Pompeys Pillar National Monument is a sandstone butte above the Yellowstone River in south-central Montana with evidence of human presence stretching back about 11,000 years. Its surface preserves Native American petroglyphs and later historic inscriptions, making it an unusual record of travel through the Yellowstone valley. The site is especially significant for the signature William Clark carved there on July 25, 1806, during the Lewis and Clark Expedition, the only remaining physical evidence found along that route. Named for Jean Baptiste Charbonneau, whom Clark called Pompey, the formation later became a National Historic Landmark in 1965 and a national monument in 2001.