ALLEGHENY MOUNTAINS or THE ALLEGHE NIES, a name formerly applied to all the Appalachian mountains (q.v.), and now, sometimes, to all that system lying west and south of the Hudson river, being steep and narrow-crested in Pennsylvania (',Soo-1,800ft.), in Maryland, Virginia and West Virginia rising to above 4,000f t. and with broader crests. It is also used to include the ridges (the Allegheny ridges) parallel to the Blue ridge; the north-western part of this region is called the Allegheny Front, and is characterized by a bold south-eastward facing escarpment and a gentle north-westward slope. The Alle gheny plateau is the north-westernmost division of the Appala chian system; it is an eroded mass of sedimentary rock sloping north-westward to the Lake plains in the region of the Great Lakes and reaching south-west from the Catskill (q.v.) region of New York State to the Cumberland plateau region in Tennessee. The Alleghenies, which once formed a barrier to western communica tion, have long since been crossed by numerous railways and high ways which follow the lower elevations or the "water gaps" of the eastward-flowing streams.