 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.