In my other life, I am a professor of geology at Northern Kentucky University. An important part of my teaching is taking students on field trips to expose them to geology first hand. Last weekend I took my introductory class to Red River Gorge in Kentucky to study the formation of natural bridges and other associated rock formations.
Red River Gorge is near Slade, Kentucky, about an hour southeast of Lexington. It is one of the largest collections of natural bridges in the country.
Red River Gorge is on the edge of the Appalachian Plateau on what is known as the Pottsville escarpment. In this area, sandstone from the Pennsylvanian Period, about 290 million years ago, is exposed. This sandstone is relatively resistant to erosion, especially compared to the Mississippian Limestone that lies below it. As a result, it holds up the edge of the plateau and creates the escarpment, where it drops down to lower elevations where the Mississippian limestone is exposed.
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| From University of Kentucky department of Geography. The blue-gray area on the east side of the state is Pennsylvanian sandstone and the lighter blue is Mississipian limestone. |
The region was uplifted along the edge of the Cincinnati arch during the formation of the Appalachian Mountains, which ended in the Permian Age, 245 million years ago. The uplift warped the continent in this area and made the layers tilted, as shown in the geologic cross-section above. This causes different rock units to be exposed in different parts of the state, with the oldest in the center of the Cincinnati arch, which is centered on Lexington, and younger rocks found in a bulls-eye pattern around it. Where the Pennsylvanian sandstone and the Mississippian limestone are exposed together, we get the escarpment.
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| The flat tops of the hills are the Appalachian Plateau. The Red River cut down through the sandstone, exposing it and allowing the formation of the natural bridges. |
The limestone was deposited in a shallow sea teeming with life of a coral reef in a shallow sea that formed in the Appalachian Basin, a low area that formed when the Appalachian Mountains pushed up and put weight on the continent, pushing it down. From the Mississippian to the Pennsylvanian, this basin started to get more shallow and the sandstone was deposited in layers by large rivers flowing off the Appalachian Mountains and into the Appalachian Basin. During the uplift of the Cincinnati arch, the sandstone was fractured in both a north-south and east-west direction.
The combination of horizontal layers in the sandstone and the fractures are responsible for the formation of the natural bridges in this area. Initially, the sandstone starts to weather by grains of sand and pebbles getting plucked out of the sand. Small holes open up in the sand, and the grains get caught up in the holes with wind and rain and tumble around in the holes opening them larger. This creates a "swiss cheese" texture in the rock.
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| "Swis cheese" texture in the sandstone as little holes open |
Eventually, these little holes grow to where they start combining together and open up recessed caves. Recessed caves are shallow but long and follow the layers in the sandstone. The layers are zones of weakness in the rock, so the rock weathers more quickly here.
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| A recessed cave can be seen under this waterfall at Rock Bridge |
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| The sandstone has horizontal layers that are more susceptible to weathering and open up recessed caves |
Because the sandstone is fractured vertically, eventually, the recessed cave will cut through to the other side, where a fracture exposes it, opening a hole in the rock and creating an arch. Initially this hole is small, but it eventually grows. Early, the edges of the arch are sharp and blocky, like at Natural Bridge. But, over time the edges of the bridge weather more smooth, and the arch becomes more rounded. This is like what is seen at Delicate Arch at Arches National Monument in Utah.
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| Students under Natural Bridge. Natural Bridge is sharp and blocky and is a young arch. |
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| Delicate Arch in Utah is an old arch |
Eventually, the top of the arch becomes too thin to support itself and it collapses forming pinnacles. The pinnacles continue to weather along the layers, and form balanced rocks. Eventually, the balanced rock will collapse, completely destroying the formations.
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| Balanced Rock at Natural Bridge State Park |
Sometimes, the bridges will form from a river running over the sandstone and cutting down into the fractures. As the fracture opens, it separates a bridge from the rest of the rock, and forms a waterfall next to the bridge. Over time, the waterfall will cut upstream, separating from the bridge. This is the case at Rock Bridge. The waterfall is now about 1/4 mile upstream from the bridge.
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| Waterfall formed by Rock Bridge |
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| Rock Bridge with a river flowing under it |
Weathering is a process of creation, but it is also a process of destruction. The formation of the arches will continue to change until ultimately they are destroyed by the same processes that created them.
