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Limestone itself is a permeable rock (allowing water to pass through) consisting of calcium carbonate. Limestone structures may include chalk or carboniferous limestone, the latter formed on the bed of the sea many thousands of years ago. Over time the rocks develop to form great landscapes, however rain water will play its part sculpturing the rock to a further extent. The structure of carboniferous limestone is a hard, grey sedimentary rock with different layers called bedding planes.
All limestone is vulnerable to chemical weathering, the alkali consistence of the rock reacts with the acidic rain causing it to dissolve slowly and gradually. The rock is removed in solution by running water which in many cases continually passes by it. The chemical weathering widens weaknesses in the rock causing great cracks and blocks to be formed, for example in the Burren. Here, clearly limestone weathered features can be seen, in particular, pavements.
Surface Land forms
Dry valleys are evidence that rivers once flowed on the top of limestone. Obviously thousands of years ago, rivers travelling at great speed would gradually wear away the surrounding rock causing drastic changes to the landscape. The valleys are usually very steep sided because of it. At the same time limestone mountains are formed where much the same thing happens, once again ice plays its part. The vertical sides are formed due to water which deepens the cracks where the freeze-thaw cycle takes place forcing apart the limestone. Limestone areas often have a flat, plateau like appearance. Where there is no soil in the landscape, the top bedding plane will be exposed as a limestone pavement. Many joints reach the surface along this pavement. However they are widened and deepened by solution to form grykes. The flat topped blocks between them are called clints and both together form limestone pavements. Other surface landforms result from limestone having collapsed. Where limestone collapses over an underground river it creates a gorge.
Underground Landforms
Carboniferous limestone areas are characterised by a lack of surface
drainage. A river which has its source and headwater on nearby impermeable
rock will flow over the surface until it reaches an area of limestone.
Various acids in the water, including carbonic acid derive from rainfall,
begin to dissolve and widen surface joints to form swallow holes, or sinks.
At the same time streams/water may find weaknesses in the surface of the
limestone (in cracks), and will eventually expand weakening away the surrounding
rock, thus forming underground caves, disappearing down the swallow holes.
It will widen the joints and bedding planes through solution. Should the
river meet an underlying rock, it will have to flow over this rock until
it reaches the surface at a spring or resurgence.
It is not just swallow holes that are formed in underground limestone landscapes. Deposition landforms also occur forming stalactites and stalagmites.
Water containing calcium carbonate in solution, continually drips form the ceilings of underground caves. Although it is cold in these areas, some evaporation does take place and the formation of these structures take place. As water drips onto the floor beneath the stalactite further deposition of calcium carbonate produce more rounded stalagmites. Pillars are the result of stalactites and stalagmites joining together. In some cases translucent curtains may form in much the same way
