DESCRIPTIVE GEOLOGY. 17 
occur within the Detrital-Sacramento Valley, which was presumably 
excavated in late Tertiary time. 
Greggs breccia. — The Greggs breccia, so called from Greggs Ferry, 
at the northern end of the region, is a name here given to a detrital 
formation filling the Grand Wash Trough and having an exposed 
thickness of about 1,400 feet. It is composed of coarse unassorted 
and poorly stratified material, consisting largely of blocks of crystal- 
line rock, similar to the granite and gneiss of the Virgin Mountains 
to the west. Toward its top the detrital material is cemented with 
lime carbonate, and in places the upper 200 feet consists of travertine 
containing few rock fragments. This travertine is best exposed 
south of Colorado River and east of Greggs Ferry, where it caps 
conspicuous cliffs, which rise 1,400 feet or more above the river. 
The formation contains no fossils, so far as observed, and its refer- 
ence to the Tertiary is based largely on the physiographic evidence 
given in detail in the description of the Grand Wash Trough and later 
in the section on " Geologic history." Briefly stated, the accumula- 
tion of the breccia antedates the cutting of Grand Canyon, which 
apparently began either in the latter part of the Tertiary or at the 
beginning of the Quaternary. 
QUATERNARY. 
Temple Bar conglomerate. — Temple Bar conglomerate is a name here 
given to a sand and gravel formation having wide distribution in west- 
ern Arizona. It occurs in the Colorado and other valleys of that 
region, filling the low places generally to an altitude of 3,000 feet or 
more. The conglomerate is typically exposed near the mouth of 
Virgin River at Temple Bar, from which it takes its name. At this 
point it consists of slightly consolidated sand and gravel exposed in 
nearly perpendicular cliffs (PL II), in which are included sheets of 
basalt. 
Where exposed along the Colorado, the Temple Bar conglomerate 
is evidently a river deposit, but it merges laterally into deposits of 
angular mountain wash in some places, and possibly into lacustrine 
deposits in others. It rests unconforrhably upon the Greggs breccia 
and older formations, and is apparently equivalent in age to the wide- 
spread detrital accumulation filling the low places of the Southwest 
generally and forming the desert plains of Arizona and parts of south- 
ern California. 
No fossils have been found in the conglomerate, and there are no 
means known at the present time by which it may be definitely corre- 
lated with other formations. In composition, geologic and physio- 
graphic relations, and general appearance it is similar to the Gila 
49964— Bull. 352—08 2 
