Abstract The type area of the Pennsylvanian System is the central part of the Appalachian Mountains, and specifically, the state of Pennsylvania. The Pennsylvanian rocks of the area are divided into the Pottsville supergroup, the Allegheny group, the Conemaugh group, the Monongahela group; and are overlain by the Dunkard group. The thick, primarily sandy, sequence of the Appalachian basin grades westward into cyclical units. Marine units are but a minor part of the system. Several biofacies are well developed—those with plant compressions, a large number of fresh-water zones, a few vertebrate communities, and thin, marine invertebrate-bearing limestones and shales. Structural deformation of the region developed through much of the Paleozoic Era, and the culminating westward thrusting and folding in the miogeosynclinal sediments is attributable to the Allegheny orogeny of post-Pennsylvanian (and perhaps pre-Permian) time. Pennsylvanian rocks of the area contain many minable coal beds, of which the anthracites, the Pittsburgh seam, and the Pocahontas coals are the more valuable; and the Kittanning and Freeport coals are of major importance. Petroleum and natural gas of local importance are produced in a restricted area within the central Appalachians.