Designated a National Historic Landmark in 1965 and a National Monument in 2001, Pompeys Pillar is nationally significant for its association with the Lewis and Clark Expedition. On their trip west, the members of the Corps of Discovery, led by Captains Meriwether Lewis and William Clark, stuck together, following their instructions from President Thomas Jefferson. Jefferson had commanded them “to explore the Missouri river” in search of the fabled “Northwest Passage,” a water route that would theoretically facilitate trade between the eastern United States and Asia. The route did not exist. On their return from the Pacific, the captains decided to split up to explore the country more thoroughly. Sacagawea and her eighteen-month-old son, Jean Baptiste Charbonneau (nicknamed “Pompey” by Clark) traveled with Clark. On July 26, 1806, Clark and his party arrived at this prominent rock formation, visible from the Yellowstone River. Clark stopped to climb the tower, which he named after young Jean Baptiste. Clark also etched his name and the date in the soft sandstone. Clark’s signature joined a number of petroglyphs—carved images of animals, shields, and other signs—created by the native peoples who made their home here, including the Crow (Apsáalooke). The Apsáalooke call this sacred site Iishpíialawaache, “where the mountain lion sits.” Because the formation is located at a natural river crossing, they often came here to trade with their allies, the Nez Perce and the Shoshones. Later Europeans also carved their names into the rock, including the captain of the steamboat Josephine in 1875 and infantrymen commanded by Colonel John Gibbon in 1876.