Pompeys Pillar National Monument

Pompeys Pillar is a sandstone rock formation that stands 150 feet (45m) above the Yellowstone River. The rock features Native American petroglyphs and the signature of William Clark of the Lewis and Clark Expedition. Clark signed the rock on July 25, 1806 and it can still be seen today. Clark named the pillar after Sacagawea's son, Jean Baptiste Charbonneau whom he called Pompy.