GEOLOGIC HISTORY. 63 
between two great periods of time. Since, therefore, the canyon 
cutting followed the uplift, it falls into the later or Quaternary 
period. 
Deposition of the Temple Bar conglomerate. — After the canyon had 
been eroded to something like its present depth some change occurred 
which caused the river to deposit sand and gravel from Grand Canyon 
to the Gulf of California. Between the mouth of the canyon and the 
Detrital-Sacramento Valley no great volume of the gravels accumu- 
lated, owing to the limited dimensions of that part of the valley; 
but in the broad Detrital-Sacramento Valley extensive deposits 
were laid. down to a depth of something like 2,000 feet. 
Similar deposits were formed in other valleys of the Southwest, 
and the low -lying interstream areas were built up with angular 
rock debris derived from the near-by mountains. As previously 
described, the detrital formations of various parts of the Territory 
can not be definitely correlated, but beds apparently equivalent to the 
Temple Bar conglomerate are found throughout the Southwest. 
During this epoch of deposition numerous volcanic eruptions 
occurred. Sheets of basalt underlie the gravels in some places and 
in others are included within them at several horizons. Toward the 
close of the epoch eruptions occurred near the mouth of Williams 
River, the molten basalt bursting upward through the filling of the 
Detrital-Sacramento Valley and spreading over its surface to a depth 
of 800 feet or more, apparently forming a volcanic dam. This sheet 
has since been greatly eroded and its original extent has not been 
determined. Lava hills north of Williams River having the appear- 
ance of volcanic necks suggest that the center of eruption and the 
real obstruction across the valley may have been between the Mohave 
Mountains and the Aubrey Hills, the sheet of compact lava south of 
Williams River being an outflow from this center. 
A dam 800 feet high thrown across Colorado River at this place 
would have created slack-water conditions not only throughout 
the entire length of the Detrital-Sacramento Valley, but far into 
Grand Canyon, and must have facilitated the deposition of sand and 
gravel, which had previously accumulated in the valley to a depth of 
about 1,000 feet. The altitude of the basalt sheet does not differ 
greatly from that of the aggraded floor of the Detrital-Sacramento 
Valley to the north, and of the sand and gravel remnants found far 
above the river in the walls of Virgin Canyon. Deposition of sand 
and gravel apparently continued long after the extrusion of these 
lavas, tilling the Detrital-Sacramento Valley to a level corresponding 
to the surface of the lava sheet, an additional depth of about 800 feet. 
It is probable that the volcanic dam did not at once divert the 
river from its former course through the Detrital-Sacramento Valley, 
for the upper part of the valley filling, so far as observed, does n,»t 
