Mammoth Cave National Park

Karst Topography
Frozen Niagara in Mammoth Cave National Park, Kentucky (Nancy Nehring/Photodisc/Getty)

The pit-marked land of south-central Kentucky is an unusual region where placid streams and even the violent, flash-flowing floodwaters disappear underground and later rise from some mysterious subterranean channel to flow normally as surface streams once more. This is a land of ancient, heavily weathered limestone.

The limestones originated as limey ooze and beds of shell fragments in the bottom of shallow seas that covered most of Kentucky more than 300 million years ago. About 200 million years ago, the region that was once a sea became land, and from that far distant date until today these rocks have been subjected to the relentless destructive action of the forces of nature.

Solution activities caused by the reaction of water with limestone in south-central Kentucky have created a geologically interesting and scenic topography to which scientists have assigned the name "karst." This word is derived from the Jugoslav "kars," which means stone, and was used to refer to the solution features in limestone that occur on a plateau in Jugoslavia and in adjacent parts of Italy, both of which border on the Adriatic Sea. Although this is considered the classic region of karst topography, karst features are found in many different parts of the world. This region in south-central Kentucky shows an assemblage of well-developed solution features in the limestone that were formed during the middle part of the Mississippian Period of geologic time.

The term "karst" is a comprehensive topographic term applied to limestone areas that possess a topography peculiar to and dependent upon underground solution and the diversion of surface waters to underground routes. Implied in this definition is the fact that carbonic rock, chiefly limestone, yields freely to the solvent action of water. Also, it is a known fact that the rate of solution is greatest when the water is charged with carbon dioxide (CO2). Rain, as it falls to earth, collects carbon dioxide from the atmosphere, and as it soaks into the ground, it collects more carbon dioxide from decaying organic matter. Water heavily charged with carbon dioxide forms a weak acid, known as carbonic acid, that reacts with limestone and dissolves it. As the acid dissolves limestone, it forms a solution of calcium bicarbonate, and as long as carbon dioxide exceeds calcium carbonate, the limestone continues to dissolve. But when the water containing calcium bicarbonate reaches an opening, such as a cave, it loses its carbon dioxide, and calcium carbonate is deposited as columns or as icicle-like forms, called stalactites and stalagmites.

Reaction of carbonic acid in the dissolving of limestone causes the cracks in limestone to become larger, fissures and joints are widened, and eventually the surface of the ground becomes pitted because of the formation of sinks. All water that falls upon the ground in regions of limestone bedrock eventually finds its way into underground channels. In areas where solution of limestone is in evidence, the most characteristic topographic feature is the sinkhole, though sinking streams, caverns, resurgences, and other features attending underground drainage in limestone areas compose the karst assembly. Many of the features of the karst assembly are not distinctly topographic, or they are only occasionally present in karst terrain.



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