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The Taconic orogeny
An orogeny is what happens when two landmasses are pressed together and the rocks between them fold and are thrust upward. Mountain ranges are rarely built in one event. The Appalachians are the product of three separate collisions across 250 million years — Taconic, Acadian and Alleghanian — each leaving its own set of rocks and faults. Debris eroded from the Taconic mountains filled the inland sea to the west and forms the thick sedimentary layers beneath much of Ohio and Pennsylvania.
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