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Geologic History

Basalt flows and ice-age floods shaped the predominant geologic features of the Columbia River Basin. Incorporate this heritage into your garden with basalt columns or boulders, paths of basalt gravel, or use basalt gravel as a rock mulch. The floods deposited erratics and flood-tumbled rock and gravels. Use one or more erratics as a centerpiece in your garden or construct a dry creek bed using the tumbled rock. Learn a bit of our geologic history here.


nature's rock garden, Umtanum Ridge

Above is one of Nature's rock gardens on display on Umtanum Ridge. Some gardeners strive to mimic the look of our shrub-steppe and hillsides. Many of our native species thrive in the nooks and crannies of rock and gravel.

Photo: Jane Abel

First came layer upon layer of lava...

Basalt Flows & Rock Formations

Between 16.7 and 5.5 million years ago, massive lava flows erupted from western Idaho, to northeast Washington, down to south east Oregon, and traveled more than 200 miles out to the coast through the gap in the Cascade Mountain Range now claimed by the Columbia River. This volcanic activity left many layers of basalt in its wake, and as these layers are exposed, one can view the different types of basalt formed by lava cooling under varying environmental conditions. Basalt wasn't the only rock formed in this process. As rock is heated and cooled, compressed and eroded, other rocks and minerals were formed.

Columbia River Basalt Group Main Regions of Basalt Exposure

Image: Columbia River Basalt Group, Main Regions of Basalt Exposure

Source: USGS images @

Columbia River Basalt Group
Additional Resources

Then came multiple, massive floods...

The Ice Age Floods

Ice sheets covered much of what is now known as Canada and the northern United States. With cyclical changes in climate between 2.5 million years ago and 10,000 years ago, these sheets likely retracted and advanced many times, leaving ice dams and super reservoirs in their retreat. Though ice dam failures occurred periodically, approximately 18,000 years ago one particular ice dam blocking the Clark Fork River in northern Idaho collapsed releasing Glacial Lake Missoula in what could be visually be compared to a tidal wave of rock, dirt, ice, and debris that carved its way through the Columbia Basin. While some areas were left with barren scablands for thousands of years, others received deep and rich soil deposits of temporary lakes giving rise to the productive agriculture of the region today.

Lake Lewis Isles
Lake Lewis Isles, Tri-Cities Washington, Ice Age Floods

During the largest floods, the Tri-Cities was inundated by over 800 feet of water. The tops of Candy Mountain, Badger Mountain, Red Mountain and Goose Hill formed islands in temporary Lake Lewis. (credit: Bruce Bjornstad & Lake Lewis Chapter, IAFI)

Erratic Boulders - Rafted in Icebergs by the Ice Age Floods

"Erratic boulders are scattered throughout the Ice Age Floods region. Many of the boulders resting on the younger Columbia River Basalt flows were transported within icebergs from the Rocky Mountains. Floodwaters from Glacial Lake Missoula swept over eastern Washington and down the Columbia River Gorge at speeds in excess of 60 miles per hour. The icebergs that carried the erratics often drifted into side channels during flood events. The stranded boulders now look out of place, grounded high on canyon walls above the scablands. Visit to learn more about the Ice Age Floods." (Nick Zentner & Tom Foster)

Additional Resources
Erratic Boulders

Background photo by Jane Abel

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