A mound of glass

This is a shot of Big Obsidian Flow, near Paulina Lake, Oregon. Created by a lava flow some 1300 years ago, is about 700 acres in size. The flow is made up of lava that cooled, and as it did so, became a pile of what's essentially glass. Walking along it you find large, shiney Obsidian rocks and boulders and pumice stones (apparently forms of glass, according to the informational placard...)