Between 14 and 16 million years ago, fissue volcanic eruptions in eastern Washington, eastern Oregon, and western Idaho produced enormous volumes of molten Columbia River basalt that flowed like water west into the Deschutes-Columbia Plateau.
As the basalt cooled and congeal, it formed the columnar cliffs that dominate the landscape today. Erosion by the Columbia River has exposed a particularly spectacular sequence of these rocks in the Columbia River Gorge on Oregon's northern boundary.
