The Folded Rocks of Sideling Hill Road Cut

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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 travellers who wished to go from Maryland to northeast West Virginia and vice versa. Travellers 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.

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Sunrise hits the south side rocks of the Sideling Hill road cut along I-68. Photo credit

A syncline is a fold in which the strata on either side dip inward toward the axis. Such folding resulted from the enormous compressional stresses developed in the Earth's crust by the collision of the North American and African continents. The 100-meter-deep cut, located on Interstate 68, about 11 km (7 miles) west of Hancock, is considered one of the best rock exposures in the United States. It provides an excellent outdoor classroom where students of geology can observe and examine various sedimentary rock types, structural features, and geomorphic relationships. A walkway over the highway and steps leading to the rock formations allow students and visitors alike to take a closer look.

The road cut exposes almost 250 meters of tightly folded syncline formed during the early Mississippian Period, about 330 to 345 million years ago. The various layers and colors of rock make large U-shapes up the side of the cut, and consist of sandstones, silt stones, mud rocks, conglomerates, shale and even coal. Some fossils, mainly plant remains and some marine invertebrate shell casts, were also found at Sideling Hill.

Previously, there was a four-story exhibit center near the road cut, where interactive displays, hands-on exhibits, photographs, graphics, maps and descriptions told the geologic history of Sideling Hill and this part of Western Maryland. The exhibit center was closed in 2009 due to state budget cut.

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The bend through the Sideling Hill Road Cut. Photo credit

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Bands of conglomerate rock, shales and coal are seen in the close-up of the Sideling Hill road cut. These bands are very wide as you can see from the size of the trees in this picture. Photo credit

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The vertical lines in this section are drill holes for setting explosives. Notice the road sign on the bottom left and guardrails at bottom for scale. Photo credit

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The walkway over Interstate 68 that allows visitors to approach the face of the hill. Photo credit

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Aerial photo of Sideling Hill. Photo credit

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A model of the Sideling Hill Road Cut at the now closed Exhibit center. Photo credit

Sources: Wikipedia / Federal Highway Administration / Washington Post / Maryland Geological Survey

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