intermittently in
the open, it crosses successive uplifts in a series of deep gorges, and
flows finally at the foot of canyon walls 1500 ft. high near its
junction with the Grand.

The Colorado in its course below the junction has formed a region that
is one of the most wonderful of the world, not only for its unique and
magnificent scenery, but also because it affords the most remarkable
example known of the work of differential weathering and erosion by wind
and water and the exposure of geologic strata on an enormous scale.
Above the Paria the river flows through scenery comparatively tame until
it reaches the plateau of the Marble Canyon, some 60 m. in length. The
walls here are at first only a few score of feet in height, but increase
rapidly to almost 5000 ft. At its southern end is the Little Colorado.
Above this point eleven rivers with steep mountain gradients have joined
either the Green or the Grand or their united system. The Little
Colorado has cut a trench 1800 ft. deep into the plateau in the last 27
m. as it approaches the Colorado, and empties into it 2625 ft. above the
sea. Here the Colorado turns abruptly west directly athwart the folds
and fault line of the plateau, through the Grand Canyon (q.v.) of the
Colorado, which is 217 m. long and from 4 to 20 m. wide between the
upper cliffs. The walls, 4000 to 6000 ft. high, drop in successive
escarpments of 500 to 1600 ft., banded in splendid colours, toward the
gloomy narrow gorge of the present river. Below the confluence of the
Virgin river of Nevada the Colorado abruptly turns again, this time
southward, and flows as the boundary between Arizona and California and
in part between Arizona and Nevada, and then through Mexican territory,
some 450 m. farther to the Gulf of California. Below the Black Canyon
the river lessens in gradient, and in its lower course flows in a broad
sedimentary valley--a distinct estuarine plain extending northward
beyond Yuma--and the channel through much of this region is bedded in a
dyke-like embankment lying above the flood-plain over which the escaping
water spills in time of flood. This dyke cuts off the flow of the river
to the remarkable low area in southern California known as the Salton
Sink, or Coahuila Valley, the descent to which from the river near Yuma
is very much greater than the fall in the actual river-bed from Yuma to
the gulf. In the autumn of 1904, the diversion flow from the river into
a canal heading in