Columbia River Gorge Geology

For more information about local geology, visit these sites:

USDA Forest Service - Columbia River Basalt

The Columbia River Plateau was created by a series of basalt flows. Six to seventeen million years ago, the flows covered 164,000 square kilometers -- portions of northeast Oregon, southwest Washington and western Idaho (figure 1).

Over 17 million year ago, cracks in the earth's surface began spewing molten basaltic lava. Basalt is a hard black rock. Its mineral grains are too small to see with the naked eye. Columbia River basalt is a volcanic rock composed of the black mineral--pyroxene and a white mineral--plagioclase. The total volume of Columbia River basalt is about 174,000 cubic kilometers with up to 5,000 meters thick. MORE

USGS - The Geologic History of the Columbia River Gorge

Thousands of volcanic eruptions piled layers of volcanic ash, lava, and mudflows over the region, creating the Ohanapecosh Formation. These rocks weathered into slippery red clay and greenish rocks visible near Stevenson, Washington.

Millions of years later, mudflows poured off volcanoes, covering the land with hundreds of feet of ash, boulders, and cobbles, creating the Eagle Creek Formation. You can see this beige formation on cliffs north of Bonneville Dam and along I-84 near exit 41. Beacon Rock, just downstream from the dam, is the ancient core of one of these volcanoes. MORE

BPA - The Geologic Story of the Columbia Basin

The rock formations of the Columbia Basin were formed by some of the most unusual and catastrophic processes in geologic history.
Early History - 40-60 million years ago:

During the early stages of the Columbia Basin formation, granite rock was slowly created by heat and pressure deep in the crust of the earth. Then the crust was uplifted, exposing the granite, creating mountains similar to the Okanogan Highlands north of Grand Coulee Dam.

Forty to sixty million years ago the formation of the outline of the Columbia Basin was complete. The land had subsided below sea level, and a large inland sea had formed. MORE

General Geology of Southeastern Washington - by Bob Carson

Whitman College is situated on the eastern Columbia Plateau, site of some of the most catastrophic events in earth history: enormous basaltic lava flows 15 million years ago and giant jökulhlaups (glacier outburst floods) 15 thousand years ago. These and related events shaped the landscape of eastern Washington, where the Earth's youngest basalt plateau was swept by the largest documented floods in geologic history. MORE

Dept. of Geology & Mineral Industries - Learn more about Oregon's geology

Between 14 and 16 million years ago, "fissure" 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 province in eastern Washington and northeastern Oregon, with some lava continuing to flow as far west as the Pacific Ocean via the ancestral Columbia River valley. MORE

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