The Geology of Cumberland Gap: A Doorway Through the Appalachians

Cumberland Gap National Historical Park runs for 20 miles (32 km) along the forested Cumberland and Brush Mountains, across Bell and Harlan Counties, Kentucky; Claiborne County, Tennessee; and Lee County, Virginia. Authorized on June 11, 1940, and covering about 24,547 acres (9,933 hectares), it stretches southwest from the White Rocks overlook, across the gap, to the Little Yellow Creek watershed beyond Fern Lake Reservoir.

It protects the geologic "doorway to the west": a deep notch in the otherwise unbroken ridge of Cumberland Mountain in the southern Appalachians.

A gap that made history

Frontiersman Daniel Boone famously explored the gap in 1775. As a prominent break in otherwise formidable mountains, it became a main corridor for westward expansion into the nation's interior. Landmarks such as the White Rocks guided settlers heading west on the Wilderness Road, which grew out of Boone's Trace. During the Civil War the gap was a strategic outpost, held in turn by Union and Confederate troops, and the Wilderness Road carried supplies, soldiers and ordnance. It remains a valuable transportation corridor today.

The park also preserves the Hensley Settlement on Brush Mountain, a community of scattered farmsteads founded by Sherman Hensley around 1903. It includes more than 40 historic structures — log cabin homes, split-rail fences, a one-room log schoolhouse — among pastures and woods with mountain views.

The White Rocks cliffs. Image from the National Park Service's page.

How the landscape formed

The park's rocks began as sediments laid down in or near shallow seas. About 300 million years ago, during the Alleghany Orogeny — the last great mountain-building event of the Appalachians — they were folded, faulted and pushed up and westward along the Pine Mountain thrust fault. The movement created a large regional structure called the Pine Mountain thrust sheet, with the Middlesboro syncline to the west and the Powell Valley anticline to the east.

Hundreds of millions of years of erosion then wore the old mountains down, leaving:

• limestone in Powell Valley;
• resistant sandstone ridges at Cumberland and Pine Mountains;
• stream-cut uplands near Middlesboro.

Why the gap is there

The gap's location traces to an older fault, the Rocky Face Fault, which formed before the last episode of Appalachian mountain building. Movement on the fault fractured and deformed the rock and weakened it. Over time the broken rock eroded away, leaving the notch.

Next to the park, and easy to see from the Pinnacle Overlook, lies the circular Middlesboro basin. Its intensely deformed rock is evidence of an ancient meteorite impact.

Overview of the park's digital geologic data, from the NPS Geologic Resources Inventory.

Caves and karst

The park holds a significant karst landscape formed mostly in Mississippian limestone, 360 to 318 million years old. Besides 30 known caves, it has sinking streams, pits, sinkholes and springs.

• Gap Cave is the park's longest known cave, with more than 18 miles of surveyed passages — and exploration continues.
• Gap Cave Spring was an important water source for early settlers passing through the gap.
• The gap and Gap Cave changed hands several times during the Civil War, and soldiers from both armies left graffiti — names, dates and drawings — in Soldier's Cave, part of Gap Cave.

Caves in the national parks are protected under the Federal Cave Resources Protection Act of 1988.

Fossils

The limestone is full of marine fossils from ancient seas hundreds of millions of years old. Much younger remains — tens of thousands of years old — turn up in caves and sinkholes. All fossils in the national parks are protected under the Paleontological Resources Preservation Act of 2009.

Old mines

Coal and iron-ore mining have been important in the region for more than a century and continue today, alongside smaller-scale quarrying of limestone and sandstone. Inside the park, disturbed lands include old coal and iron mines from around 1900, potassium nitrate mines, abandoned haul roads and a few sand and gravel quarries. None were large, but together they show how central mining was to the area's history.

Abandoned mine sites can be valuable cultural resources and habitat, but many are dangerous to people and wildlife and can harm water quality and the landscape. The National Park Service's advice near them: stay out and stay alive.

Part of a larger region

The park belongs to the Appalachian Plateaus physiographic province and shares its geologic history and characteristic rock formations with a region far beyond its borders. The NPS Geologic Resources Inventory has produced digital geologic maps and reports for the park, and a soil resources inventory has also been completed; both are available through the NPS DataStore.

Sources

• National Park Service: "NPS Geodiversity Atlas—Cumberland Gap National Historical Park, Kentucky, Tennessee, and Virginia," citing Thornberry-Ehrlich (2008, 2011), Englund (1964) and National Park Service (2016).