During the existence of this lake,
soil materials were washed from the mountains into the lake and
deposited on the lake bottom. When at length, the lake disappeared,
the lake bottom was exposed and is now the farming lands of the
Great Basin district. The soils of this district are characterized
by great depth and uniformity, an abundance of lime, and all the
essential plant-foods with the exception of phosphoric acid, which,
while present in normal quantities, is not unusually abundant. The
Great Basin soils are among the most fertile on the American
Continent.
_Colorado River district.--_The fourth soil district lies in the
drainage basin of the Colorado River It includes much of the
southern part of Utah, the eastern part of Colorado, part of New
Mexico, nearly all of Arizona, and part of southern California. This
district, in its northern part, is often spoken of as the High
Plateaus. The soils are formed from the easily disintegrated rocks
of comparatively recent geological origin, which themselves are said
to have been formed from deposits in a shallow interior sea which
covered a large part of the West.
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