longitudinal chain of valley lowlands of the Appalachian mountain system of North America. Extending from Canada on the northeast to Alabama, U.S., on the southwest, it includes the St. Lawrence River valley in Canada and the Kittatinny, Cumberland, Shenandoah, and Tennessee river valleys in the United States. In its southerly region the Great Appalachian Valley divides the Appalachian Mountains into two subequal portions; in its northern section it lies west of the Appalachian ranges of New England and Canada, separating them from the Adirondack group. The valley contains rich farmland, especially in the Cumberland and Shenandoah valleys.