Pobinson — Tertiary Peneplain of the Plateau District. 109 



Art. XI. — The Tertiary Peneplain of the Plateau Dis- 

 tricts and Adjacent Country, in Arizona and New Mexico', 

 by H. H. Robinson. 



Introduction.— The, existence of a base-level of erosion, or 

 as it is now interpreted, a peneplain, in the Grand Canyon 

 District of Arizona at the close of the period of the great 

 denudation and just previous to the inauguration of the can- 

 yon cycle, was strongly insisted upon by Dutton in his "Terti- 

 ary History of the Grand Canyon District." His point of 

 view may be illustrated by the following quotation : 



" Thus there is a general accord of testimony that at the 

 period of the older basaltic eruptions very large bodies of Per- 

 mian strata lay upon the Carboniferous platform. In truth, 

 it seems as if the summit of the Permian then constituted the 

 surface of the country, just as the summit of the Carboniferous 

 does now. The fact that the older basalts, wherever found, 

 rest upon the same geological horizon, suggests to us the fur- 

 ther inference that the region near the (Colorado) river was 

 then flat and destitute of deep canyons and valleys, such as 

 now exist there, and, therefore, destitute of great hills, buttes, 

 or mesas. The meaning of this is a base-level of erosion .... 

 This, it is true, looks at first like drawing a very broad and 

 rather remote inference from a very slender basis, and would 

 not be justified at all if it were not in general harmony with a 

 wide range of facts. Many facts take form and coherence 

 around it which would otherwise seem mysterious."* 



It is desired to present in this article evidence from the 

 vicinity of the San Francisco Mountains, which are located on 

 the San Fi*ancisco Plateauf south of the Grand Canyon, in sup- 

 port of Dutton's conclusion, and to show that the peneplain he 

 described existed not only in the Grand Canyon District, but 

 extended over the greater part of the southern Colorado Pla- 

 teau and into the region south of the present plateau, now 

 known as the Mountain District of Arizona. And it is further 

 desired to point out the influence that this peneplain, in con- 

 nection with later faulting, may have had upon the drainage 

 system of the Grand Canyon District. 



Tertiary History of Southern Plateau District. — Before 

 describing the several localities where this ancient peneplain is 

 now exposed, a brief summary of the Tertiary history of the 



*Mon. II, p. 224, U. S. G. S., 1882. 



f Name applied to that portion of the Colorado Plateaus, exclusive of 

 Coconino Plateau, bounded on the north by the Colorado Eiver, on the east 

 by the Little Colorado and on the south and west by the Aubrey Cliff. 



An. Jour. Sci.— Fourth Series, Vol. XXIV, No. 140.— August, 1907. 

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