54 
RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
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detrital plain, into which it has cut a canyon several hundred feet 
deep. At the point where the river crosses the direct southward con- 
tinuation of the Big Sandy Valley the canyon has 
been broadened to a basin several miles wide. 
About 12 miles east of Planet this basin narrows 
to a gorge about 8 miles long, known as Williams 
Canyon. The southern wall continues westward 
to Colorado River, but the northern wall is broken 
down west of Planet. 
Although Williams River crosses the Temple 
Bar conglomerate, filling the Detrital-Sacramento 
Valley, it has not excavated a broad basin in it, 
as it has in the gravels filling the Big Sandy Val- 
ley, the difference being due to the protection 
afforded the conglomerate by the sheets of basalt. 
Rock formations. — The oldest rock masses ex- 
posed in Williams Canyon are gneisses and met- 
amorphosed sediments consisting of quartzites, 
slates, and limestones associated with beds of iron 
ore and veins of copper. Within valleys eroded 
in these rocks occur beds of the Temple Bar con- 
glomerate. In the Detrital-Sacramento Valley, 
which crosses the Williams Valley, a thickness of 
nearly 1,000 feet is exposed under a sheet of basalt 
800 feet thick. (See fig. 16.) The lowest part of 
the detritus extends beneath the bed of Williams 
River, but the underlying gneiss and granite are 
exposed at the sides of the old valley. The hori- 
zontally bedded sand and gravel have been 
disturbed locally, probably by the eruption of the 
basalt which burst upward through them. 
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Geologic section in Williams Canyon, 2 miles east of Colorado River. 
Feet. 
Massive flow of basalt 800 
Alternating layers of sand and basalt 40 
Sand and gravel intersected by basalt dikes 900 
Granite gneiss 50 
Between the gravels and the massive flow of 
basalt at the top is a transition zone 40 feet in 
thickness, containing at the point where the sec- 
tion was made (PI. VI, B) five thin sheets of basalt 
separated by beds of sand. Each sheet of basalt was outpoured 
upon what was then the surface, and each in turn was covered by 
sand. The sand is metamorphosed for a few inches beneath each 
