The Sideling Hill is a long, steep, narrow mountain ridge that runs north to south across western Maryland in Washington County, in the United States. For many centuries, this mountain had blocked the path of many travelers who wished to go from Maryland to northeast West Virginia and vice versa. Travelers had to decide whether they wanted to go around it or over it, but both routes being treacherous, resulted in many mishaps.
The first tunnel was blasted through the rocks of Sideling Hill in 1873–74 for the East Broad Top Railroad. This was followed by at least a couple more. The original railroad tunnel ceased operation in 1956, and the one opened in 1940 is now abandoned. A few decades later, when the Maryland State Highway Administration was laying down Interstate 68 across the state of Maryland, they decided that another tunnel through Sideling Hill would be too expensive. Instead, they decided to cut a deep notch across the hill and lay the road through it. After excavating 10 million tons of rock, engineers discovered they had exposed an unusual geologic structure — a syncline of tightly folded rock strata dating back more than 350 million years.