DETAILS OF HIGHLANDS. 19 
Topographic features. —The Grand Wash Cliffs are very precipitous, 
as shown in section P-P' ', PL V, rising 4,000 feet or more above the 
plains to the west. The drainage of the plateau near the cliffs is 
northward to Colorado River, and the continuity of the westward- 
facing escarpment is broken by few canyons. 
The lower or granitic portion of the cliffs, though not so nearly 
perpendicular as the upper or limestone portion, is steep, and the 
slopes meet thedetrital surface of the Hualapai Valley at high angles. 
Numerous detrital cones have formed at the foot of the escarpment, 
but they are comparatively small. In a few places the cones join 
laterally, forming an alluvial slope, but in general the escarpment 
rises abruptly from the nearly level floor of the valley. 
Rock masses. — At the mouth of Grand Canyon the cliffs are com- 
posed principally of Carboniferous limestone (Redwall) and Cam- 
brian sandstone and shale (Tonto), with a little granite exposed at 
the base. In Music Mountain, at the southern end of the Grand 
Wash Cliffs, the same series occurs, but the granite is more exten- 
sively exposed, the base of the Tonto being about 4,000 feet higher 
than at the mouth of Grand Canyon. 
The granite is coarse grained and is intersected by dikes and 
sheets of intrusive rocks which terminate abruptly at the contact 
between the granite and the overlying sandstone of the Tonto. This 
contact, as seen in the face of the escarpment, is a practically straight 
line and represents a base-leveled surface upon which the Cambrian 
sediments were deposited. 
The sandstone at the base of the Tonto at the mouth of Grand Can- 
yon is 80 feet thick and the overlying shale is about 600 feet thick. 
About 12 miles southwest of the mouth of the canyon, at the base 
of the tilted block through which Iceberg Canyon is cut, the sand- 
stone is again 80 feet thick, but the overlying shale is only 200 feet. 
A few miles north of Music Mountain, in the pass occupied by the 
road from the Hualpai Valley to the plateau, the sandstone was 
found to be 50 feet thick and the overlying shale 300 to 400 feet thick. 
The Devonian of the Grand Canyon region, described by Walco'tt, a 
was not identified by the writer in the Grand Wash Cliffs. The 
highest formation observed was the Redwall limestone, which forms 
the rim of the cliffs, and though more or less eroded, still has a thick- 
ness of 1,000 feet or more, according to locality. 
COTTONWOOD AND AQUARIUS CLIFFS. 
Location, — At Music Mountain the escarpment divides, the upper 
or sedimentary part receding toward the east and the lower or granitic 
part continuing southward under the names of the Cottonwood and 
the Aquarius cliffs. The Cottonwood Cliffs extend from Music 
a Walcott, C. D., Am. Jour. Sci., 3d ser., vol. 20, 1880, p. 222. 
