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Columbia River PlateauThe Columbia River Plateau lies across parts of the states of Washington, Oregon, and Idaho. During late Miocene and early Pliocene times, one of the largest basaltic lava floods ever to appear on the earth's surface engulfed about 63,000 square miles (160,000 km²) of the Pacific Northwest. Over a period of perhaps 10 to 15 million years lava flow after lava flow poured out, eventually accumulating to a thickness of more than 6,000 feet (1.8 km). As the molten rock came to the surface, the earth's crust gradually sank into the space left by the rising lava. The subsidence of the crust produced a large, slightly depressed lava plain now known as the Columbia Basin or Plateau. The ancient Columbia River was forced into its present course by the northwesterly advancing lava. The lava, as it flowed over the area, first filled the stream valleys, forming dams that in turn caused impoundments or lakes. In these ancient lake beds are found fossil leaf impressions, petrified wood, fossil insects, and bones of vertebrate animals. Portions of this article adapted from works of the United States Government, which are in the public domain See also External links USGS Page on Columbia Plateau
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