Lower Falls was shaped where volcanic heat and gases below the falls softened the rhyolite rock, allowing the Yellowstone River to carve faster there than upstream, sculpting a ledge and creating the waterfall in a canyon wall of hardened rhyolite lava exposed by the river's excavation of the canyon. Reaching the brink requires descending a steep, winding trail about 600 feet in 3/8 mile to an overlook at the edge, where the Yellowstone River can be seen plunging over the falls.