Absence of rock in place--that is, of ridges or strata of rock rising
through the soil above or nearly to the surface--has determined the
character not only of the Plains but of much of the roll of the great
rivers east and south of them. Even at the very base of the Rocky
Mountains, the Chugwater shows a milky though rapid current, while the
North Platte brings a considerable amount of earthy sediment from the
heart of that Alpine region. After fairly entering upon the Plains,
every stream begins to burrow and to wash, growing more and more turbid,
until it is lost in 'Big Muddy,' the most opaque and sedimentary of all
great rivers. I suspect that all the other rivers of this continent
convey in the aggregate less earthy matter to the ocean than the
Missouri pours into the previously transparent Mississippi, thenceforth
an unfailing testimony that evil company corrupts and defiles.
Louisiana is the spoil of the Plains, which have in process of time
been denuded to an average depth of not less than fifty and perhaps to
that of two or three hundred feet. I passed hills along the eastern base
of the Rocky Mountains where this process is less complete and more
active than is usual,--hills which are the remaining vestiges of a
former average level of the plain adjacent, and which have happened to
wear away so steeply and sharply that very little vegetation ever finds
support on their sides, which every rain is still abrading.
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