Sideling Hill is part of the Appalachian Mountains, specifically the Ridge-and-valley Appalachians. It crosses Virginia, Maryland and Pennsylvania.
The Sideling Hill road cut is a 340-foot deep road cut where Interstate 68 cuts through Sideling Hill, about 6 miles west of Hancock, Maryland in Washington County, Maryland. It is notable as one of the best rock exposures in Maryland and indeed in the entire northeastern United States. Almost 810 feet of strata in a tightly folded syncline are exposed in this road cut. Although other exposures may surpass Sideling Hill in either thickness of exposed strata or in quality of geologic structure, few can equal its combination of both. There is an Exhibit Center to help provide the public with a better understanding of the geology of the cut. A pedestrian walkway bridge crosses I-68 for better access to the cut, along with a picnic area and rest area facilities.
A tunnel was partially bored through the mountain near Breezewood, Pennsylvania by the South Pennsylvania Railroad in the 1880s; it was left abandoned when the railroad folded. When the Pennsylvania Turnpike Commission began constructing a toll highway on the right-of-way of the old South Penn Railroad in the 1930s, the tunnel was completed and used by traffic from the Turnpike's opening in 1940 until, the volume of traffic having become too great for the two-lane, single-tube tunnel to handle, it was bypassed in 1968.
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