When sea creatures die, their flesh rots quickly but their shells and skeletons are left behind. In the past, huge numbers of shells and skeletons have collected on the sea bed and formed thick layers. As the layers became covered in other materials, the weight on the shells and skeletons became so great that they were squashed together to form limestone.

Most of the skeletons belong to animals that were very small and so their remains cannot be seen with the naked eye. The fossils you see in the limestone have formed from the bodies of the larger animals.

Limestone is used as a building material. Limestone also contains useful chemicals and can be made into a fertiliser. It is used to extract iron from its ore in a blast furnace and to make cement and glass.