Pompeys Pillar National Monument is located on the banks of the Yellowstone River, 30 miles east of Billings, Montana’s largest city. The Monument was established in 2001 to protect the only remaining physical evidence of the Lewis and Clark Expedition: Captain Clark’s signature, which he carved into a 150-foot sandstone butte on July 25, 1806. In his journals, Clark named the Pillar “Pomp’s Tower” in honor of the infant son of Sacagawea, the Shoshoni woman who accompanied the expedition and contributed greatly to its success. The historic signature remains today, and visitors can walk on a boardwalk to see it. There is also evidence of Native Americans, early explorers, fur trappers, the U.S. Cavalry, railroad development and early homesteaders, many of whom left their history embedded in this same sandstone pillar.