748 ■ H. H. ROBINSON 



THE ADDITIONAL EROSION CYCLE 



The erosion cycle to which the remainder of this article will be 

 largely devoted followed after the development of the extensive pene- 

 plain at the close of the period of great denudation but before the 

 uplift which introduced the canyon cycle of erosion, and is to be 

 associated with the former cycle rather than the latter. It was 

 characterized by the widespread removal of a notable thickness of 

 Permian and Triassic strata and the development of a thoroughly 

 mature topography on the underlying resistant upper Aubrey cherty 

 limestone — the present surface rock of the region. It will be called 

 the post-peneplain cycle of erosion. The preceding cycle, which 

 closed with the reduction of the region to a peneplain, will be desig- 

 nated as the peneplain cycle, while the latest cycle, as formerly, 

 will be called the canyon cycle of erosion. 



A striking feature of the Grand Canyon District, which impresses 

 even the casual observer, is the pronounced contrast between the 

 broad expanse of the smooth or but gently undulating surface of 

 the plateau and the deep and precipitous walled canyons carrying 

 the present drainage, a contrast that is highly suggestive of different 

 conditions of origin. 



In the region south of the canyon, with which the writer is 

 particularly acquainted, the surface of the plateau is etched by 

 an extensive system of shallow valleys of thoroughly mature form. 

 They were first described by Newberry, in 1858, in the following 

 terms : 



Where we crossed this divide it had the character of an elevated plateau, 

 of which the surface has been considerably modified by erosion, and now pre- 

 sents many broad and shallow excavated valleys [p. 58]. 



Davis, in speaking of these valleys on the Coconino Plateau, 

 says: 



.... We were much impressed with the maturity of their graded sides 

 and floors, in contrast to the youthful expression of the precocious canyon. 

 .... Furthermore, the contrast between the rapid wasting of the cliff in the 

 canyon waUs and the slow change of the mature valleys on the plateau strongly 

 suggests that the processes represent different cycles of erosion [a, p. 120, and 

 Fig. 2]. 



These old valleys are also well developed farther south in the 

 vicinity of the San Francisco Mountains. Figs, i and 2 illustrate 



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