30 [RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
The current is swift as far south as the mouth of Black Canyon, the 
fall being about 400 feet, or about 5 feet to the mile. There are 
several rapids of considerable size, formed by bowlders washed into 
the river from tributary canyons. The largest of these are: (a) 
Near Pierce Ferry in the Grand Wash Trough, caused by bowlders 
from the Greggs breccia entering from the south; (b) at the mouth 
of Grand Wash, caused by bowlders from this wash; (c) 3 miles east 
of the Nevada- Arizona boundary, caused by bowlders from a small 
wash entering from the north; (d) Red Granite Rapids, 3 miles west 
of the Nevada-Arizona boundary, at which point there is a long 
bowlder strainer a on the Arizona side which deflects the river sharply 
against the red granite on the Nevada side; (e) Hualpai Rapids, 
formed by the bowlders from Hualpai Wash; (f) Reverse Rapids, in 
Boulder Canyon; (g) Roaring Rapids, in Black Canyon. 
MOUTH OF GRAND CANYON. 
Colorado River emerges from the Plateau region about 7 miles 
east of the Nevada- Arizona boundary. The Grand Wash Cliffs, which 
form the western border of the plateau, rise to an altitude of about 
4,500 feet, or 3,500 feet above the river. A few miles north of the rim 
of the canyon Shiwitz Plateau rises to an altitude of about 6,300 feet. 
Grand Canyon is very narrow at its mouth (PL VII, B), and nearly 
perpendicular walls of Redwall limestone, sandstone and shale of the 
Tonto formation, and granite (see pp. 14-15) rise from the water's 
edge on either side. Springs emerge from these formations in many 
places and deposit carbonate of lime. The lower portion of the can- 
yon walls is covered in many places with calcareous tufa and pendants 
of travertine. In some places the travertine projects irregularly, 
inclosing picturesque cavern-like chambers; in other places it forms 
basins built out into the river. 
Massive beds of travertine were observed on both sides of the 
canyon at an elevation of 500 feet or more above the river. These 
may have been built out from the canyon sides by springs and later 
partly eroded away, but their horizontal bedding and location on 
opposite sides of the canyon at about the same altitude (see section 
A, fig. 3) indicate that they are probably remnants of a mass which 
formerly filled the canyon to a depth of several hundred feet. They 
are probably part of an extensive travertine formation having its 
greatest development near Hualpai Wash. 
GRAND WASH TROUGH. 
The Grand Wash Trough, lying between the Grand Wash Cliffs and 
Iceberg Canyon, is a depression formed by faulting and by the tilting 
a "Strainer" is a term used locally to designate an accumulation of bowlders forming a porous dam 
in the river, through which the water finds its way as through a strainer. 
