After Old Faithful, this magnificent canyon is probably the park's best-known feature, roughly 20mi long, 800-1,200ft deep and 1,500-4,000ft wide. The brilliant color of its rhyolite rock is due to iron compounds "cooked" by hydrothermal activity. Weathering resulted in the oxidation of the iron, producing the yellow, orange, red and brown colors. At the end of the last ice age, scientists believe ice dams formed at the mouth of Yellowstone Lake. When breached, they released tremendous amounts of water down the Yellowstone River course, carving the canyon.