Pompey’s Pillar is the smallest National Monument in the country. It only measures a mere 51 acres.

Basically, the Pompey’s Pillar is a huge rock formation measuring 150 feet above the Yellowstone River. It is primarily composed of mixed siltstone and sandstone and is believed to have formed from the Late Cretaceous Epoch.
The Pompey’s Pillar is popular for its Native American petroglyphs. It is considered as a significant landmark by natives. However, it is also a historic site for bearing William Clark’s name engraved on it. This mark is the only remaining physical evidence of the trail followed by the Lewis and Clark Expedition, which William Clark was a co-leader of.

Pompey’s Pillar is available for visits seasonally. You can check their website or call their office to learn more about their schedule.