42 
RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
M 
fine sand and clay. At the northern end of the Mohave Valley, near 
Bulls Head, there are two distinct divisions of the gravel. (See PI. 
Ill, B.) The lower division consists mainly of well-stratified and 
firmly packed sand and silt; the upper of stratified but very loose 
sand and gravel. The two divisions appear to be perfectly conform- 
able, and the difference in character is probably due to some change 
in the river, such as 
an increase in its 
carrying power, en- 
abling it to bear the 
finer material away 
and deposit only 
the coarser mate- 
rial; or to a change 
in course, bringing 
the gravel-bearing 
current over what 
had formerly been 
a flood plain. 
The gravels in the 
old channel east of 
Pyramid Canyon 
were examined with 
consi d e r a b 1 e care 
and found to consist 
of limestone, mar- 
ble, sandstone, and 
metamorphic and 
igneous rocks' of 
great variety. Many 
of the limestone 
pebbles have beau- 
tifully etched sur- 
faces, which Gilbert a describes as "carved with a network of ver- 
micular grooves into a most beautiful arabesque design." These 
etched pebbles were observed in the Chemehuevis gravel throughout 
the region described. 
THE NEEDLES. 
South of the Mohave Valley occurs a mass of eruptive rock resting 
on the flanks of the Mohave Mountains (section N, fig. 9), and charac- 
terized by sharp pinnacles of erosion, from which the group has been 
called The Needles. Mohave Canyon, about 8 miles long, has been 
cut in this eruptive mass to* a depth of about 2,000 feet. In gen- 
Fig. 9.— Diagrammatic sections across Colorado River. L, Pyramid 
Canyon; M , in the Mohave Valley; N, in the Needles Mountains; 
O, at the mouth of WiUiams River. 1, Chemehuevis gravel; 2, gran- 
ite; 3, Temple Bar conglomerate; 4, eruptive; 5, basalt. 
a Gilbert,. G. K., U. S. Geog. Surv. W. 100th Mer., vol. 3, 1875, pt. 1, pi. 9 and p. 83. 
