CAVES COMMONLY FORM in areas of limestone, although on coastlines they also occur in other rocks. Limestone is made of calcite (calcium carbonate), which dissolves in the carbonic acid naturally present in rainwater, and in humic acids from the decay of vegetation. The acidic water
trickles down through cracks and joints in the limestone and between rock layers, breaking up the surface terrain into clints (blocks of rock), separated by grikes (deep cracks), and punctuated by sink-holes (also called swallow-holes or potholes) into which surface streams may disappear. Underground, the acidic water dissolves the rock around crevices, opening up a network of passages and caves, which can become large caverns if the roofs collapse. Various features are formed when the dissolved calcite is redeposited; for example, it may be redeposited along an underground stream to form a gour (series of calcite ridges), or in caves and passages to form stalactites and stalagmites. Stalactites develop where calcite is left behind as water drips from the roof; where the drops land, stalagmites build up.
