Thunder Rocks are tied to a local legend in which guardian spirits of rocks competed to throw the biggest rock to the top of the mountain, and a nearby Iroquois tribe, hearing the thunderous noises of the contest, went to the mountaintop, found the rocks, and named them Thunder Rocks. Geologically, the rocks formed from sediments eroded from the Acadian Mountains and carried westward by a river about 325 million years ago, where gravel bars and floodplain deposits later became Olean conglomerate, a sedimentary rock made of coarse sand and white, rounded quartzite pebbles. Their irregular and rounded pebbles indicate deposition in a stream environment. The deposits were buried, compressed, and cemented into solid rock, then later revealed by uplift and erosion that shaped the Allegheny Plateau and its deep valleys. Thunder Rocks are native bedrock, not glacially transported, and are among the youngest Paleozoic rocks in the park, found on its highest hills while older Devonian rocks are exposed in the valley walls.