Hellroaring Mountain is an 8,374-foot summit in Yellowstone National Park’s Absaroka Range, rising in Montana with a lower south slope extending into Wyoming above Hellroaring Creek and the Yellowstone River drainage. Its steep relief is striking, but its greatest significance is geological: unlike much of Yellowstone’s volcanic terrain, it consists of Precambrian gneiss, granite, and schist. As Yellowstone’s largest granite outcrop, it preserves rock about 2.7 billion years old, among the oldest exposed in the world, offering an unusually deep window into Earth’s history. Named by prospectors in 1867, the mountain’s official form was settled in 1965.