Hickory Run Boulder Field: A Frozen River of Stone

A few miles north of the interstate, inside Hickory Run State Park, lies one of the most disorienting natural features in Pennsylvania: a flat, treeless field blanketed in boulders, some of them twenty-six feet long, stretching four hundred feet wide and eighteen hundred feet end to end. No soil. No plants. Just stone, sitting in a valley as if someone poured a concrete mixer the size of a neighborhood.

Geologists call it a boulder field, and research suggests the boulders at the surface have sat in place for roughly six hundred thousand years, surviving multiple ice ages rather than forming in just the most recent one. During the last ice age, a glacier at least a mile thick covered the western portion of what is now the park. The eastern section, including the boulder field site, lay just beyond the glacier's edge in what is called a periglacial zone, a belt of land where freeze-thaw cycles operated in extreme overdrive. Water seeped into cracks in the bedrock, froze, expanded, and fractured the rock repeatedly over millennia. The resulting boulders slowly migrated downslope under gravity and the churning action of ice, eventually filling the valley to a depth of at least twelve feet.

What is remarkable is the flatness. A moving mass of boulders tends to heap up at angles. The exceptional levelness of the Hickory Run field suggests the boulders were carried by a periglacial stream, a shallow flood of meltwater running just below the surface of the frozen ground, that spread them evenly across the valley floor.

The National Park Service purchased Hickory Run in nineteen thirty-five as a recreation demonstration area, and in nineteen forty-five the land transferred to Pennsylvania as a state park. In nineteen sixty-seven, the federal government designated the boulder field a National Natural Landmark, one of only twenty-seven such designations in all of Pennsylvania. It remains the largest formation of its kind in the Appalachian Mountains.