DETAILS OF VALLEYS. 
37 
Boulder Canyon is apparently younger than Virgin Canyon. The 
walls are but slightly eroded by lateral washes, although the open 
fissures and shattered condition of the rock in many places are 
favorable to the development of such ; none were observed containing 
beds of the Temple Bar conglomerate, as is the case in Virgin Canyon. 
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LAS VEGAS WASH. 
West of Boulder Canyon, and extending southward to the head 
of Black Canyon, the river flows through a debris-filled basin in 
which Colville and 
Las Vegas washes 
join the river from 
the north and west. 
Detrital beds re- 
sembling the Tem- 
ple Bar conglomer- 
ate and containing 
sheets of lava are 
capped by a thick 
lava sheet in Forti- 
fication Hill at an 
altitude of 3,500 
feet, or nearly 2,500 
feet above the river 
(section I, fig. 6). 
The gravels are well 
stratified and are 
faulted and tilted 
in places. 
BLACK CANYON. 
Fig. 6.— Diagrammatic sections across Colorado River. I, Near Las 
Vegas Wash; J, in Black Canyon; K, at Little Round Island. 1, Ba- 
salt; 2, Chemehuevis gravel; 3, Temple Bar conglomerate; 4, granite; 
5, andesite and rhyolite. 
A few miles south 
of Fortification Hill 
the river leaves the detrital basin and enters Black Canyon, a deep, 
narrow rock gorge about 18 miles long. The walls rise steeply from 
the water's edge (see PL VIII, B), and are composed to some extent 
of coarse crystalline rock, but mainly of massive rhyolite (section J, 
fig. 6). Black Canyon is very similar to Boulder Canyon in its 
general youthful appearance and in the absence of notable tribu- 
tary washes and remnants of gravel beds. 
COTTONWOOD VALLEY. 
A few miles north of Eldorado Ferry the river emerges abruptly 
from Black Canyon into a broad open basin, the central part of 
