Formed from the remains of organic matter, usually seashells and plants. It was formed under the sea 220-280 million years ago.
It is a hard, grey sedimentary rock, with a large number of joints(vertical cracks) and bedding planes (horizontal cracks)
Carboniferous limestone is an example of a pervious rock, as it allows water to flow through the joints and bedding planes.
The main processes, which affect it, are carbonation and solution.
The two best areas of carboniferous limestone in Britain are the Yorkshire Dales and the Peak District.
Carboniferous limestone produces distinctive karst scenery. These areas are dry on the surface due to the permeability of the rock, but have mainly been shaped by the action of water. The water attacks the many joints and bedding planes in the rock, through the chemical weathering processes of carbonation and solution. There are a number of distinctive features seen in karst scenery areas, both on and beneath the surface.
- Limestone pavements are large areas of exposed limestone. When the overlying rock was eroded the pressure release on the limestone below caused it to crack even more. Hence limestone pavements are characterised by large gaps between the rock, called grikes. The remaining blocks of rock are called clints.
- Limestone cliffs or scars are produced at the edge of the area of limestone. Often near vertical and highly jointed.
- Swallow holes and sink holes are where rivers flow down into the rock. Sink holes are relatively small, whilst swallow holes are larger. Both have been formed either by the constant chemical attack of the water on the joints in the limestone, or by the collapse of a cavern below.
- Caverns are underground caves that have been hollowed out by the action of underground streams and by carbonation and solution. They have three distinctive features. Stalactites hang from the roof of the cavern, and are basically lime deposits. Stalagmites grow from the floor, and are also lime deposits. Where a stalactite and stalagmite have joined you get a pillar.
- Underground streams flow down through the limestone carving out caverns, until the y reach the impermeable layer of rock below the limestone layer. Once at this point the stream flows under the limestone until it re-emerges. This is called resurgence.
- Gorges are created where the roof of a large underground cavern falls in, to create a steep sided gorge with a river running in the bottom.
- Dolines are formed when the roof of a small underground cave falls in. The ground above the cave subsides into it causing an indentation on the surface.
- Dry valleys, such as Cheddar Gorge, were formed in periglacial times, when the ground froze, so the rivers ran over the surface of the limestone rather than flowing down through it. These rivers carved out steep sided valleys. Once the climate had warmed again the rivers disappeared underground leaving a dry valley behind. These also are common features of Chalk landscapes.
- Poor, thin soils mean that the only type of farming possible in limestone areas is sheep grazing.
- The tourist industry is a very important source of income to limestone areas. Most people come to walk in the hills and see the spectacular karst scenery. The local people are cashing in on this by opening cafés, guest houses and other tourist facilities.
- Limestone is an excellent building stone, and has been used in some very well known buildings, such as the Houses of Parliament. Obviously, this means that there are often a large number of quarries.
- Limestone, whether crushed or used as lime, can be very useful in a number of areas. It is used as an industrial cleanser, farmers use it as fertiliser and it forms an important ingredient in cement making.