From the record
“Pompeys Pillar National Monument is a rock formation located in south central Montana, United States. Designated a national monument on January 17, 2001, and managed by the U.S.”
Wikipedia article Pompeys Pillar National Monument, CC BY-SA 4.0.
One of 48 National Register listings published here for Yellowstone County · listed in 1966 · category: Site · built in 2001 · a National Historic Landmark.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service
