NEW EROSION CYCLE IN GRAND CANYON DISTRICT 747 



of great denudation. The peneplain was developed over much of 

 the region on the soft Permian and Triassic strata and owes its 

 preservation to a capping of basalt. The extent of the peneplain 

 and the fact that it is developed across the basset edges of strata 

 varying in hardness from a compact sandstone to a barely consoli- 

 dated marl, as at Black Point in the Little Colorado Valley and 

 elsewhere, force the conclusion that a long period of time is repre- 

 sented, that the base of control was an oceanic body of water, 

 and that consequently the region stood approximately at sea- 

 level. 



The history of the region, following Huntington and Gold- 

 thwait, may now be tabulated as follows : 



I. Period of folding and flexing. 



II. Erosion period. 



III. The first faulting. 



IV. Inter-faidt cycle of erosion. Region reduced to a peneplain at close. 



V. The later faulting. 



VI. The post-fault canyon cycle of erosion. 



Stripping of weak strata down to the upper Aubrey cherty limestone. 

 Cutting of deep canyons. Refreshing of cliff profiles. 



As bearing on what is to follow it may be noted that Davis, 

 and also Huntington and Goldthwait, considered that the peneplain 

 developed' on the Permian and Triassic formations marked the 

 close of the period of great denudation and that the stripping of 

 these strata and the consequent exposure of the upper Aubrey 

 limestone — the present surface rock of the region — occurred after 

 the uplift which introduced the canyon cycle. Button also 

 recognized the existence of a peneplain on Permian strata beneath 

 the basalt cap of Mount Trumbull and at other localities (p. 224), 

 and he also considered that the present surface of the plateau was 

 likewise developed at a base-level of erosion (p, 118). He did not 

 give these two planes of erosion distinct interpretative values, but 

 used them as common evidence that the region stood at a low 

 elevation at the close of the period of the great denudation. The 

 proper discrimination between these two planes of erosion, as will 

 appear in the sequel, gives the clue to an essential feature in the 

 history of the Grand Canyon District which has not thus far been, 

 fully recognized. 



