DETAILS OF VALLEYS. 39 
Near Round Island partly consolidated gravel beds similar in 
general appearance to the Temple Bar conglomerate and intersected 
by basalt dikes (see section K, fig. 6) occur above andesitic lavas. 
The gravels are well stratified, tilted steeply to the east, and exten- 
sively eroded. Horizontally bedded Chemehuevis gravel lies in 
depressions eroded in older upturned beds which in turn are younger 
than the andesites. For these reasons the older gravels are pro- 
visionally correlated with the Temple Bar conglomerate. 
The Cottonwood Valley is the northernmost of a series of large 
basins which extend thence southward to the Gulf of California. 
The flood plain in the center of the valley is terminated laterally 
by bluffs of the Chemehuevis gravel. Between these bluffs and the 
mountains on either side are long corrugated slopes covered with 
angular rock debris or wash from the highlands. The slopes in the 
Mohave Valley were examined with more care than elsewhere, and 
are described in a following section. (See pp. 41-42.) 
At Eagle Rock, near the southern end of Round Island, the river 
leaves the flood plain and flows through a narrow rock gorge about a 
mile long and 150 feet deep. The rock is andesite, and its relation 
to the gravel formations is shown in fig. 7. The gravel-filled channel 
to the east connects the Round Island Valley with the Cottonwood 
Valley proper. In other words, Colorado River, during one of its 
periods of canyon cutting, described under " Geologic history" 
(pp. 62-67), failed to reexcavate its old gravel-filled channel at Eagle 
Rock and cut a new channel in the rock west of the old one. 
PYRAMID CANYON. 
South of the Cottonwood Valley the granite and gneiss of the 
bordering mountains extend across the valley of the Colorado, and 
through these rocks the river has cut a canyon about 300 feet deep 
and 8 miles long. This is known as Pyramid Canyon, the name 
being derived from a pyramid-shaped rock near the northern end. 
P^ast of the canyon, and separated from it by a narrow rock ridge 
(see PL IX), is an old channel filled with sand and gravel — Cheme- 
huevis gravel — remnants of which occur 400 feet or more above the 
river, overlying and abutting against the granite and gneiss, as shown 
in PL III, B. 
One of the proposed dam sites of the Reclamation Service is 
located at the southern end of Pyramid Canyon, near Bulls Head 
rock, the southern extremity of the rock ridge. On account of the 
investigations of the Reclamation Service in this vicinity much 
exact information is available. On this account the Bulls Head 
region is perhaps the best locality for the description and illustration 
of the late physiographic history of Colorado River. Unfortunately 
