Somber, bare tree silhouettes in April at Rockbridge State Nature Preserve, Hocking County, Ohio, USA. Rockbridge's natural bridge is more than 100 feet long and 10 to 20 feet wide, and gracefully arches 50 feet across a ravine. This sandstone arch is the largest natural bridge in Ohio. It originated millions of years ago, when Ohio lay under a warm inland sea. Rivers flowed into the vast body of water, carrying both fine and coarse grained sands which settled to the sea bottom. Over the centuries, the accumulating sand thickened, compressed and formed the hard sedimentary rock known as Black Hand sandstone. Eventually, great pressure from beneath the earth's surface caused the land in eastern North America to rise, forming the Appalachian Mountains. The inland sea soon drained away, exposing the newly uncovered rock layers to steady erosional processes. The natural bridge soon began emerging. Wind, rain and percolating groundwater worked together for centuries, carving a deep cave-like recess in the softer midportion of the Mississippian Black Hand sandstone. Gradually, erosional forces also worked along a natural joint plane behind the brink of the cliff to define the bridge.
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