Sinkholes (dolines) – Closed depressions. They can be cone- or bowl-shaped and circular or elliptical in plan view. The diameter is usually greater than the depth. Average dolines vary in size from 2-100 m deep and from 10-1000 m in diameter. Often regarded as the fundamental unit of karst relief. (Sweeting, 1973)
Five major classes of dolines have been recognized by Jennings (1985):
Collapse features - closed depressions formed by subsurface collapse that may or may not be the result of subaerial karst. Dissolution by circulation of deep-seated hydrothermal fluids and structural collapse above tectonic pullaparts are other possible mechanisms for the formation of these features.
Polygonal karst – Landscape is completely pitted by closed depressions, which form a cellular network when delimited on the basis of their topographic divides. (Williams, 1972)
Williams (1972) proposed a growth model for the evolution of polygonal karst: (A) The starting point in this model is an uplifted horizontal surface, fissured by a system of shear and tension joints. (B) Depressions acting as local stream-sinks are initiated at sites of maximum fracturing, where intersection of joints encourages vertical drainage and corrosion. Later depressions are formed at slightly less favorable sites that require more time to establish vertical circulation. (C) Newer, smaller depressions are captured by older, deeper depressions. (D) Eventually the surface is completely pitted by depressions, the mutual divides of which form the cellular network of polygonal karst.