Karst Topography and Underground Drainage

The dissolution of limestone is a very slow process. For instance, Kennard and Knill (1968) quoted mean rates of surface lowering of limestone areas in the British Isles that ranged from 0.041 to 0.099 mm annually, and Sowers (1996) suggested rates of 0.025 to 0.040 mm a for the eastern United States. More recently, Trudgill and Viles (1998) quoted calculated erosion rates of calcite of 0.06 to 0.11 mm a- at pH 5.5, and 2.18 to 2.69 mm a at pH 4.0. Nevertheless, solution may be accelerated by man-made changes in the groundwater conditions or by a change in the character of the surface water that drains into limestone. 

Sinkholes are characteristic features of karst terrain and may develop where solution-opened joints intersect (Waitham and Fookes, 2003). They may lead to an integrated system of subterranean galleries and caves. The latter are characteristic of thick massive limestone or dolostone formations. Sinkholes vary in diameter but usually are a few metres or tens of metres across and may descend up to 500 m in depth below the surface (Fig. 3.22). 

Surface drainage is usually sparse in areas of thick limestone. Dry valleys are common, although they may be occupied by streams during periods of intense rainfall. Underground streams may appear as vaclusian springs where the water table meets the surface. Occasionally, streams that rise on impervious strata may traverse a broad limestone outcrop without disappearing. 

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