Geology of Mammoth Cave National Park, Kentucky

Limestone Bedrock Leads to Cavern Network and Karst Landscape

Sep 22, 2009

Mammoth Cave was formed when acidic groundwater dissolved limestone bedrock. The cavern system has led to Kentucky's distinctive karst topography.

According to the National Park Service, Mammoth Cave is the world’s longest cavern. 367 miles of the cave system have been explored since its discovery in the 1790s, and geologists estimate that the entire system may be as many as 1000 miles long.

Limestone is the key to Mammoth Cave’s geology. The presence of this rock, along with the abundance of fossils in the cavern, has allowed scientists to envision the park’s paleoenvironment. Limestone is also necessary to the creation of caves and the overlying karst topography.

Geologic History of Mammoth Cave Area

Approximately 350 million years ago during the Mississippian Period, what is now the southeastern United States was located near the equator and covered by a shallow sea. This marine environment existed for about 70 million years and was home to many animals with calcium carbonate shells, such as crinoids, blastoids, and gastropods. The abundance of this fauna led to the large-scale accumulation of calcium carbonate on the sea floor. Over time, this accumulation formed over 700 feet of limestone and shale deposits.

A sandstone formation 60 feet thick overlies the limestone and shale formations. An ancient river system deposited large amounts of clastic sediment into the shallow sea, creating this sandstone.

Sea level began to fall about 280 million years ago and was followed by additional tectonic uplift in the area. This uplift and subsequent river erosion has led to the Mammoth Cave area’s modern landscape. The Green River flows through an almost flat limestone plain. There is a 300-foot escarpment over the river plain, at the top of which is a plateau. The plateau is characterized by sandstone-capped ridges separated by steep, limestone-floored valleys.

Formation of Mammoth Cave

While notable for its size, Mammoth Cave formed in much the same way as other cave systems. According to Stephen Marshak in his book Earth: Portrait of a Planet, limestone rock dissolves easily in acidic environments, and groundwater is naturally acidic. As rain falls, it reacts with carbon dioxide in the atmosphere to form carbonic acid. Additionally, the rain water becomes more acidic as it percolates through organic matter in the upper layers of the soil. Just below the water table, the groundwater’s acidity is highest and its flow the fastest, leading to large-scale dissolution of limestone bedrock. When the water table drops, the groundwater drains away, leaving behind an empty cave. Below Mammoth Cave’s hollow caverns, cave formation continues at or near the water table. The area’s groundwater flows out into the Green River.

However, just because Mammoth Cave is now empty does not mean geologic processes are not still at work. Groundwater trickling through the cave leads to the growth of speleothems, or cave formations. Carbon dioxide evaporates out of the water as it drips from the cave ceiling or along its walls, making the water less acidic. As this happens, any calcium carbonate dissolved in the water will precipitate, creating speleothems. Mammoth Cave’s speleothems consist of calcium carbonate and calcium sulfate. The cavern’s most prominent features include stalactites, stalagmites, and gypsum flowers. Flowstone in the cave appears multicolored due to oxidized iron components or tannic or humic acids that have interacted with the surrounding carbonate rock.

Karst Topography of South Central Kentucky

Cave formation in the subsurface leads to karst topography above ground. Kentucky’s karst features are so pronounced that many geologists point to the area as the defining example of a karst landscape. The state’s karst environment covers over 400 square miles and is typified by sinkholes, disappearing streams, and emerging springs. The National Park Service points out that sinkholes in the Mammoth Caves area create an “undulating” landscape. In fact, sinkholes are so numerous that the plain through which the Green River flows is often called the Land of 10,000 Sinks or the Sinkhole Plain.

A Textbook Cave and Karst Environment

Large deposits of limestone formed in an ancient shallow sea have led to the formation of the world’s longest cave and most classic example of karst topography. Mammoth Cave’s sheer size has earned the cavern its name, and it continues to grow as acidic groundwater dissolves the surrounding bedrock.

The copyright of the article Geology of Mammoth Cave National Park, Kentucky in Geography is owned by Dianne Turgeon. Permission to republish Geology of Mammoth Cave National Park, Kentucky in print or online must be granted by the author in writing.