 Five major rock formations are present in Leonard Harrison State Park, from the Devonian and Carboniferous periods. The youngest of these, which forms the highest points in the park and along the gorge, is the early Pennsylvanian Pottsville Formation, a gray conglomerate that may contain sandstone, siltstone, and shale, as well as anthracite coal. Low-sulfur coal was once mined at three locations within the Pine Creek watershed. Below this is the late Mississippian Mauch Chunk Formation, which is formed with grayish-red shale, siltstone, sandstone, and conglomerate. Millstones were once carved from the exposed sections of this conglomerate. Together the Pottsville and Mauch Chunk formations are some 300 feet (91 m) thick.
