Sideling Hill is a long, steep ridge in the Ridge-and-Valley Appalachians of Maryland, West Virginia, and Pennsylvania, shaped as a classic syncline with hard sandstone on its crest and softer rocks on its slopes. That geology created both the ridge and the narrow valleys beside it, while leaving much of the crest forested. Because few routes could cross such difficult terrain, Sideling Hill became the site of notable transportation works, including railroad and turnpike tunnels, the Sideling Hill Road Cut, older National Road grades, and nearby river and canal crossings near Hancock. Its landscape vividly links Appalachian geology with the history of engineering and travel.