Columnar Basalt. Picture Gorge Basalt of the Columbia River Basalt Group.
When large basalt flows cool, parts of them contract and form vertical fractures, which result in polygonal shaped columns --in much the same way that drying mud cracks into polygons as it shrinks. The part of the flow that forms these columns is called the "colonnade". This photo shows two colonnades formed by the cooling of two separate flows.

The Picture Gorge Basalt is one of the 5 main units of the Columbia River Basalt Group, a vast accumulation of basalt that erupted from about 17 to 6 Ma in eastern Oregon and Washington states. Click here for more photos of the Columbia River Basalt Group.

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