52 RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
32° to the east and end abrubtly against the granite at the fault line. 
This evidence of faulting, considered in connection with the faulting 
and block tilting in the Hualpai Mountains, indicates that the Big 
Sandy Valley was formed by a downthrow of the eastern edge of the 
Hualpai block. (See fig. 2 and section S-S', Pi. V, p. 24.) 
SIGNAL CANYON. 
At the town of Signal the Big Sandy leaves the debris-filled trough 
and passes through a rock gorge known as Signal Canyon. The rocks 
exposed are coarse-grained granites in the northern part of the canyon 
and andesites in the southern part. These rocks had been deeply 
dissected previous to the accumulation of the older detrital beds 
(just described) which later filled the old valleys and were in turn 
buried beneath effusive rock. Later, during the erosion of Signal 
Canyon, these old valleys were reexcavated in part, leaving on the 
mountain sides conspicuous shelves of lava-covered detritus. (See 
PL III, A.) 
The broad, debris-filled trough here called the Big Sandy Valley 
continues southward west of the Artillery Mountains to Williams 
Canyon, where the detritus is seen to extend lower than the bed of 
Williams River. 
DETRITAL-SACRAMENTO VALLEY. 
Location and character. — The Detrital-Sacramento Valley extends 
from Colorado River on the north to Williams River on the south, 
between the Cerbat and Hualpai mountains to the east and the Black 
Mountain Range to the west. Thus defined it is about 130 miles long 
and .5 to 15 miles wide. The northern part of the valley, drained by 
Detrital Wash, is locally known as the Detrital Valley, and the cen- 
tral part as the Sacramento Valley. (See PL XI.) The southern 
part bears no name, but since it is the southern continuation of the 
Sacramento Valley, the whole is here called the Detrital-Sacramento 
Valley. 
The valley was apparently formed mainly by erosion, although its 
form has been modified by crustal movements; such is notably the 
case west of Chloride, where sheets of basalt, outpoured within the 
valley, have been tilted eastward. The andesites and rhyolites on 
either side, from the southern end of Black Mesa northward to Colo- 
rado River, are similar in character and occur at about the same level. 
They are probably remnants of a single effusive mass through which 
the Detrital-Sacramento Valley was eroded. 
The depth to which this valley was cut is not definitely known. 
At the northern end it was deeper than the present Colorado Valley 
(altitude 800 feet), and in Williams Canyon (altitude 505 feet) it 
extends beneath the bed of the river. The maximum depth between 
