Forbes Road cuts across two geologic provinces of the Appalachian Mountains: the folded sedimentary rocks of the Valley and Ridge and the essentially horizontal strata of the Allegheny Plateau. The road crossed three major ridges: Allegheny Ridge, Laurel Ridge, and Chestnut Ridge. Uplift, combined with differential erosion of Late Paleozoic rocks has created the relief. The relief of these ridges posed a major engineering obstacle in the 18th century.
The two forts constructed along the road, Forts Bedford and Ligonier, used the local geology for defensive advantage. In the summer of 1759, Fort Pitt was built near the site of Fort Duquesne. The settlement that grew up around it became Pittsburgh. Much of Forbes Road became part of the Pennsylvania portion of the Lincoln Highway. This first transcontinental automobile highway in the United States was begun in 1913.