GEOLOGIC HISTORY. 65 
to the mouth of Williams River, a distance of about 125 miles. This 
diversion may have been due in part to the volcanic dam winch had 
been thrown across the Detrital-Sacramento Valley near Williams 
River; or it may have been caused wholly by the normal deflection 
of the river over the aggrading surface — an action well illustrated in 
the frequent changes of course over the flood plains and delta of the 
Colorado at the present time. It is noteworthy in this connection 
that the volcanic dam near Williams Canyon, the highest remnants of 
the detrital beds, and the mountain ridges through which the river 
cut at Boulder and Black canyons, do not differ greatly in altitude. 
Second canyon cutting. — A second epoch of canyon cutting was 
brought about by some change which caused the streams throughout 
the Southwest to erode their channels. The streams which had not 
been deflected from their former courses reexcavated their old valleys; 
those that had been deflected cut new canyons. Colorado River 
reexcavated its old valley from Grand Canyon westward to the 
Detrital-Sacramento Valley, where it left its former course, passing 
westward across the Black Mountain Range and thence southward to 
the mouth of Williams River, where it returned to the old valley 
which it now occupies. 
Viewed from the standpoint of existing topography, a more diffi- 
cult course could scarcely have been selected. Instead of reexca- 
vating the Detrital-Sacramento Valley, where the only hard rock to 
be eroded was the volcanic dam, it eroded four rock canyons (Boulder, 
Black, Mohave, and Aubrey) and crossed four debris-filled basins 
(Las Vegas, Cottonwood, Mohave, and Chemehuevis) before return- 
ing to its former course. The excavation of the detritus in the basins 
was naturally more rapid than the work in the hard rock of the ridges 
separating them, and the result is the conspicuous alternation of 
short, sharp canyons and basin-like valleys characteristic of low T er 
Colorado River. 
During this epoch the downcutting was apparently very rapid, the 
rock canyons being narrow, their walls rising abruptly from the 
water's edge, as in Black Canyon (PL VIII, B) and in Boulder Can- 
yon. The rapidity of the downcutting is perhaps most strikingly 
shown where the river has cut through the Temple Bar conglomerate, 
as at Temple Bar, leaving the poorly consolidated gravels standing in 
perpendicular cliffs hundreds of feet high. (See Pis. II and VIII, A.) 
The occurrence of these high cliffs of unconsolidated gravel and the 
absence of extensive weathering are not wholly in accord with the 
suggestion that the change from the aggrading to the degrading con- 
dition of the river was due entirely to a change of climate. 
Deposition of Chemehuevis gravel. — Some influence not certainly 
known brought the second epoch of erosion to a close and caused 
Colorado River and other streams of the Southwest again to' fill their 
49964— Bull. 352— OS 5 
