Originally Posted by DBR96A
Pittsburgh was founded in 1758 and incorporated in 1771, so it predates the railroads and canals. If you look at the geology of Pennsylvania, you'll notice a series of very long ridges in the middle of the state that were virtually impossible to navigate during colonial times. The only way around the ridges were water gaps, and the most convenient water gap for settlers moving west was to the south at the Potomac River, which forms the border between Maryland and Virginia.
However, the Potomac River still passes nowhere near Pittsburgh, and it even turns back to the southwest closer to its headwaters. Even worse, the terrain changes markedly at the Allegheny Front, and becomes even more difficult on the Allegheny Plateau, which is a heavily dissected high plateau that's been eroded to the point of resembling an endless low mountain range. (Geologically speaking, it's actually a broad series of escarpments, rather than an actual mountain range.)
The good news is, the Allegheny Front is a major water divide in the Eastern U.S., and luckily enough, the first tributary to the interior river valleys rises just a few miles to the west of it. The Casselman River carves its way through some of the most difficult terrain on the Allegheny Plateau, and eventually flows to the Youghiogheny River (yock-a-GAY-nee), which flows to the Monongahela River (ma-NON-ga-HAY-la), which flows to the Ohio River at the confluence of the Allegheny River in Pittsburgh.
The most treacherous part of the route to Pittsburgh from the rest of Pennsylvania was between the Potomac River and the Casselman River, where settlers had to find their way up and over one long mountain ridge, and then the Allegheny Front. From there, they would just follow the river tributaries to Pittsburgh. Long story short, traveling to Pittsburgh from the east involved turning southwest in order to turn northwest.
This southwesterly turn made the route more convenient for settlers from Virginia as well, which is why Virginia attempted to claim a large amount of territory to its northwest, including a piece that is now the southwestern corner of Pennsylvania, west of Laurel Ridge, and south of the Ohio, Allegheny and Kiskiminetas Rivers (KIS-kim-in-NEET-as). Pennsylvania and Virginia resolved this border dispute by extending the Mason-Dixon Line to the west for 40 miles or to the Ohio River, whichever came first. This extension placed Pittsburgh firmly in Pennsylvania.
Oddly, the westward extension of the Mason-Dixon Line ended at 40 miles instead of the Ohio River, but the Ohio River was only another 12 miles away at that point, which explains part of the funky shape of the state of West Virginia.
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