Geology

Beneath the surface of south-central Kentucky lies a world that is virtually unparalleled. It is a labyrinth characterized by mile upon mile of dark, seemingly endless passageways. The geological process resulting in Mammoth Cave began hundreds of millions of years ago and continues to this very day.

How the Caves Formed

350 Million Years Ago: The region we now call south-central Kentucky was 10 degrees south of the equator and submerged beneath a warm and shallow sea. For 70 million years, sediments—including calcium carbonate shells from sea creatures—accumulated on the ocean floor ultimately depositing 1,200 to 1,400 feet of limestone.

300 Million Years Ago: A river, flowing into the ocean from the north, deposited 50 to 100 feet of sand and silt, creating a layer of sandstone and shale over the existing limestone.

280 Million Years Ago: The sea level began to drop and the continent began to rise, exposing the layers of limestone and sandstone. Forces within the earth caused the surface to buckle and twist, causing tiny cracks between and across the layers of limestone and sandstone. At the same time, river systems as we know them developed on the surface.

3 Million Years Ago: Forces of erosion had left a sandstone-capped ridge (insoluble to water) above the Green River. Beyond this ridge to the south there is a limestone plain called, "Pennyroyal Plateau," filled with sinkholes. When it rained, water seeped through the sinkholes into the tiny cracks and crevices within the limestone. Combining with carbon dioxide (and thus becoming a weak acid), the water slowly made its way through the limestone toward the Green River. Ultimately, on its journey toward the river, the water traveled under the sandstone-capped ridge by dissolving away larger and larger passages from its limestone bed, in the process forming an intricate and interconnected river system.

The Last Million Years: As the Green River continued to cut deeper into its bed, the water table continued to drop. To keep up, new underground drains formed in the limestone bed, creating new channels beneath the original ones. Water drained from the higher passages, leaving behind air-filled passageways that visitors recognize—cave. Thus, the oldest cave passages are the closest to the surface and the youngest horizontal passages are the deepest underground.

Now: At the present water table, cave passages continue to form in the deepest depths of Mammoth Cave.

Why is Mammoth Cave So Long?

A unique combination of circumstances have come together to make Mammoth Cave the longest cave in the world. First, the karst setting (a limestone region with sinkholes, disappearing streams and underground streams) is ideal for the formation of caves. Second, the Green River Valley slowly deepened throughout the ice age, causing multiple levels to form. Third, the limestone is made up of many different layers with different characteristics. As the underground water sought lower and lower levels, each layer provided a different path of flow, resulting in numerous interconnecting passageways. Fourth, vertical shafts formed when water flowed off the edge of the sandstone cap and seeped into the limestone below, which add to the cave's complexity. Fifth, the sandstone caprock on the plateau above protects the older upper level passages from destruction. If that sandstone didn't exist, portions of the caves would quickly erode and collapse.

Cave Features

Speleothems: Cave formations caused by the deposition of dissolved minerals in crystalline form. Gypsum flowers, stalactites and stalagmites are examples of speleothems.

Stalactite: A speleothem that hangs from cave ceilings. They form when water containing calcium carbonate drips into an air-filled passage. Remember: stalactites hang from the ceiling.

Stalagmite: A speleothem that rises from the floor when water containing calcium carbonate drips onto the floor of an air-filled passage. Remember: stalagmites grow from the ground.

Gypsum: A calcium sulfate mineral found in dry sections of a cave that is colorless, white or yellowish and found in powder or crystal form. Gypsum can form spectacular flower-like structures that seem to ooze and curl from the walls, ceilings and floor of a cave.

A Scientific Treasure Chest

Mammoth Cave is truly a scientific treasure chest. In addition to being the world's longest known cave, it contains a clear and complete record of geomorphic and climatic changes over the past 10 million to 20 million years, the most diverse cave ecosystem in the world and the greatest variety of sulfate minerals of any cave.