NEW EROSION CYCLE IN GRAND CANYON DISTRICT 757 



The history of the Grand Canyon District, with the post- 

 peneplain cycle of erosion introduced, may now be summarized 

 as follows: 



I. Period of folding and flexing. 



II. Erosion cycle. 



III. The first period of faulting. A period of extensive faulting. 



IV. The peneplain cycle of erosion. This cycle closed with the widespread 

 development of a peneplain. Relief produced by faulting (III) entirely obliter- 

 ated. Widespread volcanic activity, marked by the eruption basalt, occurred 

 shortly after the development of the peneplain and while the region still stood 

 close to sea-level. 



V. The second period of faulting. Faulting probably of less magnitude 

 than that of the first and third periods. 



VI. The post-peneplain cycle of erosion. Widespread stripping of Permian 

 and Triassic strata and development of a mature topography on the under- 

 lying beds, principally the upper Aubrey limestone, at a horizon ranging from 

 zero to 1,000 feet below the level of the peneplain. Further retreat of the high 

 cliffs on the north and east sides of the district. Land stood at no great height 

 above the sea. 



VII. The third period of faulting, with broad regional uplift. Region 

 raised from 4,000 to 6,000 feet above the position it occupied at the close of 

 the post-peneplain cycle. 



VIII. The canyon (present) cycle of erosion. Marked by the develop- 

 ment of a canyon system of drainage of extreme youthfulness. Refreshing 

 of cliff profiles. Erosion otherwise very slight. 



The history above outlined is considerably more complex than 

 that presented by Dutton and is believed to be complete so far as 

 the principal events are concerned. It differs from previous 

 interpretations in introducing the post-peneplain cycle of erosion, 

 and it is felt that this cycle rests on evidence fully as conclusive as 

 that which establishes the separate existence of the peneplain and 

 canyon cycles of erosion. It removes the anomaly in the previous 

 explanations of the widespread stripping of a very considerable 

 thickness of Permian and Triassic strata and development of a 

 mature topography on the resistant upper Aubrey limestone in the 

 same cycle that has witnessed the cutting of the youthful canyons. 



POSITION OF EVENTS IN GEOI.OCxIC TIME 



While the history of the Grand Canyon region, on the whole, 

 possesses a very reasonable definiteness, the problem of placing the 

 several events which comprise it in geologic time is quite unsatis- 



