DETAILS OF VALLEYS. 45 
which are composed of granite overlain by eruptive rock (section 0, 
fig. 9). The eruptives west of the canyon were not examined closely, 
but resemble in general appearance the rhyolite and andesite to the 
north and the great masses of similar rock composing the Chocolate 
Mountains to the south. The eruptive rock east of the river is the 
basalt which caps the Temple Bar conglomerate, as described and 
illustrated from Williams Canyon (fig. 16). 
East of Aubrey Canyon, and exposed in the bluffs of Williams 
Canyon (see p. 54), is an old valley filled to a depth of about 1,000 
feet with the Temple Bar conglomerate and covered with 800 feet 
of basalt. Aubrey Canyon is younger than this gravel -filled valley, 
having been cut through the basalt sheet covering the Temple Bar 
conglomerate. 
The rock channel in Aubrey Canyon has been cut to a considerable 
depth beneath the present river level, and filled with sand and 
gravel. Borings made for the Reclamation Service near the head of 
the canyon failed to reach bed rock at a depth of 75 feet. In the open 
basins the river is building flood plains, and the sediment accumu- 
lated within the canyon evidently corresponds with that in the open 
basins, but the maximum depth of this most recent deposit is no- 
where indicated. 
Here, as elsewhere along Colorado River, there is abundant evi- 
dence of a complex physiographic history. A broad valley, described 
elsewhere as the Detrital-Sacramento Valley (see p. 52), was formed 
east of Aubrey Canyon and filled to a depth of 1,000 feet or more with 
sand and gravel — the Temple Bar conglomerate. This was covered 
with 800 feet or more of basalt, and a new canyon was later eroded 
west of the old one (section 0, fig. 9). Still later this canyon was 
filled with several hundred feet of sand and gravel — Chemehuevis 
gravel — which were partly carried away by renewed erosion that 
cut the canyon to a depth lower than the present river level. This 
channel was in turn filled to its present condition. 
GREAT COLORADO VALLEY. 
The Great Colorado Valley was examined only in the immediate 
vicinity of the river during a somewhat hurried excursion by boat, 
but the data obtained are supplemented to some extent by the 
investigations of the Reclamation Service. The valley is the largest 
of the basins through which Colorado River flows, extending, with 
varying width, from Aubrey Canyon southward to the Choco- 
late Mountains, a distance of about 75 miles. The Riverside and 
Halfway mountains form a partial boundary on the west, and the 
Dome Rock and other mountains on the east; but between the moun- 
tain groups occur broad grades, such as the Cactus Plain. 
