DETAILS OF VALLEYS. 51 
the Aubrey Hills, has a slight inclination to the east, but is in general 
an undissected and undrained detrital plain. 
Bock formations. — The oldest rocks in the Big Sandy Valley are 
the granites exposed in the bordering mountains and cliffs, which, 
because of their resemblance to the granite beneath the Cambrian 
sediments in the Grand Wash Cliffs, are thought to be of pre-Cambrian 
age. They are overlain in places by Tertiary andesites. 
The trough formed by these older rocks is partly filled with rock 
debris which has been later exposed by erosion. The oldest detrital 
beds have been upturned and eroded to depths of 800 feet or more. 
They are composed of clay, sand, and gravel, and in places contain 
H 
},... MM ■ , JvTg^ r^^ -^ 7 ^ :- ■ ■ ■ -' -■■•■.-■■■■■-,.■• I __'._' _:,,'..< - - | | S g c^ - 
Fig. 14.— Sketch section across Big Sandy Wash near mouth of Deluge Wash, showing the older gravels 
tilted eastward, eroded, and overlain by younger gravels, and again eroded and partly covered by 
recent flood-plain deposits. 1, Flood-plain deposits; 2, Temple Bar (?) conglomerate; 3, gravel. 
granite bowlders 5 to 8 feet in diameter. In other places, as at 
Deluge Wash (PI. IV, B), they are composed of clay and fine, well- 
stratified sand and volcanic ash. 
Within the valley eroded in the oldest tilted detrital beds, and 
resting with horizontal bedding upon their eroded edges, is a second 
series of gravel deposits. These are perhaps best exposed near the 
mouth of Deluge Wash, where they are about 50 feet thick. This 
second gravel accumulation was in turn eroded and a third deposit 
laid down. (See fig. 14.) 
In general character and stratigraphic position the oldest detritus 
corresponds with the Temple Bar conglomerate, the middle with the 
FlG. 15. — Sketch section across Big Sandy Wash at the mouth of Trout Creek, showing the older 
gravels overlying granite, tilted eastward, and terminating against the granite of Cottonwood 
Creek. I Flood-plain deposits; 2, Temple Bar (?) conglomerate; 4, granite. 
Chemehuevis gravel, and the youngest with the flood-plain deposits 
of the Colorado Valley, but no definite correlation can be made at 
the present time. 
Structure. — The Big Sandy Valley is a trough formed by faulting. 
The fault zone along the Grand Wash and Cottonwood cliffs continues 
southward to the Aquarius Mountains. Recent movement along this 
/one is indicated by the displacement of portions of the andesite sheet 
at the northern end of the valley and by the faulted and tilted gravel 
beds near the mouth of Trout Creek (fig. 15), where the strata incline 
