PAPennsylvania Geography

The only curved state line in the country

Every other boundary between two American states is a straight line, a river, or a ridge. Pennsylvania's southeastern corner is an arc. The Twelve-Mile Circle was struck with a radius of twelve miles from the cupola of the courthouse in New Castle, Delaware, under the terms of William Penn's charter, and where that arc runs it is the border between Pennsylvania and Delaware. It is the only circular state boundary in the United States, and it left behind a stray scrap of ground called the Wedge, caught between the circle and the Maryland line, which three states argued over until 1921.

The southern boundary is the Mason-Dixon Line itself. Charles Mason and Jeremiah Dixon surveyed it between 1763 and 1767 to end eighty years of lawsuits and occasional gunfire between the Penn and Calvert families, and they fixed it at 39 degrees 43 minutes north, setting stone markers every mile with the Penn arms carved on the north face and the Calvert arms on the south. Only later did the line become shorthand for the divide between North and South.

The 202,187 acres Pennsylvania bought to reach a lake

Look at the northwestern corner and there is a blunt triangle pushing up to Lake Erie. Pennsylvania did not start out with it. New York, Massachusetts and Connecticut all had claims on that ground, the federal government pressed them into surrendering, and in 1792 Pennsylvania bought the 202,187-acre Erie Triangle from the United States at 75 cents an acre, a total of $151,640.25. Without the purchase the state would have no Great Lakes frontage whatsoever. With it, Pennsylvania has roughly 51 miles of Lake Erie shore and a working port.

That port is sheltered by Presque Isle, a curving sand spit anchored to nothing. Waves and currents shift its sediment steadily eastward, so the peninsula has crept along the shoreline over the past few centuries and the Army Corps of Engineers spends money there every year holding it in place. The upshot is that Pennsylvania keeps both a Great Lakes port and an ocean port, since Philadelphia sits on the tidal Delaware about 90 miles from the open Atlantic.

Water leaves Pennsylvania four different ways

Very few states send their rain to four separate destinations. Pennsylvania does.

Basin	Main rivers	Where the water ends up
Susquehanna	Susquehanna, West Branch, Juniata	Chesapeake Bay
Delaware	Delaware, Lehigh, Schuylkill	Delaware Bay
Ohio	Allegheny, Monongahela, Ohio	Gulf of Mexico via the Mississippi
Lake Erie	Conneaut Creek, Elk Creek	The Atlantic via the St. Lawrence

The Susquehanna alone drains close to half the state. At 444 miles it is the longest river on the American Atlantic seaboard, and it delivers roughly half the fresh water entering Chesapeake Bay, which is why farm runoff in central Pennsylvania turns into a Maryland and Virginia problem. It is also older than the mountains it crosses. The river held its course while the Appalachian ridges rose around it, so rather than detouring it saws straight through them in a series of water gaps.

At Pittsburgh the Allegheny comes down from the north and the Monongahela comes up from the south, and where they meet at the Point they both stop existing and become the Ohio. No other major American river begins in the middle of a downtown.

Ridges you can pick out from orbit

The Ridge and Valley province across central Pennsylvania is the most legible piece of folded rock in the eastern United States. Long parallel ridges of resistant sandstone run northeast to southwest and then hairpin around the noses of plunging folds, producing a zigzag pattern that shows up clearly in satellite imagery. Kittatinny Mountain is the easternmost of them, and rivers punch through the wall at the Delaware Water Gap, the Lehigh Gap, and at Harrisburg, where the Susquehanna cuts four ridges in a row.

West of the folds the Allegheny Front rises sharply and the country levels out into the Allegheny Plateau, which covers more of the state than anything else. Mount Davis, the high point at 3,213 feet, is a broad swell on that plateau in Somerset County rather than a peak, and a road runs most of the way up. The low point is sea level on the tidal Delaware. Pennsylvania also holds a sliver of Atlantic Coastal Plain in Philadelphia and a narrow band of Central Lowland along Lake Erie, so a state ranked 33rd in area still manages to contain seven physiographic provinces.

Ice, anthracite and a canyon

The Wisconsin ice sheet pushed into the northeast and the northwest but stopped short of the middle and south of the state. That old ice limit still shows up on any map of lakes. The Pocono country and the Erie corner have natural glacial ponds; central and southern Pennsylvania have essentially none, and every large body of water down there sits behind a dam. Conneaut Lake in Crawford County, a glacial kettle, is the largest natural lake in the state.

Beneath the northeastern hills lies something Pennsylvania has almost entirely to itself. Nearly all the anthracite in the United States occupies a field of roughly 480 square miles around Scranton, Wilkes-Barre, Hazleton and Pottsville, squeezed and cooked into near-pure carbon by the same continental collision that folded the ridges above it. Softer bituminous coal fills the western plateau, and the well that launched the modern oil industry was drilled at Titusville in 1859.

Pine Creek has carved a gorge 47 miles long and as much as 1,450 feet deep across the northern plateau. The forest around it is dark enough at night that Cherry Springs State Park was certified as an international dark sky park, one of very few east of the Mississippi. Pennsylvania's recorded temperature extremes are 111 degrees at Phoenixville on 10 July 1936 and 42 below zero at Smethport on 5 January 1904, and the entire state keeps Eastern Time.

Common questions

Geography of the bordering states

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