Mississippian - About 330 million years ago · Countywide
Long before Preston County existed, a shallow tropical sea spread across the region. Sediment accumulating in this marine setting became the limestone-rich Greenbrier Formation. These ancient layers remain part of the county’s deeper bedrock, exposed only in places where folding lifted the rocks and rivers later cut deep valleys. They connect the landscape of today with a time when this area lay beneath the sea.
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Read full entry and research notes →Pennsylvanian - About 323–299 million years ago · Countywide
During Pennsylvanian time, rivers and shifting lowland environments deposited sand and mud across the region. Burial and cementation turned these materials into sandstone and shale. These layers became part of the bedrock of the future county. Much later, erosion exposed resistant sandstone at places such as Coopers Rock, leaving cliffs and rock masses that visitors can see today.
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Read full entry and research notes →Late Carboniferous–Permian - About 325–260 million years ago · Countywide
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.
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