62 RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
crustal movements or may have occurred as accompaniments of the 
initial movements and later have been tilted to their present position. 
Effect of uj)lift on Colorado River. — The lines of faulting and the 
axes of uplift were developed across Colorado River and no doubt 
materially changed its course. Whatever this may have been 
previously, the uplift of the plateau finally established its direction 
by causing it to erode the Grand Canyon. In the low-lying region 
west of the plateau the river flowed across the Grand Wash Trough, 
previously filled with the Greggs breccia, across the truncated edges 
of the upturned sediments now exposed in Iceberg Canyon, across a 
second debris-filled trough near Hualpai Wash, through the crystalline 
rocks of the Virgin Mountains, and into the previously formed Detrital- 
Sacramento Valley, which it apparently followed thence to the sea. 
The reasons previously given in the detailed descriptions for 
believing this to be the river's former course may be briefly sum- 
marized as follows: Boulder and Black canyons west of the Detrital- 
Sacramento Valley are apparently younger than Virgin and Iceberg 
canyons. Remnants of the Temple Bar conglomerate, which occurs 
typically as the filling of the Detrital-Sacramento Valley, are found in 
Virgin and Iceberg canyons in positions that indicate deposition 
after these canyons were eroded; no such occurrence was observed 
in the canyons west of the Detrital-Sacramento Valley. There are 
gravel formations similar to the Temple Bar conglomerate west of the 
Black Mountain range, but they were apparently deposited previous 
to the erosion of Boulder and Black canyons. 
QUATERNARY (?) EVENTS. 
First canyon cutting. — During the first epoch of erosion that 
followed the uplift of the plateau Colorado River eroded Grand 
Canyon to a depth of 5,000 feet or more beneath the Mohave pene- 
plain. West of the plateau it cut to a depth of somewhat more than 
1,400 feet beneath this plain in Iceberg and Virgin canyons. The 
erosion in the Detrital-Sacramento Valley was probably correspond- 
ingly great, but little is known of this on account of the detrital 
filling of the valley which has not been removed. 
The reasons for departing from the usage of former writers who 
assign the erosion of Grand Canyon to late Tertiary time are twofold : 
First, the Temple Bar conglomerate, deposited immediately after the 
first epoch of canyon cutting, is regarded as a Quaternary deposit for 
reasons stated in the section next following; second, the crustal 
disturbances resulting in the elevation of the plateau and the erosion 
of the Grand Canyon were of such magnitude and far-reaching 
influence in effecting radical changes in the geography and geology 
of the region that, considering this region alone, without reference to 
established subdivisions, they appropriately constitute the separation 
