Glass Mountain, on the Inyo National Forest, is one of the tallest peaks in Mono County, California. The peak lies 20 mi southeast of the shoreline of Mono Lake and is the highest point on the 4 mi long sinuous Glass Mountain Ridge. The Glass Mountain Ridge forms the northeast boundary of Long Valley Caldera. It consists of a sequence of lava domes, flows, and welded pyroclastic flows of rhyolite composition that were erupted between 2.1 and 0.8 million years ago. Obsidian, a naturally occurring volcanic glass, can be found on the mountain.