Late Carboniferous–Permian - About 325–260 million years ago

Continental collision folds the rocks beneath Preston

As the present-day African part of Gondwana collided with ancestral North America, compression helped build the Appalachians and assembled smaller continents into the supercontinent Pangaea, a vast landmass joining continents that are now separated. Sedimentary layers were bent into upward arches called anticlines and downward troughs called synclines. In the Preston County region, structures such as the Ligonier Syncline and Preston Anticline preserve this folded arrangement. Later erosion reshaped the surface, so folds in the bedrock do not always match the hills and valleys above them. Chestnut Ridge is a familiar exception: the ridge corresponds to the anticline of the same name.