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The Colorado Plateau has been the site of accumulation and preservation of non-marine sediments since late Paleozoic time. The climatic conditions have been desert-like for long periods and wind-blown sand is a common sedimentary type. Much of the alluvial material did not leave the source area and is still near its place of origin. The deep and intricate erosion permits excellent three-dimensional views of the sedimentary bodies.
Extensive eolian formations occur in the Permian, Triassic, and Jurassic systems. These are mainly interpreted as superposed dune fields. In many instances the edges of the formations are abrupt and comparison with modern sharply defined dune areas is obvious. Tangential cross-bedding with occasional contorted masses characterize these deposits. Chief interest attaches to the determination of wind directions; apparently the source of most of the sand lay to the north and northwest.
Fluvial deposits are common after the Pennsylvanian. These offer excellent opportunity to study sedimentary variations resulting from differences in climate, weathering, distance of transport, provenance, and energy relations of stream systems. The common occurrence of uranium deposits in the fluvial sandstones has stimulated geologic investigation. The petroleum possibilities of these beds are also receiving increased attention.
Practically every type of deposit seen in process of formation in modern rivers can be detected in the consolidated rocks. The overbank or flood-plain deposits are of less variety and interest than the channel deposits. All types of bars and channel-fill deposits are present but those formed during the building of alluvial plains are most common. Apparently the final composition of a typical fluvial formation depends on the gradient of the streams, the total amount of sediment supplied, and the relative amounts of fine and coarse material. Internal structures of channel sandstones show great variety and can be related to stream volume and velocity. Ripple-mark, festoon cross-bedding, rib-and-furrow, and lineation are the most common.
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