The Allegheny Mountain Range (also spelled Alleghany and Allegany) — informally, the Alleghenies — is part of the vast Appalachian Mountain Range of the eastern United States and Canada. It has a northeast-southwest orientation and runs for about 400 miles (640 km) from north-central Pennsylvania, through western Maryland and eastern West Virginia, to southwestern Virginia.
The word "Allegheny" was once commonly used to refer to the whole of what are now called the Appalachian Mountains. John Norton used it (spelled variously) around 1810 to refer to the mountains in Tennessee and Georgia. Around the same time, Washington Irving proposed renaming the United States either "Appalachia" or "Alleghania".. In 1861, Arnold Henry Guyot published the first systematic geologic study of the whole mountain range. His map labeled the range as the "Alleghanies", but his book was titled On the Appalachian Mountain System. John Muir, in his book A Thousand Mile Walk to the Gulf (written in 1867), used the word "Alleghanies" for the southern Appalachians. Other people used the word "Appalachians".
There was no general agreement about the "Appalachians" versus the "Alleghanies" until the late 19th century. The term "Appalachian" became commonly used for the whole range, first by geologists and eventually, everyone.
Although there are no official boundaries to the Allegheny Mountain region, it may be generally defined to the east by the Great Valley (locally called the Cumberland Valley in Pennsylvania and the Shenandoah Valley in Virginia); to the north by the Susquehanna River valley; and to the south by the New River valley. To the west, the Alleghenies grade down into the dissected Allegheny Plateau (of which they are sometimes considered to be a part). The westernmost ridges are considered to be the Laurel and Chestnut Ridges in Pennsylvania and Laurel and Rich Mountains in West Virginia.
The mountains to the south of the Alleghenies -- the Appalachians in westernmost Virginia, eastern Kentucky, and eastern Tennessee -- are the Cumberlands. The Alleghenies and the Cumberlands both constitute part of the Ridge and Valley Province of the Appalachians.
The highest ridges of the Alleghenies are just west of the Front, which has an east/west elevational change of up to 3,000 feet. Absolute elevations of the Allegheny Highlands reach nearly 5,000 feet, with the highest elevations in the southern part of the range. The highest point in the Allegheny Mountains is Spruce Knob (4,863 ft/1,482 m), on Spruce Mountain in West Virginia. Other notable Allegheny highpoints include Thorny Flat on Cheat Mountain (4,848 ft/1478 m), Bald Knob on Back Allegheny Mountain (4,842 ft/1476 m), and Mount Porte Crayon (4,770 ft/1,454 m), all in West Virginia; Dans Mountain (2,898 ft/883m) in Maryland, Backbone Mountain (3360 ft/1024 m), the highest point in Maryland; Mount Davis (3,213 ft/979 m), the highest point in Pennsylvania, and the second highest, Blue Knob (3,146 ft/959 m).
Because of intense freeze-thaw cycles in the higher Alleghenies, there is little native bedrock exposed in most areas. The ground surface usually rests on a massive jumble of sandstone rocks, with air space between them, that are gradually moving down-slope. The crest of the Allegheny Front is an exception, where high bluffs are often exposed, revealing an exceptional view.