Image courtesy Pennsylvania Department of Conservation and Natural Resources
Pennsylvania may be the quintessential Appalachian state. It straddles the entire range, starting from the Atlantic coastal plain on the extreme southeast corner, where young sediments are shown in dark green (Tertiary) and yellow (recent). The oldest rocks (Cambrian and older) at the core of the Appalachians are depicted in orange, tan and pink. The collisions between the North American and Europe/African continents pushed these rocks into steep folds. (The green-gold strip represents a crustal trough where today's Atlantic Ocean began to open much later, in Triassic and Jurassic time. The red is thick intrusions of basalt.)
To the west, the rocks grow progressively younger and less folded as the full range of the Paleozoic Era is represented from the orange Cambrian through the Ordovician, Silurian, Devonian, Mississippian, and Pennsylvanian, to the greenish-blue Permian basin in the southwest corner. All these rocks are full of fossils, and rich coal beds occur in western Pennsylvania.
The American petroleum industry began in western Pennsylvania, where natural oil seeps were exploited for many years in the Devonian rocks of the Allegheny River valley. The first well in the United States drilled specifically for oil was in Titusville, in Crawford County near the northwest corner of the state, in 1859. Soon afterward began America's first oil boom, and the region is littered with historic sites.