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NATURAL HERITAGE

Living on Karst

A Reference Guide for Landowners in Limestone Regions

Produced by the Cave Conservancy of the Virginias, June 1997



KARST

A land area that includes sinkholes, springs, sinking streams, and caves.

What is Karst?

Karst is defined as a landscape with sinkholes, springs, and streams that sink into subsurface caverns. The word "karst" was developed in Europe, where early geologists first studied the nature of groundwater flowing through limestone hills and valleys.

Approximately 10% of the earth's surface (and 20% of the U.S.) is composed of karst; however, approximately 25% of the world's population lives on these areas! The hollow nature of karst terrain results in a very high pollution potential. Streams and surface runoff enter sinkholes and caves, and bypass natural filtration through soil and sediment. Groundwater can travel quite rapidly through these underground networks - up to thousands of feet, or even miles, per day - transmitting contaminants to wells and springs in the vicinity.

In karst areas, the fractured limestone rock formations have been dissolved by flowing groundwater to form cavities, pipes, and conduits. Sinkholes, caves, sinking streams, and springs signal the presence of underground drainage systems in karstlands.

Unless watersheds are protected, these direct connections between the surface and the subsurface can threaten the quality of our drinking water. The safest watersheds are those in which all residents understand the karst landscape and work together to reduce soil erosion, high-density development, agricultural and urban storm water runoff, overgrazing, improper waste disposal, and pollution.

How Karst was Formed

Between 570 million and 320 million years ago, the geographic area now occupied by the eastern United States was predominantly covered by a calm, shallow, tropical sea. The sea was populated by microscopic (and larger) organisms that lived, died, and sank to the bottom of lagoons, or were washed into deeper parts of the basin by storms. Over the eons, the deposits of calcium-rich shells and skeletons solidified into the bedrock that we call limestone (CaCO3), dolomite (CaMg(CO3)2) and gypsum (CaSO4.2H2O). These rocks are soluble in dilute acids. Water becomes slightly acidic when it takes up carbon dioxide while passing through decaying organic debris in the surface soils. The interaction of acidic water with soluble rocks such as limestone produces the characteristic landscape known as karst.

During the Appalachian Orogeny, a series of mountain-building events in the central and eastern U.S., rocks were alternately buried, uplifted, faulted, folded, and fractured. The geologic stresses of mountain-building and subsequent erosion created cracks and fissures in the rock through which rainwater and groundwater entered and actively dissolved the organic limestone. Within the past 10 million years, caves, conduits, and underground drainage systems have been dissolved into this rock by moving water. Surface water and streams are captured by underground channels. These channels convey the water to springs which sustain the water flow, cool temperatures, and aquatic habitats of our rivers.