Pompeys Pillar is one of the most famous sandstone buttes in America. It bears the only remaining physical evidence of the Lewis and Clark Expedition, which appears on the trail today as it did 200 years ago. On the face of the 150-foot butte, Captain William Clark carved his name on July 25, 1806, during his return to the United States through the beautiful Yellowstone Valley.

Captain Clark named the Pillar "Pompeys Tower" in honor of Sacagawea's son Jean Baptiste Charbonneau, whom he had nicknamed "Pomp." Nicholas Biddle, first editor of Lewis and Clark's journals, changed the name to "Pompeys Pillar."

Native Americans called the Pillar "the place where the mountain lion lies." Some observers suggest that a sandstone formation, that is a part of the Pillar, which resembles a mountain lion's head, is the reason for the name. Another theory cites live mountain lions being spotted in the area.