Loessial

When Washtucna Coulee could not contain the great surge, floodwaters rose to make one of the most significant records on the entire plateau. In two places, they topped the divide between the nearly parallel preglacial Palouse and Snake valleys, crossing it to enter the Snake Canyon. They swept away a loessial cover more than 100 feet thick from 80 square miles of summit area and scored the basalt thus exposed into a congeries of canyons, cataracts, and rock basin.