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 The
Great Plains, a broad, flat, mostly green band down the center of
the map, is a vast east-tilted surface formed by deposition of sediment
eroded from the uplifting Rocky Mountains
in the Neogene and Paleogene,
beginning about 65 million years ago. This region is now being eroded,
especially on its eastern margin, by numerous east-flowing rivers
that are exposing the older rocks beneath. To the west, these older,
largely Cretaceous, sedimentary
rocks contain some of the continent's most spectacular dinosaur fossils.
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