The Grand Canyon of the Yellowstone, the first major canyon below Yellowstone Falls in Yellowstone National Park, extends about 24 miles through steep walls 800 to 1,200 feet deep and up to three-quarters of a mile wide. Brought to wider public attention by the Cook–Folsom–Peterson Expedition of 1869 and the Washburn-Langford-Doane Expedition of 1870, it became one of Yellowstone’s defining sights. Its vivid yellow, red, and white walls reflect Yellowstone’s volcanic and thermal past: after the Yellowstone Caldera eruption, lava flows, faulting, and hydrothermal alteration softened the rhyolite, and the Yellowstone River, possibly aided by late glacial outburst floods, cut the present canyon.