Address: 44°49'20.3"N 110°43'40.6"W
Located in Yellowstone, on the left side of Grand Loop Road heading south from Mammoth Village, the half-mile-long Obsidian Cliff can be seen. The free sight is a National Historic Landmark and archaeological site because it is the product of 180,000-year-old rhyolite lava flow which crystallized. If you stop to take a closer look (through binoculars because exploration of the site is off-limits) you can see shiny black volcanic glass and obsidian in the layers. Early natives used this rare material to create tools and weapons. North of the cliff is the Obsidian Cliff interpretive kiosk.