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To the west, the Allegheny Mountains grade down into the dissected Allegheny Plateau and Cumberland Plateau. To the east of the Allegheny Mountains (and east of the Allegheny Front) are the Ridge-and-valley Appalachians.

Much of the Monongahela National Forest in West Virginia is in the Allegheny Mountains. These mountains also include the Dolly Sods Wilderness, Laurel Fork Wilderness , and Cranberry Wilderness.

The bedrock of the Alleghenies is mostly sandstone and metamorphosed sandstone, quartzite, which is extremely resistant to weathering. Prominent beds of resistant conglomerateIn geology, a conglomerate is a rock consisting of other stones that have been cemented together. Conglomerates are sedimentary rocks consisting of subangular clasts and are thus differentiated from breccias, which consist of angular clasts. Both conglome can be found in some areas, such as the Dolly Sods area. When it weathers, it leaves behind a pure white quartzite gravel.

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, exposing an exceptional view.