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The Columbia Plateau is a basin-like subprovince of the Columbia Intermontane Physiographic Province (Freeman and others, 1945; Thornbury, 1965). The Blue Mountains area of southeastern Washington and northeastern Oregon is the southern upwarped part of the province, whereas the region north of the Blue Mountains, east of the Cascade Mountains, south of the Okanogan Highlands, and west of the Idaho Rockies makes up the less deformed part of the province. The entire province is characterized by great late Cenozoic outpourings of basaltic lava. Because of gentle dips on the lava flows in the northern and eastern sections of the plain, the term “Columbia Plateau” has been applied to this region. Waitt and Swanson (1987) propose that the area be named “Columbia Plain” to indicate its analogy to the Snake River Plain, which is also characterized by basaltic lava flows.

The Miocene tholeiitic flood basalt that characterizes the Columbia Plateau is named the Columbia River Basalt Group (Swanson and others, 1979). Its volume is approximately 1.7 × 105 km3, and it extends over an area of approximately 1.6 × 105 km2 (Tolan and others, 1987). Most of the flows date between 17.5 and 14.5 Ma, but basalt eruptions from linear vents in the eastern part of the province continued to 6 Ma on a reduced scale (Swanson and others, 1979). Sedimentary interbeds occur between some basalt flows, especially near the margins of the basalt plain (Waters, 1955;

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