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A karst terrain is formed by the dissolution of limestone, dolomite or gypsum. The minerals that form these sedimentary rocks are especially susceptible to erosion and dissolution. Since water always follows the path of least resistance, faults, fractures and joints are natural flow routes. These routes are gradually enlarged as water moves through limestone deposits that are interbedded with permeable layers of bedrock. The land surface gradually gains a unique and distinctive topography.  A karstic landscape is characterized by caves, sinkholes, and underground streams. Caves often form near the water table. As the water table drops, the ceilings of some caves become too heavy and unstable without the support of water. Those caves then collapse creating sinkholes that can be seen at the surface. The size of sinkholes can vary greatly, from a few feet wide and a couple feet deep, to miles in diameter and thousands of feet deep. Some of these sinkholes are filled with water while others are dry; it all depends on the level of the water table. Another unusual characteristic of a karst landscape is the lack of surface drainage. Rivers may flow through a karstic terrain, but karst can not support the creation of a large river system. For example, a stream in a non-karst landscape will flow at the surface until the water table falls below the bottom of the streambed. However, in a karst environment there are disappearing streams, where the stream abruptly flows vertically downward to the water table through faults, fractures, joints or sinkholes. Conversely, ground water can reappear when the water table reaches the surface as springs or seeps. The EPA defines a spring as a natural outflow of water at the surface of the land or into surface water. Sometimes, if the water table varies greatly, a sinkhole may also become a spring where water will actually bubble up through the fissures in the sinkhole. |
