Pompeys Pillar National Monument

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Pompeys Pillar National Monument is a rock formation. The pillar itself stands 150 feet (45 m) above the Yellowstone River and consists of sandstone from the late Cretaceous Hell Creek Formation, 75 - 66 million years ago.

The pillar features an abundance of Native American petroglyphs, as well as the signature of William Clark, co-leader of the Lewis and Clark Expedition. Clark's inscription is the only remaining physical evidence found along the route that was followed by the expedition.

The pillar gets 50,000 visitors annually. Archeological evidence suggests that the outcropping has been witness to 11,000 years of human involvement in the area. Consequently, in addition to the pictographs and the signature of William Clark, hundreds of other people have carved their initials into the rock, including early pioneers to the area.