NEW EROSION CYCLE IN GRAND CANYON DISTRICT 763 



since the beginning of the canyon cycle of erosion they must have 

 been very unfavorable for the existence of any glaciers during 

 the post-peneplain cycle when the region stood nearer sea-level, 

 unless the estimates of temperature during glacial time are much 

 in error. It may be concluded, then, that San Francisco Mountain 

 has experienced only the one period of glaciation which occurred 

 in late Quaternary time. 



The Tertiary history of the Grand Canyon District, with geologic 

 dates assigned as given in the preceding pages, may be summarized 

 in conclusion as follows: 



I. Period of folding and flexing during the latter half or at the close of the 

 Eocene. 



II. Erosion period during the Miocene. 



III. First period of faulting at close of the Miocene. A period of extensive 

 faulting. It is correlated with the faulting that gave rise to the Basin Ranges 

 of southern Nevada as tilted block mountains. 



IV. The peneplain cycle of erosion during the Pliocene. The Miocene 

 and Pliocene erosion, which are considered as constituting the latter and greater 

 part of the Period of the Great Denudation, closed with the widespread develop- 

 ment of a peneplain. This is correlated with the mature topography and 

 local peneplains of the Basin Range country of southern Nevada and of Arizona. 

 Relief produced by previous faulting (III) largely and at some localities entirely 

 obliterated. Widespread volcanic activity, marked by the eruption of basalt, 

 occurred shortly after the development of the peneplain and most probably 

 while the region stiU stood close to sea-level. 



V. The second period of faulting at the close of the Pliocene. Movements 

 probably of less magnitude than those of the first and third periods. 



VI. The post-peneplain cycle of erosion during the first part of the Quater- 

 nary. Widespread stripping of Permian and Triassic strata and development 

 of a mature topography of low relief, principally on 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, during the 

 middle or latter part of the Quaternary. Region raised from 4,000 to 6,000 

 feet above the position it occupied at the close of the post-peneplain cycle of 

 erosion. 



VIII. The canyon cycle of erosion during the latter part of the Quaternary. 

 Marked by the development of a canyon system of drainage of extreme youth- 

 fulness. Refreshing of cliff profiles. Erosion otherwise very slight. Colder 

 atmospheric conditions prevailed during part of this cycle, at least, as indi- 

 cated by the existence of a small glacier on San Francisco Mountain. 



