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  Karst Description
   
  Excerpt modified from William E. Davies text (in Davies and others, 1984)
   
  Distinctive surficial and subterranean features developed by solution of carbonate and other rocks and characterized by closed depressions, sinking streams, and cavern openings are commonly referred to as karst. The term was used first to describe the region of Carso in northeastern Italy and northern Yugoslavia, where solution landscape was studied in the 19th century. Originally the term defined surface features derived by solution of carbonate rocks, but subsequent use has broadened the definition to include sulfates, halides, and other soluble rocks. The term has been expanded also to cover interrelated forms derived by solution on the surface in the subsurface. A further expansion of the concept of karst was the introduction of the term "pseudokarst" to designate karstlike terrain produced by processes other than the dissolution of rocks (Burger and Dubertret, 1975). When used in its broadest sense, the term encompasses many surface and subsurface conditions that give rise to problems in engineering geology. Most of these problems pertain to subterranean karst and pseudokarst features that affect foundations, tunnels, reservoir tightness, and diversion of surface drainage.
   
  Environmental aspects of karst lead to additional problems in engineering geology, especially in site selection. Subterranean openings may be the habitat of unique and, in some cases, endangered fauna. The openings are also conduits for water and refuse disposal from the surface or, in caves, for pollutants that can be carried for great distances. Many caves contain features of beauty and scientific interest that can be important esthetic factors in site selection for structures, transportation routes, and impoundments. Subterranean openings in karst range in size from minute voids to large caverns. Most of the openings are formed by solution processes along fractures, joints, and bedding planes. Caves and related solution features are common in most carbonate and gypsum terrains in the United States, except in the area formerly covered by Pleistocene ice sheets (Davies and LeGrand, 1972).
   
  The systematic study of karst in the United States started with W. M. Davis’ (1930) theory on the origin of caves by deep-seated solution. Bretz (1942) obtained data, from studies in flat-lying carbonate rocks in the Midwestern States that supported Davis’ theory. After World War II, studies of karst in the United States became widespread beginning with investigations in the Appalachian Mountains. Based on these studies, many of which were in areas of folded rock, older theories were modified with emphasis on maximum solution activity in a zone directly beneath a uniform water table (Davies, 1960). Since 1948, the exploration of caves and studies of landforms in carbonate terrains have produced a vast amount of data on karst. Reports of these explorations and studies were primary sources in compiling a national map (Davies and others, 1984) on the subterranean aspects of engineering geology of karst and pseudokarst.
   
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  REFERENCES CITED
   
  Bretz, J. H., 1942, Vadose and phreatic features of limestone caverns. Journal of Geology, v 50, no.6, pt2 , p. 675-811.
   
  Burger, A., and Dubertret, L., 1975, Hydrogeology of Karstic terrain: Paris, International Association of Hydro geologists, p. 159.
   
  Davies, W. E.. 1960, Origin of caves in folded limestone (Appalachian Mountains) in Moore, G W., ed.. Origin of limestone caves; a symposium with discussion. National Speleological Society, Bulletin, v. 22, pt 1, p. 5- 18.
   
  Davies, W E., and LeGrand, H. E., 1972, Karst of the United States; in Herak, M. and Stringfield, V. T.. eds., Karst; important karst regions of the northern hemisphere. New York Elsevier Publishing Co., p. 467-505.
   
  Davies, W.E., Simpson, J.H., Ohlmacher, G.C., Kirk, W.S., and Newton, E.G., 1984, Engineering aspects of karst: U.S. Geological Survey, National Atlas, scale 1:7,500,000.
   
  Davis, W M.. 1930, Origin of limestone caverns. Geological Society of America, Bulletin, v. 41, no.3, p, 475-628.
   
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