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Welcome to Friends of Pompeys Pillar Nestled on the banks of the Yellowstone River Pompeys Pillar National Monument rises 120 feet into the sky. This sandstone outcropping covering roughly 2 acres at its base is not just a geologic wonder but a site of cultural and historic significance. Although perhaps best known for the inscription left behind by William Clark of the Lewis and Clark expedition this site boasts the markings of thousands of years of human occupation. We invite you to take a journey through our website to explore and learn more about the history of Pompeys Pillar National Monument.