DETAILS OF VALLEYS. 41 
deep, the old one; but in the absence of soundings within the canyon 
north of Bulls Head, the depth of the new channel remains in doubt. 
Borings 100 feet deep in the older channel do not penetrate through 
the gravels. 
A brief statement of the physiographic history of the river, as 
here recorded, may give a clearer understanding of the phenomena 
described. The river at some former time occupied the old course 
to the east (PI. IX) and eroded its channel to a depth considerably 
below the present river level. Later the Colorado deposited the 
Chemehuevis gravel, filling its valley with sand and gravel to a 
level several hundred feet above the present bed. When the river 
again began to erode, it feexcavated the old valley throughout the 
greater part of its course, but in Pyramid Canyon, at Eagle Rock 
(described in the previous section), and at other places farther south 
to be described in following sections, it left its old course and cut 
rock gorges at one side. 
MOHAVE VALLEY. 
The Mohave Valley is a large basin extending from Bulls Head 
southward to The Needles, a distance of about 35 miles. The center 
of the basin is occupied by a broad flood plain having an area of 
about 50,000 acres. This is bordered on either side by terraced 
gravel bluffs, from which long, graded, alluvial slopes extend to the 
bordering mountains, joining the slopes at altitudes of 2,500 to 3,000 
feet. 
The rock masses in the Mohave Valley were examined principally 
east of the river. The rocks of Black Mesa to the east are mainly 
andesite and rhyolite, resting upon granitic gneiss. Overlying these 
older rocks occurs a formation of partly consolidated sand and gravel, 
which in composition and general appearance resembles the Temple 
Bar conglomerate. It is exposed in steep cliffs in the washes of the 
alluvial slope, and the strata are generally nearly horizontal. In 
some places, however, the layers are faulted and tilted (see PI. X, B). 
These stratified sands and gravels are covered with coarse rock debris, 
due partly to wash from the hills and partly to surface concentra- 
tion of the coarser material of the conglomerate. 
Resting upon this formation and occupying spaces eroded in it is 
the Chemehuevis gravel, forming conspicuous terraces, three of 
which are prominent and traceable continuously for considerable 
distances; in some places five are distinguishable. The lowest is 
about 50 feet above the 1 river, and is represented by the broad shelf 
upon which old Fort Mohave stands; the second is about 100 feet 
higher, and the others occur at intervals of 50 to 100 feet. 
The Chemehuevis gravel varies considerably in character from 
place to place from beds of large, well-rounded pebbles to those of 
