Sediments are small particles of rock transported by water, ice and wind. Most eventually reach the sea bed where over the years successive layers of sediments accumulate. The weight of materials above compresses the sediments below into sedimentary rocks. These rocks are laid down in layers, or beds, with lines of weakness, or bedding planes, between layers.
When sand is compressed, sandstone rock is formed. Clay forms from the accumulation and compression of deposits of mud.
Limestone and chalk consist of calcium carbonate which comes from the remains of plants and animals. For example, the shells of sea creatures are made of calcium carbonate; when these animals die, masses of shells accumulate on the sea floor, building layers of limestone rock.
A lot of limestone was formed during the Carboniferous period (280-345 million years ago) because at that time much of Britain was a warm shallow sea, rich in plant and animal life.
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