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.
I've also prepared a 1200x900 pixel version of this map (290 KB) that includes the explanation. All the text is readable. The explanation also includes the many different resourcesstone, gravel, coal, oil and moreextracted from everywhere in the state. A larger version at 3100x2400 pixels (525 KB) has every pixel of the state's original PDF file. You can also get that map and many others from the state Department of Conservation and Natural Resources.
More Pennsylvania resources on About.com:
Pennsylvania Geography, State Symbols & Facts
Pennsylvania National Parks
Pennsylvania State Parks
Pennsylvania Rock Climbing & Caving
Pennsylvania Bed & Breakfasts