Most of Pennsylvania is part of the Appalachian Mountains, including the south central and northeastern areas of the state. Much of the rest of the state is very hilly, partly due to the closeness to mountains and partly due to the steep river valleys in the state. The Monongahela, Allegheny, and Ohio rivers are in the western part of the state, and the Susquehanna is in the central part of the state. The northwestern and southeastern parts of the state are mostly flat and low-lying.