DETAILS OF VALLEYS. 58 
these extremities is not known. At Yucca (altitude 1,804 feet) the 
bottom of the gravels was reached in a well at a depth of 905 feet, 
or at an altitude of about 900 feet above sea level. The general alti- 
tude of Black Mesa, a few miles west of Yucca, and of the mesa east 
of the valley near Kingman, is about 3,500 feet. Assuming that the 
mesas are remnants of the surface which formerly extended across 
the Detrital-Sacramento Valley, the known depth of erosion at Yucca 
is about 2,500 feet. Since the valley is known to be deeper than this 
at both the northern and southern ends, it is probable that the well 
at Yucca does not penetrate the filling at the deepest point and that 
the depth of the old valley is greater than 2,500 feet. 
Detrital filling.— The Temple Bar conglomerate filling this ancient 
valley is exposed at the north in the banks of Colorado River (PL II) 
and at the south in Williams Canyon (PL VI, B). It is composed of 
horizontally bedded sand, gravel, and wash, and contains sheets of 
basalt at several horizons. The composition of the filling at the 
extremities of the valley is described under " Temple Bar" (p. 35), 
and under "Williams Valley 7 ' (p. 54), and need not be repeated here. 
Where exposed in the washes throughout the valley and penetrated 
by wells the material does not differ greatly in character. Hori- 
zontally bedded sand and gravel containing sheets of basalt and 
having the same general appearance as the beds at Temple Bar and 
in Williams Canyon were observed within the valley up to altitudes of 
2,500 feet or more, and sheets of basalt within the detrital beds 
were penetrated in the well at Drake. For these reasons the detrital 
filling is correlated with the Temple Bar conglomerate. 
Considerable erosion has taken place within the Detrital-Sacra- 
mento Valley since the filling was completed. Sacramento Wash is 
1,000 feet or more lower than the valley floor farther north. This 
difference in elevation is probably due in part to surface warping, but 
is regarded as due mainly to erosion. 
In general the sands and gravels of the Detrital-Sacramento Valley 
are covered with angular rock debris, consisting in part of fragments 
of basalt, andesite, rhyolite, and granite, working their way over the 
surface as wash from the hills, and in part of the coarser material 
originally deposited with the sand and gravel and accumulated 
by surface concentration during the later degradation as the finer 
material was washed away by the rains or blown away by the winds. 
WILLIAMS VALLEY. 
Location and character. — Williams River, formerly known as Bill 
Williams Fork, is formed by the junction of two streams (Big Sandy 
Wash and Rio Santa Maria) and extends from this junction westward 
to Colorado River. The south fork, or Rio Santa Maria, emerges 
through a narrow canyon from the plateau to the east upon a broad 
