Geology
Deposition of sediments in the Badlands began 69 million years ago when an ancient sea, the Western Interior Seaway, stretched across what is now the Great Plains. After the sea retreated, successive land environments, including rivers and flood plains, continued to deposit sediments. Although the major period of deposition ended 28 million years ago, significant erosion of the Badlands did not begin until half a million years ago.
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