NEW EROSION CYCLE IN GRAND CANYON DISTRICT 759 



Verde Valley into the Black Mesa on its east side. At many points 

 in the Black Hills and the mountains the remnants of the plain 

 are capped by a basalt identical in character with that which 

 covers the peneplain on the Black Mesa. Also the amount of 

 erosion that has occurred since the basalt was erupted on the 

 plain east of the Bradshaw Mountains compares very favorably 

 with that of the post-peneplain cycle in the near-by present plateau 

 region.^ As both areas were situated in local drainage basins under 

 similar climatic control and as the strata involved, judging from 

 more recent results in the Grand Canyon of the Colorado, possessed 

 approximately equal erodability, it may be concluded that the 

 topography of the two areas was of contemporaneous origin. 



The date at which the Basin Ranges originated as tilted block 

 mountains possesses, therefore, a critical value, since it marks the 

 time between which and the present the most clearly known events 

 in the history of the Grand Canyon District occurred. This date 

 is fixed by the age of certain formations — Pah-Ute of King and 

 Siebert of Spurr — which were involved in the range-making move- 

 ment. A Miocene age was originally assigned to these beds by 

 King^ and this has since been concurred in by Spurr,^ Ball,'' and 

 Ransome.s The determination is not so positive as might be 

 desired, nor is it certain that the entire Miocene is represented. 

 In the absence of evidence to the contrary, however, it will be 

 assumed that the sedimentary beds involved in the range-making 

 do represent the whole of the Miocene. The probable correctness 

 of this assumption is indicated by following consideration. If the 

 strata involved in the mountain-making represented only the earlier 

 part of the Miocene and the faulting which gave rise to the ranges 

 as tilted block mountains occurred, for instance, in the middle of 

 the Miocene, then the succeeding formation should be of late 

 Miocene age, since the region during this time was a land area and 

 the deposits were of a local nature. On the contrary, however, the 

 next youngest formation is of Pliocene age ; it rests unconformably 

 on the older tilted strata of the ranges generally in a horizontal or 



' H. H. Robinson, op. cit., 120. 4 U.S.G.S., Bull. 308 (1907), 32. 



2 Explor. 40th Parallel, I, 412-24. s U.S.G.S., LXVI (1909), 66. 



3 U.S.G.S., XLII (1905), 66. 



