DETAILS OF VALLEYS. 47 
Like the other basins described, the Great Colorado Valley contains 
gravels representing three distinct epochs of accumulation. The 
oldest, interpreted as belonging to the Temple Bar conglomerate, 
was observed near the northern end of the valley, where it is overlain 
by lava, as at Headgate Rock. Farther to the south, in the middle 
of the valley, it was not identified. 
The younger or Chemehuevis gravel occurs throughout the Great 
Colorado Valley in conspicuous terraced bluffs. The beds vary much 
in composition, from a sandy silt to coarse gravel, the finer material 
greatly predominating. The sand is regularly stratified, cross-bedded, 
and not distinguishable in physical character or composition from 
the beds forming at the present time over the flood plain. The 
Chemehuevis gravel extends for an undetermined distance back from 
the terraced bluffs, and form corrugated slopes illustrated in fig. 12. 
The washes entering at the sides of the valley have steeply graded 
floors, often extending back many miles toward the hills, and are ter- 
minated laterally by nearly perpendicular walls. These floors often 
join laterally, forming wide graded slopes. 
From the foot of the gravel bluffs (fig. 12) and extending through- 
out the valley with a maximum width of 12 miles and a length of 75 
miles are flood plains building up by the deposition of sand and silt. 
Over this great area Colorado River is continually shifting its chan- 
nel, sometimes gradually by slow cutting and filling and sometimes 
suddenly by turning through one of the numerous abandoned 
channels. 
The annual floods carry immense quantities of silt, which they 
spread over the flood plain, building it up with great rapidity. Where 
the caving banks of the river expose vertical sections it was noted 
that the roots of living arrow weeds had been buried to a depth of 6 
feet or more. In other words, there have been accumulations of silt 
6 feet or more in depth during the life of this shrub. The material 
is well stratified, sometimes with horizontal lamination, but often 
showing a conspicuous oblique lamination similar to that found in 
the Chemehuevis gravel. Much of the silt is very fine and is held in 
suspension for a long time. When deposited in thick beds it dries 
and cracks in columnar form, some of the columns being 2 to 3 feet 
in diameter and separated by cracks several inches wide and 2 feet or 
more deep. 
CHOCOLATE MOUNTAINS. 
South of the Great Colorado Valley, for a distance of about 30 
miles, Colorado River flows in a canyon or narrow valley through the 
Chocolate Mountains — a name applied generally to a complex group 
consisting of the Spire Range, the Picacho group, the Castle Dome 
Mountains, and the Purple Hills. 
