36 RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
The composition, stratigraphic relations, and general appearance 
of the older or cemented breccia suggest that it is probably a time 
equivalent of the Greggs breccia. The gravels above it form the 
Temple Bar conglomerate, which derives its name from this locality. 
The younger or Chemehuevis gravel is also present at Temple Bar, 
but is not conspicuous. 
VIRGIN VALLEY. 
Between Temple Bar and Boulder Canyon, a distance of about 12 
miles, Colorado River crosses the old debris-filled valley which to the 
south of the Colorado is known as the Detrital-Sacramento Valley 
and to the north as the valley of the Virgin. 
The filling of this old valley is apparently composed of two distinct 
formations, but their relation was not satisfactorily determined. 
The older one consists of alternating layers of sand and clay, and 
contains extensive beds of gypsum. Rock salt occurs in the Virgin 
Valley a few miles north of the Colorado, in beds similar to those 
containing the gypsum. These salt beds apparently lie beneath the 
gypsum, as shown by the salt well near the mouth of Virgin River, 
described by Gilbert. a -The salt well is a crater-like depression 
about 300 feet across and 65 feet deep, situated in the midst of a 
level gravel plain. It is not connected with any surface drainage, 
and is evidently formed by a local caving of the surface probably due 
to the removal by solution of underlying beds of salt. It is filled 
with salt water to a depth of 15 to 20 feet. 
The gypsum and gypsiferous clays are conspicuously exposed 
along the river, where they are deeply dissected over a large area 
locally known as the badlands of the Virgin. The presence of gyp- 
sum and salt indicates that this formation is not a part of the Temple 
Bar conglomerate, which overlies it with apparent unconformity 
(section G, fig. 5) and which does not contain gypsum or salt in its 
typical exposures so far as observed. The relation of the gypsiferous 
shales to the underlying rocks and to the overlying conglomerates 
was not satisfactorily determined. The stratigraphic position, how- 
ever, suggests that this formation may be a time equivalent of the 
Greggs breccia. 
BOULDER CANYON. 
Boulder Canyon is a sharp gorge cut by Colorado River through 
the Black Mountain Range. It is about 5 miles long and 2,000 feet 
or more in depth, the walls rising steeply from the water's edge. 
The rocks exposed in the canyon are syenite, gneiss, rhyolite, and 
basalt. The basalts and their associated gravel deposits near the 
entrance to the canyon are faulted and tilted to some extent, and 
faults and open fissures occur in the canyon walls. 
a Gilbert, G. K., Final Rept. U. S. Geol. Survey W. 100th Mer., vol.3, 1875, pt. 1, pp. 109-110. 
