76o H. H. ROBINSON 



but slightly disturbed position. On this basis, then, the Basin 

 Ranges originated as tilted block mountains at the close of the 

 Miocene. 



It is to be noted that the relief produced by the extensive 

 faulting, which followed in the present plateau region next after 

 the Eocene folding, was entirely reduced in the development of the 

 peneplain. A very considerable interval, therefore, must have 

 intervened between the time of the faulting and the final degrada- 

 tion of the region to a peneplain. In view of the magnitude of the 

 faulting and its time relation to the peneplain, it seems permissible 

 to correlate it with that which gave rise to the Basin Ranges as 

 tilted block mountains, and which likewise followed a period of 

 folding. 



After the faulting at the close of the Miocene the newly formed 

 block mountains of the Basin Range region were attacked by erosive 

 forces and reduced to mature forms. Contemporaneously local 

 peneplains were developed, under favorable conditions, about the 

 footslopes of the ranges,^ while at greater distances — in the present 

 plateau region — a highly developed peneplain covered thousands 

 of square miles. There is at present no direct evidence to tell 

 when this erosion cycle came to an end, so that it is necessary to 

 make an assumption as to this date. It is the writer's opinion, 

 based on the character and extent of the erosion in the several 

 cycles through which the Grand Canyon region has passed, that 

 the close of the peneplain cycle should be placed at the end of the 

 Pliocene. This appears to be the most probable date when the 

 volume of the erosion during the peneplain cycle and the wide- 

 spread base-leveling are compared with the extent and nature of the 

 erosion during the post-peneplain and canyon cycles. The point 

 is one that is difficult to settle and may always remain, perhaps, a 

 matter of individual judgment. 



As the result of the foregoing assignment of dates, the post- 

 peneplain and canyon cycles of erosion, with the pronounced 

 faulting that came between them, are placed in the Quaternary. 

 This is at variance with previous ideas. Button, for instance, 

 spread the canyon cutting alone over both the Pliocene and Quater- 

 nary;^ a previous correlation by the writer confined it to the 



^ Ball, op. cit., 41. 2 ii)ifi^^ chap. xii. 



