Pumpkin Ridge to Pyles Canyon, and 15 mi wide, east to west, from Cove to the Grande Ronde River's canyon. Its name, fittingly, means, "great circle."

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.

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