Called Iishiia Anaache or "Place Where the Mountain Lion Dwells" by the Apsaalooka (Crow) people, Pompey's Pillar was a well-known landmark to the Plains Indians. It was here, at a strategic natural crossing of the Yellowstone, or Elk River as it was known to the Apsaalooka, that the Indian people met to trade and exchange information. They painted pictographs and etched petroglyphs onto the sheer cliffs of the feature. Apsaalooka legend reports that Pompey's Pillar was once attached to the sandstone bluffs on the north side of the river. At one point, however, the rock detached itself from the cliffs and rolled across the river to its present site.
Pompey's Pillar was also a significant landmark for Euro-American explorers, fur trappers, soldiers and immigrants. It was discovered by Canadian North West Company employee Francois Larocque in 1805. A little less than a year later, on July 25, 1806, it was visited by a 12 man detachment under the command of William Clark that included Sacajawea and her infant son. Clark carved his name and the date on the rock and named it in honor of Sacajawea's son. He was just one of hundreds of individuals who for generations have left their marks on the rock.