Plateau fine-grained volcanic rock

Dark gray fine-grained volcanic rock, often loosely called basalt, is a common toolstone on the Columbia Plateau. Terry Ozbun points out that most artifacts casually called basalt are really andesite, dacite or similar fine-grained, silica-rich rocks, because true basalt has too little silica to flake well. By silica content the series runs from basalt (under 52 percent) through andesite and dacite to rhyolite. The finer-grained, more silica-rich rocks fracture better and give sharper edges, and they hold an edge better than obsidian in rough use. In southeastern Washington, west-central Idaho and northeastern Oregon, workshops in Miocene Columbia River Basalt flows such as Pataha Canyon in the Blue Mountains provided basaltic andesite for large bifacial blanks.

Extrusive volcanic rocks, most of the Plateau stone coming from Miocene flows of the Columbia River Basalt Group. Low-silica basaltic andesites of the Grande Ronde Basalt can be separated chemically from high-silica andesites and dacites of the Saddle Mountain Basalt. Tough, dark rock outcrops as nodular clasts or tabular boulders at workshops.