huntington and Goldthvvait,* in 

 connection with their study of the Hurricane and Grand Wash 

 faults, have described an area of some 3,000 square miles, to 

 which they have given the name Mohave peneplain. The 

 evidence for planation, as in the San Francisco Mountain 



* Op. cit. , pp. 226-245. 



