Index map
The Great Plains Province, a broad yellowish
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 Early Tertiary time, beginning about 65 million
years ago. This upraised region now is being eroded, especially
on its eastern margin, by many east-flowing rivers that expose
the older rocks beneath. To the west, these older, largely Cretaceous,
sedimentary rocks contain some of the Nation's most spectacular
dinosaur fossils.
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