Pompeys Pillar National Monument is a rock formation located in south central Montana, United States.

Exhibits in the 5,700-square foot center relate the journey of Captain William Clark and his detachment, including Sacagawea and her son Pomp, down the Yellowstone River Valley in 1806.

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.

[3] Clark wrote that he climbed the sandstone pillar and "had a most extensive view in every direction on the Northerly Side of the river".

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.