Pennsylvania is bisected in an S-curve (or roughly diagonally) by the barrier ridges of the Appalachian Mountains from southwest to northeast, forcing pretwentieth century ground travel most often on or near the ancient Amerindian trails along the higher terrains of the local watersheds with limited penetration and connectivity often only through water gaps. As rough looking as the first map appears, the valley bottoms throughout the entire central part of the state and parts of lower New York state are connected by the Susquehanna River and its tributaries—virtually the whole length of which was improved into the Pennsylvania Canal System in the 1830s. To the northwest of the folded mountains is the Allegheny Plateau which lies above the cliff-like Allegheny Front escarpment, which continues into southwestern and south central New York, and is pierced in only a few places known as the gaps of the Allegheny.
This Allegheny Plateau is so dissected by valleys formed by small streams and springs that it also seems mountainous, but with elevation differences between valley floors and hilltop peaks most often less than a few hundred feet. The plateau is underlain by sedimentary rocks of Mississippian and Pennsylvanian age, which bear abundant fossils, mineral deposits such as iron, as well as natural gas, coal and petroleum. These regions fostered 17th-19th century industries locally even into the 1930s days of mass production. To the south and east of the escarpment/plateau region, the folded mountains and alternating valleys are known as the Ridge-and-Valley Appalachians. These extend from the South of the Appalachians to northern New England save where cut by water gaps. In Northeastern Pennsylvania from the east of Harrisburg on the Susquehanna River, these ridge and valley features contain the richest and most widespread deposits of high energy clean burning anthracite coal in the world—the Coal Region—without which the industrial revolution in the United States would have been severely retarded and delayed.
In 1859, near Titusville, Edwin L. Drake drilled the first oil well in the U.S. into these sediments.[2] 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 over top the mountains. At its furthest extent it spread as far south as Moraine State Park, about 40 miles (64 km) north of Pittsburgh.
