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Big South Fork Through Time


How Arches Are Formed

     Geologist Todd Knoedler explains how Big South Fork was formed and discusses how it has changed over time due to erosion and other forces. He also notes the diversity of flora and fauna in the area.           Big South Fork National Recreation Area contains more natural arches than any other region in the eastern United States. In this segment geologist Todd Knoedler explains how these beautiful arches were formed and discusses the types of rocks that make up the arches.     

Background Essay: Geological Wonderland

The Big South Fork National River and Recreation Area in southeastern Kentucky is a geological wonderland, including over 400 miles of dramatic cliffs, 2,000 rock shelters, and more natural sandstone arches than in any other region in the eastern United States. The rock formations in the Big South Fork region tell the geological story of the Appalachian Mountains and demonstrate numerous processes in the rock cycle.

A sea of hard and soft rock

Let’s begin the story about 750 million years ago. At that time Kentucky was covered by a vast inland sea. Rivers flowing into the sea carried clay, silt, gravel, and fine and coarse-grained sands, depositing them on the ocean floor in sandbars, deltas, and beaches. Over millions of years, the weight of the sediment caused the sea floor to sink, and the sea deepened. The layers of sand were covered by finer-grained sediment and compressed into thick, hard layers. The pebbles and grains of sand were cemented together by dissolved minerals in the water. This process created the sedimentary rock known as sandstone. As this process was repeated, layers of hard sandstone alternated with layers of clay and silt. The layers of clay and silt were compressed and cemented into shale and other soft sedimentary rocks, creating a multi-layered deposition thousands of feet thick with alternating layers of hard and soft rock.

By the Pennsylvanian Period (about 300 million years ago), the ocean trough was full of sediment, and vast swamplands near sea level dominated the area. Areas known as “coal swamps” contained a wealth of plant life that is now extinct. Giant ferns up to 50 feet in height flourished and died. As the layers of dead ferns decayed, they formed peat. When the peat was compacted by layers of sediment, it formed coal. In other areas, the shells of ancient marine animals formed limey deposits that later became limestone.

Continents collide and mountains rise

About 275 million years ago, the continent that would later become Africa collided with the land mass that would later become North America. The collision of the two enormous land masses lasted millions of years and caused great pressures that buckled the sedimentary beds of rock upward from the sea bottom. The bedrock was bent and folded upward in the ridge and valley formation that is the framework of the Appalachian Mountains. At that time, the Appalachian Mountains were much higher than they are today, probably as tall as the highest peaks in Alaska today.

The water of the inland sea was also uplifted, and streams began to flow down the steep mountains. Heavy rainfall swelled the streams, which carried sediment away, eroding the mountains. The mountains were worn down into a giant plain known as the Appalachian Peneplain.

About 70 million years ago, during the Cretaceous Period (the age of dinosaurs), the Appalachian Peneplain was uplifted into an enormous plateau. Like a tilted tabletop, the plateau slanted from the highest elevations in the east (which became the Great Smoky and Blue Ridge Mountains) to lower elevations toward the westward edge of the plateau. Again, water was uplifted with the land and more rain fell, continuing the process of erosion.

From rocks to rapids

One of the hardest sedimentary rocks in the Big South Fork area is known as Rockcastle Conglomerate, which often serves as the cap rock. In the places where the force of the water was strong enough to carve right through this layer and then wear away the softer layers below, steep sided gorges and dramatic cliff lines were formed. In places where the Rockcastle Conglomerate resisted erosion, ledges were formed when the softer layers below were eroded, resulting in waterfalls. In the riverbeds, alternating layers of hard, erosion-resistant rock and softer rock created whitewater rapids. Whitewater enthusiasts come from all over to enjoy “running the rapids” at the Big South Fork National River and Recreation Area.

Others come to see the spectacular sandstone arches. These natural arches are often found at the edge of the tableland surface, where erosion-resistant sandstone is able to bear its own weight when the softer layers below erode away. Archaeologists come to study the remains left behind by pre-historic Native people who used the area’s many rock shelters as temporary shelters beginning as early as 10,000 years ago. The rockshelters, too, were formed by erosion.

Sculpted by weather

Erosion is not the only force at work sculpting the landscape of Big South Fork. Weathering also plays a role. When rain water seeps into cracks in the rock and freezes, it expands in volume by 9%, widening the crack. When it thaws, the water seeps deeper into the crack. When it freezes again, it further weakens the rock. As this cycle is repeated, the rock may split along the joint into angular pieces that slide into the valleys as boulders. During the Ice Age, the freeze-and-thaw cycles caused many boulders to break away. The last Ice Age lasted 100,000 years and ended about 10,000 years ago.

Biological weathering also plays a role. Lichen and moss will grow on essentially bare rock, creating a more humid chemical microenvironment. Where they attach to the rock, they also exert a physical force. Sometimes the cracks in a rock fill up with natural materials, such as leaves, dead insects, or dead moss. As these things decompose, they create an organic mixture in which a seed can take root. Seedlings of trees and plants that sprout from cracks in the rock exert an even stronger physical force as their roots develop. Animals and insects that burrow in the soil at the edge of a rock face allow more water to infiltrate the rock and expose it to the oxidation process. All of these biological activities change, or “weather,” the rock.

Geological processes are slow, but they produce spectacular results like the arches and cliffs found in the Big South National River and Recreation Area. Because it is protected as a national park, people can enjoy the scenery, water sports, hiking, fishing and hunting in the Big South Fork gorge and know that it will still be there for future generations to enjoy.


Big South Fork National River and Recreation Area

Cumberland Plateau

Formation of the Appalachian Mountains

Rocks and the Rock Cycle

Weathering and Erosion

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