GEOLOGIC HISTORY. 61 
argument in favor of its formation mainly by river erosion is the 
presence, throughout its length, of sands and gravels similar to those 
known to have been deposited by aggrading streams. 
Relation of Colorado River to the Detrital-Sacramento Valley. — The 
influence of Colorado River has not been considered thus far for the 
reason that no clear evidence has been obtained of its presence within 
the area described, The erosion previously accomplished may have 
been the work of the Colorado, or may have been accomplished by 
streams now extinct. But with the erosion of the Detrital-Sacra- 
mento Valley the recorded history of the Colorado in western Arizona 
apparently begins. The river changed its course within the area 
described during Quaternary time, and there are indications that 
still greater changes occurred in late Tertiary time; but in order to 
determine the nature and extent of the changes, it would be necessary 
to examine a large part of the Plateau region, where, on account of 
recent faulting, uplift, and subsequent erosion, the investigation 
would be difficult. There was apparently no great difference in 
elevation between the Basin region and the southern part of the 
Plateau region during mid-Tertiary time, the Mohave peneplain 
extending continuously over both. The evidence of peneplanation 
in the Meadow Valley and in the Toquerville region, and the general 
trend of the broad valley of the Virgin heading into this region, 
suggest that the Colorado may have flowed across what is now the 
Plateau region to the north of its present course and through the 
valley now occupied by the lower part of the Virgin into western 
Arizona, where it excavated the Detrital-Sacramento Valley. A large 
part of Tertiary time must have been consumed in eroding this great 
valley. If the formation of the peneplain be correctly assigned to 
the Miocene, and the extrusion of the rhyolites and younger andesites 
to the close of this epoch, the erosion of the Detrital-Sacramento 
Valley must have been accomplished in the Pliocene. 
Uplift of the plateau. — The Plateau region seems to have been 
not notably higher than the area west of it until the end of the epoch 
during which the Detrital-Sacramento Valley was excavated. But 
at this time it was greatly elevated and faulted. Extensive dis- 
placements occurred along its western border, forming the Grand 
Wash, Cottonwood, and Aquarius cliffs. Probably the tilting of 
the great crust blocks like that of the Hualpai Mountains occurred 
about the same time. The block at Iceberg Canyon, previously 
upturned, was again tilted, together with the overlying Greggs 
breccia. The Cerbat Mountains were probably uplifted to some 
extent, but the principal movement there seems to have been down- 
ward, forming Hualpai Valley. About the same time flows of 
basalt occurred in the Detrital-Sacramento Valley west of Chloride, 
in the Cerbat Mountains, and elsewhere, which may antedate the 
