Pompeys Pillar is on the banks of the Yellowstone River. The town is named for the nearby rock outcropping, which is today Pompeys Pillar National Monument. The sandstone pillar rises 200 feet above the Yellowstone River 30 miles east of Billings. During his return trip, William Clark of the Lewis and Clark Expedition climbed the Pillar and carved his signature and the date in the sandstone. It is the only site on the trail where visible evidence of the Lewis and Clark Expedition may be viewed by the public.