Sewanee’s Natural Bridge

 

Natural Bridge

The Sewanee Natural Bridge.

Once a part of the Domain (campus) of the University of the South in Sewanee, Tennessee, the Sewanee Natural Bridge was given to the Tennessee Conservation Department in 1976. At more than 100 feet long and 25 feet high this bridge illustrates  the interplay between water and rock structure in the development of a relatively seldom seen feature along the edge of the Cumberland Plateau.

The bridge is the result of erosion by ground water and surface water along vertical joints (geology-speak for cracks) and horizontal partings in the rock. The rock is the Warren Point Sandstone, which forms the prominent cliffs all along the Cumberland Plateau in this area. Formed over 300 million years ago and buried deeply by other rock layers the Warren Point, along with other layers of the Cumberland Plateau, has experienced uplift over the last 5 million years or so. This uplift has lead to the removal by erosion of thousands of feet of rock that once lay atop this layer. This uncovering of the Warren Point released vast amounts of pressure caused by the weight of the overlying rock.  The release in pressure in turn caused the Warren Point to expand and crack along a series of vertical, parallel joints. These joints were natural conduits for the downward flow of rain water, which resulted in the gradual wearing away of the rock along the joints.

needles-joints

Aerial view of vertical joints in sandstone in Canyonlands National Park. Notice that there are 2 sets of joints at high angles to one another. Photo by Haakon Fossen.

The rock in today’s bridge represents the rock between 2 joints that was more resistant to erosion than the surrounding rock. That explains the origin of the thin band of rock in the bridge, but how did the empty space under the bridge form? A powerful clue is found at the base of the main cliff face just uphill from the bridge. Here a spring once flowed with more vigor from a horizontal parting in the sandstone. This groundwater eroded away the rock beneath the bridge.

cliffspring

Small overhang with spring from which water originated that eroded out rock under the bridge.

What the bridge may have looked like in its early stages of formation is hinted at by another nearby location, Widow’s Crack, where a younger natural bridge is forming. Here there is still an active spring eroding away the sandstone under the incipient bridge.

widow's crack

The incipient natural bridge at Widow’s Crack. 

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