Geography of Pennsylvania - The Mountains

The Mountains

Pennsylvania is bisected diagonally by ridges of the Appalachian Mountains from southwest to northeast. To the northwest of the folded mountains is the Allegheny Plateau, which continues into southwestern and south central New York. This plateau is so dissected by valleys that it also seems mountainous. The plateau is underlain by sedimentary rocks of Mississippian and Pennsylvanian age, which bear abundant fossils as well as natural gas and petroleum.

In 1859, near Titusville, Edwin L. Drake drilled the first oil well in the U.S. into these sediments. Similar rock layers also contain coal to the south and east of the oil and gas deposits. In the metamorphic (folded) belt, anthracite (hard coal) is mined near Wilkes-Barre and Hazelton. These fossil fuels have been an important resource to Pennsylvania. Timber and dairy farming are also sources of livelihood for midstate and western Pennsylvania. Along the shore of Lake Erie in the far northwest are orchards and vineyards.

During the most recent Ice Age, the northeastern and northwestern corners of present-day Pennsylvania were buried under the southern fringes of the Laurentide ice sheet. Glaciers extended into the Appalachian valleys of central Pennsylvania, but the ice did not overtop the mountains. At its furthest extent it spread as far south as Moraine State Park, about 40 miles (64 km) north of Pittsburgh.