Limestone is a sedimentary rock comprising at least 80% calcium carbonate (CaCO₃). Limestones can be deposited in both marine and fresh water environments, the former being more common, and can take many forms such as those described below.

Chalk is a pure, soft, well-jointed limestone composed primarily of the calcareous remains of micro-organisms called coccoliths and foraminifera. Pure white chalk contains more than 90% CaCO₃ and is usually found in thick beds. In Britain it stretches from Yorkshire down to the famous cliffs of Dover. These where deposited in the Cretaceous (~100 million years ago) when Britain was submerged under a deep sea.

Colour: white, grey or yellow.

Mineralogy: mainly calcite

Occurrence: chalks are pelagic limestones formed in shallow, open seas.

Texture: fine grained and finely porous.

Structure: usually well bedded in thick extensive successions, nodules of flint and marcasite are common

Fossiliferous limestones consist of a large number of fossils held together mainly in a calcite cement.

Carboniferous limestone is an example of a shelly limestone and is found predominantly in the Peak District and Yorkshire Dales where it forms a distinctive landscape known as karst. The limestone was deposited in the Carboniferous period, over 300 million years ago, and tells us that Britain once lay in warmer latitudes. This tropical environment was similar to the Caribbean of today with lots of algae, corals, and brachiopods.

Colour: variable.

Mineralogy: finely divided calcite mud containing larger crystals from animal skeletons.

Composition: calcareous.

Occurrence: widely distributed.

Texture: variable.

Structure: bedding often apparent, the fossils may be complete or fragments.

Oolitic limestone is a carbonate rock made up mostly of ooliths (or ooids) which are sand-sized carbonate particles that have concentric rings of CaCO₃. These rings are formed around grains of sand or shell fragments that were rolled around on the shallow sea floor, gathering layer after layer of limestone.

Portland limestone is an example of an oolitic limestone. This limestone formed in a shallow sea, rather like the modern Bahamas, near the end of the Jurassic period (~135 million years ago). The rock has an even structure rather like cod roe and it can therefore be cut or sculpted in any direction. This feature, coupled with hardness, colour and durability, gives the limestone its quality as a building stone. To find out more to do with Portland limestone and how it was once quarried and how it has been used you may find the link to the Jurassic Coast useful: www.swgfl.org.uk

Colour: white through yellow.

Mineralogy: usually calcite ooliths in varing amounts of a finer grained calcite matrix.

Composition: calcareous.

Occurrence: in warm, shallow seas.

Texture: spheres or oval shapes having a concentrically zoned inner structure.

Structure: often current bedded with varing amounts of fossil fragments.