Geology
During late Miocene and early Pliocene times, one of the largest flood basalts ever to appear on the earth's surface engulfed about 63,000 square miles (160,000 km2) of the Pacific Northwest, forming a large igneous province. Over a period of perhaps 10 to 15 million years, lava flow after lava flow poured out, ultimately 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 the large plateau—a large, slightly depressed lava plain sometimes also known as the Columbia Basin. 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. Entities found in these lake beds include fossil leaf impressions, petrified wood, fossil insects, and bones of vertebrate animals.
Read more about this topic: Columbia Plateau