124 Robinson — Tertiary Peneplain of the Plateau District. 



all are the eroded remnants of a single peneplain, with the 

 possible exception of the Little Colorado Valley area. There are 

 a few isolated facts regarding the actual elevation of the pene- 

 plain above the present plateau surface and the relation of the 

 different parts to one another, that point toward the existence 

 of two separate peneplains, the older of wide extent, the 

 younger local in character. The conclusions that would fol- 

 low were such the case, involving the extension of the older 

 peneplain over an area much larger than that described and 

 introducing greater detail into the geologic history of the 

 region, make it desirable not to go into this matter until 

 the facts that appear to support it have been verified and much 

 more field work has been done., 



Influence of the peneplain and later displacements upon ike 

 drainage system of the Grand Canyon District. — That the 

 Grand Canyon District was formerly thoroughly peneplained 

 appears certain, and the surface was developed for the most 

 part upon the soft strata-marls, shales and sandstones— of the 

 Moencopie and Shinarump 'formations. The slope, as deter- 

 mined by Huntington and Goldthwait for the Mohave pene- 

 plain, was toward the south, and it would seem natural that 

 the principal streams, following the maximum slope, likewise 

 should have had courses to the south or southwest. After the 

 faulting, which raised the peneplain above base-level, the well- 

 recognized tendency of streams to readjust themselves under 

 such conditions would become operative, and an entirely new 

 drainage system would develop. 



The effect of the faulting was to raise the eastern part of 

 the district above the western and apparently to produce a 

 slight, though not constant, tilting of the surface to the north. 

 The streams that would develop on this new surface might at 

 first be controlled, for instance, by the lava-capped areas, but 

 eventually they would cut down to the Upper Aubrey cherty 

 limestone, or other resistant rock. Upon this they would either 

 he superposed or become adjusted to the structure produced 

 by the folding and faulting. The large streams would most 

 likely be partly superposed, partly subsequent, while the small 

 streams would be subsequent, provided the rate of uplift were 

 not too rapid. 



The Colorado River may be considered as a superposed 

 stream in the vicinity of the Kaibab Plateau. The peneplain 

 died out against the flanks of the Kaibab at what is now an 

 elevation of about 7,000 feet, though originally, of course, the 

 elevation was much less. When the faulting and tilting occur- 

 red a depression was formed between the peneplain and upland 

 that marked out the course of a master stream. As a result 

 of this uplift there would be a slight ponding on the east side 



