GEOLOGY OF THE APPALACHIAN TRAIL IN PENNSYLVANIA

With 6 states to the north, and 7 to the south, and containing the mid-point of the trail, Pennsylvania is the keystone of the Appalachian Trail (AT). But it is also the place where the geology changes significantly from New England to the southern Appalachians. The 230 miles of the trail in PA has been described as where boots go to die, because of its rocky nature. The trail passes 2 geologically-significant National Natural Landmarks (NNL) designated by the NPS, and passes near several others.

Because of the changes in geology, the trail in PA probably more varied than in any of the other states on the trail. It crosses 5 physiographic sections in the Ridge & Valley Province, and rocks & sediments ranging in age from Cambrian to Pennsylvanian, plus Jurassic and Pleistocene. In Pennsylvania, the AT passes from glaciated to unglaciated terrain. North of PA, the AT is largely in the New England Province, which ends in PA, passing into the Ridge and Valley in NJ. From the Delaware Water Gap to Swatara Gap, the AT parallels strike along a single ridge of deformed quartzite and conglomerate of the Shawangunk and Tuscarora Fms. The AT then turns NW into the folded Appalachians, crossing several folds and the southern tip of the anthracite region, turns SW along the Pocono Fm ridgetop of Peters and Cove Mts, and crosses one of the Susquehanna Water Gaps, a NNL. Along Cove Mt., the AT turns south, crossing Blue Mt, and enters the Great Valley. Its route across the Great Valley follows on, or adjacent to Ironstone Ridge, formed by a Jurassic-age diabase dike, passing onto South Mountain, the northern terminus of the Blue Ridge. The trail continues on the Blue Ridge for the remainder of its length to Georgia.

The PA Geological Survey published, in 1983, a report on the geology of the AT in PA, by J Peter Wilshusen. It describes the geology of 40 sites along the trail, and includes a geologic map along the AT corridor. This report may serve as a model for elsewhere on the AT, and a digital version could be based on it.