Geology
The Grande Ronde Valley is part of the Columbia River Plateau. The Columbia River Plateau was created by a series of basalt flood eruptions. These happen when a long crack in the Earth's crust spews floods of lava. The molten rock then hardens and creates layers of basalt. The Grande Ronde Valley floor began to sink from this plateau at a rate of about .07-.18 millimeters per year about nine million years ago.
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