For generations of travelers, this towering sentinel delineated the Great Plains to the east from the Rocky Mountains to the west. Forged by volcanic eruptions 75 million years ago, the Shonkinite formation was then carved by erosion. A signpost on the Old North Trail, the Western Hemisphere’s oldest road, the rock signified to Indigenous people entry to bison-rich hunting grounds. Meriwether Lewis, leader of President Jefferson’s Corps of Discovery, observed the landform on July 16, 1805, while scouting for the Missouri’s entrance to the Rocky Mountains. He named the large rock “immediately in the gap which the missouri makes on it’s passage from the mountains” the tower. From near the summit, he observed “immence herds of buffaloe.” He also identified an Indian road, the first such road mentioned in the journals. That evening his detachment camped near an island Lewis named Pine Island. On the morning of July 17, 1805, William Clark and the main body met up with Lewis. The expedition maneuvered through Pine Island rapids and then proceeded upstream. Tower Rock represented a transition point for the expedition, where they exited the plains for the more unforgiving environs of the Rocky Mountains. After the Corps passed through, the Blackfeet’s dominance in the area made visits from trappers and traders infrequent but did attract a Jesuit mission in nearby Cascade. The tribe considers the space sacred and traditionally used the rock as a place for ceremony and prayer. The Montana Department of Fish, Wildlife and Parks established Tower Rock State Park in 2004.