At the brink of Upper Falls, the Yellowstone River plunges 109 feet between solid rock walls, revealing the force of the river and the geologic history of the canyon. About 484,000 years ago, the rock here was hot, molten rhyolite lava flowing across the land, and today Upper Falls cuts through that hardened lava flow. In 1867, prospector A. Bart Henderson wrote of stepping onto a rock and suddenly looking down into depths that seemed like the bowels of the earth, where the Yellowstone plunged amid a country of lava and boiling springs.