Big South Fork National River and Recreation Area - Geology

Geology

The Big South Fork’s most prominent feature is the river gorge cutting through the softer Mississippian age rock beneath the hard Pennsylvanian capstone of the Cumberland Plateau. Water is the most influential agent of geologic change in the Big South Fork region. Over time water action has left many unique and amazing geologic features ranging from the river gorge with its magnificent bluffs to the natural arches and unusual hoodoos.

Due to the substantial amount of annual rainfall of the region and the action of the Cumberland River and surrounding tributaries the water acts to erode the softer Mississippian rock composed of limestone, shale, and calcareous sandstone from beneath the much harder and erosion resistant capstone composed of Pennsylvanian sandstone. Flowing water hollows out the softer layers beneath and forms waterfalls and gorges. Where there is hard capstone intact, arches can form creating natural bridges across streams or a dry ravines. Direct erosion widens a joint and forms a cavity below the more resilient rock thus creating a void between the hard capstone and the area below. As result, water eroded arches are formed in the Big South Fork. Hoodoos are a rare but intriguing feature occurring in the Big South Fork. These hoodoos form in a similar manner to those found in the western United States. Where tough capstone still exists on the side of a hill for instance, it prevents the erosion of the softer material below. The result is a naturally formed erect columnar rock where once was located a hill.