western section of the Appalachian Mountains, U.S., extending southwestward from the Mohawk River valley in central New York to the Cumberland Plateau in southern West Virginia. Generally sloping toward the northwest, the plateau has been dissected by streams to form the Catskill, Allegheny, and other mountain ranges. The Allegheny, Delaware, and Susquehanna rivers drain its northern portion, while the Ohio River system drains the southern part. The plateau is mainly covered by hardwood forest.

With the discovery of coal, a large influx of settlers led to an early breakdown of the isolation of this part of the Appalachians. The regional economy depends heavily upon the extraction of coal, natural gas, and petroleum.