ALLEGHANY MOUNTAINS or ALLE GHANY PLATEAU, a name sometimes used to designate the entire Appalachian Mountain system, but more properly applied to the western range of this system in Pennsylvania, Mary land, Virginia and West Virginia. They begin near the New York and Pennsylvania borderó the Catskills forming a northern outlieró and extend in a southwesterly direction into West Virginia, where the line of elevations is con tinued by other ranges across Tennessee. In the northern part the mountains have an ele vation of about 2000 feet (over 4,000 feet in the Catskills). but they gradually increase in alti tude southward until in Virginia they rise to 4,500 feet above the sea. Throughout their ex tent they present a remarkably even crest-line with few gaps and isolated peaks. On the east ern side the Alleghany Front or Alleghany escarpment is an abrupt slope overlooking the Great Valley, a depression from 50 to 100 miles wide, lying between the eFronts and the Blue Ridge. On the west, the plateau has a gentler
bounding slope. The range forms the water parting between the streams draining into the Atlantic Ocean and the Gulf of Mexico. The former receives the drainage from the eastern slope principally through the Delaware, Poto mac and James rivers, while the Ohio River collects most of the waters on the western side. Except at the eastern edge, where the rocks partake of the folding of the Appalachian Mountains proper, the sediments are nearly horizontal and the region is rather a deeply dissected plateau than a true mountain range. Limestone, sandstones and conglomerates are the predominant formations and range from the Cambrian to the Carboniferous systems. Immense coal-seams occur in the higher part of the series. See APPALACHIANS.