fift RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
valleys with sand and gravel. The open basins which had been 
excavated along the Colorado while the canyons were being cut in 
the harder rock were filled to a depth of several hundred feet with 
material which has been called the Chemehuevis gravel, and which 
extends from the mouth of Grand Canyon to the Gulf of California. 
In the midst of this epoch some change occurred which caused the 
accumulation of coarser material in the vicinity of Bull Head. This 
change, however, may have been local, as the division into lower and 
upper gravels is not conspicuous in other places, so far as observed. 
During the accumulation of the Chemehuevis gravel Colorado River 
evidently shifted its course within narrow limits over the aggrad- 
ing valley floor, as it had done over a much wider area during the 
accumulation of the Temple Bar conglomerate. 
Third canyon cutting. — When the river again began to erode its 
channel it reexcavated for the most part the old valley, but in a 
number of places abandoned its former course and cut rock gorges, 
leaving the gravel-filled channel at one side, as in Cottonwood Valley 
(p. 37), Pyramid Canyon (p. 38), Mohave Canyon (p. 42), and 
Chocolate Canyon (p. 48). 
For a third time the river eroded to some unknown depth below its 
present bed. Borings to a maximum depth of about 200 feet at the 
dam sites of the Reclamation Service failed to penetrate through the 
gravels. 
Formation of flood plains. — In very recent geologic time Colorado 
River changed again from a degrading to an aggrading stream, and is 
once more building up its course. In the broad parts of the valley 
the accumulation is evidenced by the wide bottom lands over which 
the river frequently changes its course, sometimes gradually by 
lateral cutting and filling, and sometimes suddenly by cutting a new 
channel during a flood, forming numerous sloughs, lagoons, and 
oxbow lakes. 
The great depth of the debris-filled channels within the newest 
canyons, as shown by the soundings for bed rock, raises the question, 
What proportion of the depth should be attributed to the normal 
cutting of the river and what proportion to permanent filling? 
Cooley a shows that Missouri River at Nebraska City probably 
erodes its bed to a depth of 70 to 90 feet. Near Omaha the depth of 
scour and fill during the year 1883 amounted to about 40 feet. On 
this subject Chamberlin b remarks : "In the drift-filled bottoms of 
the great branches of the Mississippi system it is wholly within bounds 
to regard at least the upper 40 or 50 feet of the deposit over which the 
a Report Chief Eng. U. S. A. for 1879-80, pt. 2, pp. 1060-1071. See also Chamberlin and Salisbury, 
Text Book of Geology, vol. 1, p. 185. 
& Chamberlin, T. C, Jour. Geol., vol. 11, 1903, p. 72. 
