112 Robinson — Tertiary Peneplain of the Plateau District. 



tions took place principally before or during the period of 

 glaciation that is marked by the morainal material and waste 

 fans of San Francisco Mountain, while a few of the cones and 

 flows are of very recent geological age, and may possibly date 

 from historic time. To this same period of eruption are 

 assigned the recent basalts, not only on the plateau, but in the 

 surrounding Basin Range country. They may be easily dis- 

 tinguished in the great majority of cases from the older basalt 

 capping the peneplain by their freshness, nneroded condition, 

 and the fact that they have not suffered displacement by the 

 faulting of the second period. 



The history of the Plateau as outlined differs from that of 

 Dutton* for the Grand Canyon District in carrying the period 

 of the great denudation through the Pliocene and of restrict- 

 ing the erosion of the present Grand Canyon to the Quater- 

 nary. The fact that the relief produced by the faults of the 

 first period has been effaced, while that resulting from faults 

 of the second period has not, appears to make this change 

 necessary, if but one period of peneplanation exists. The date 

 of the second period of faulting, which inaugurated the canyon 

 cycle of erosion at the close of the Pliocene, is the same as 

 that assigned by Dutton for the cutting of the inner gorge of 

 the Grand Canyon, and is coincident with the very general 

 uplift that affected the western part of the United States at 

 that time. All recent work in the plateau region has shown 

 that numerous complications exist that were not originally sus- 

 pected and that have not as yet received sufficient attention to 

 permit their being given proper weight. With the history of 

 the region thus incompletely, if not incorrectly, outlined, the 

 description of the peneplain developed at the close of the 

 period of the great denudation may be taken up. 



Fig. 1. Black Point Monocline. 

 1 Upper Aubrey, 2 Moencopie, 3 Shinarunip, 4 Basalt. 



San Francisco Plateau. — On the San Francisco Plateau 

 the peneplain is graphically displayed in natural cross-section 

 at Black Pointf in the Little Colorado Valley by the erosion 

 of the strata along the Black Point monocline. 



The strata visibly involved in the monocline extend from 

 the Upper Aubrey through the Moencopie and certainly well 



* Op. cit. (a), chap. xii. 



f A high point eight miles north of Black Falls on the Little Colorado 

 River ; it is uot shown on the maps. 



