Beneath the surface of south central Kentucky lies a world that is virtually unparallel anywhere else on earth. It is a labyrinth characterized by mile upon mile of dark, seemingly endless passageways. The geological process resulting in this world that we call Mammoth Cave began hundreds of millions of years ago and continues today. Indeed, the complexity of the system adds to its uniqueness and the story of its formation is noteworthy.
The Ancient World
350 million years ago the world was very different than today. The North American continent was located much further south; at that time Kentucky would have been about 10 degrees south of the equator, and a shallow sea covered most of what is now the southeastern United States. The warm waters supported a dense population of tiny organisms whose shells were made of calcium carbonate (CaCO3). As these creatures died, their shells accumulated by the billions on the floor of the ancient sea. In addition, calcium carbonate precipitated from the water itself. The build-up of material continued during the next 70 million years until some seven hundred feet of limestone and shale was deposited. Late in the deposition of the limestone, about fifty to sixty feet of sandstone was deposited over much of the area by a large river system that emptied into the sea from the north. About 280 million years ago, sea levels started to drop and continents began to rise, exposing the layers of limestone and sandstone. The stage was set for the formation of caves found in central and western Kentucky. Forces at work beneath the earth's crust caused it to slowly rise, buckle and twist, causing tiny cracks between and across layers of limestone and sandstone. At the same time river systems as we know them today slowly developed. By about three million years ago, a sandstone-capped plateau stood above the Green River, and a low, almost flat limestone plain extended southeast of what is now Interstate 65.
Rainwater, acidified by carbon dioxide in the soil, seeped downward through millions of tiny cracks and crevices in the limestone layers. This weak carbonic acid (the same acid as in soda pops) dissolved creating a network of tiny micro caverns along the cracks. As the land continued slowly rising, the Green River eroded its channel deeper. The water in the network of micro caverns drained through the limestone under the plateau toward the river. Just as rivulets converged into streams above ground, water flow paths through the limestone also converged into incrementally larger flow paths. As rainwater continued to enter the system and more limestone was dissolved, the micro caverns enlarged. Because the major drains carried the most water, they enlarged the most. Caves were forming. As the Green River cut deeper, the water table continued dropping to the same level as the Green River. New underground drains formed at levels lower than the older ones, and the older channels emptied. Thus, the oldest cave passages are the closest to the surface, and the youngest horizontal passages are the deepest underground. At the current water table, cave passages are still forming today.
As you approach the vicinity of Mammoth Cave, several clues suggest the existence of caves. Road cuts along highways have vertical exposures of layered grayish rock, often broken into irregular blocks at the top where erosion has widened vertical cracks across layers. Between the layers, you may see the tiny openings in the limestone that are the first stage in the formation of a cave. The landscape along the highway also has special characteristics. You will not see surface streams. Instead, you will see myriads of crater like depressions called "sinkholes." These sinkholes are places where run-off may quickly enter the limestone aquifer. Cave drains carry the dissolved limestone away, and the surface soil settles, creating the bowl-shaped depression. If the sinkholes drains become plugged with soil, then the water cannot drain underground and a pond forms. Occasionally the drain becomes unplugged and a pond as large as several acres will disappear overnight. This kind of landscape is called karst topography. It is found along and to the southeast of Interstate 65 near Mammoth Cave National Park and referred to as the Sinkhole plain. At its southeast edge surface streams sink underground joining the drainage of thousands of sinkholes. Continuing northwest, they become part of the underground rivers in the world of Mammoth Cave.