Columbia River Basalt Group map shows main regions of basalt exposure in Washington, Oregon, Idaho and Nevada, USA. (usgs.gov)
The Swinomish Reservation is roughly 15 square miles (39 km2; 9,600 acres) in size, 7,450 acres (3,010 ha) of which are upland and 2,900 acres (1,200 ha) of which are tidelands.
