Classification of limestones

'Massive limestones' are those that are made of thick layers (called 'beds') of rock. In England, these limestones are the older and harder types, such as the Carboniferous limestone. The massive Carboniferous limestones originally accumulated in a warm shallow sea when Britain lay much closer to the equator than it does now.

Broadly speaking, there are three classifications of limestones, according to how they formed:

Shelf limestones

Shelf limestone

Shelf limestones are massive, hard and pale grey in colour. They are made up of fragments of shells and skeletons of marine organisms (such as crinoids, corals and brachiopods) and held together by calcite cement. Shelf limestones were originally deposited in very shallow seas.

Basin limestones

Basin limestone

Basin limestones are much darker in colour and more thinly bedded than shelf limestone. They contain horizons of shale or mudstone, which were originally deposited as muds. Basin limestones have very few fossils and are believed to have accumulated in much deeper waters than the shelf limestone.

Reef limestone

Reef limestone

Reef limestones are white to pale grey in colour and are very fossiliferous, but lack bedding. They occur as isolated patches within the other two limestones, between one and several hundred metres across. They probably formed in shallow tropical seas in a similar way to coral reefs today.

Types of limestone gallery

In addition to these three general categories, there are many different individual types of limestone. Some examples of limestones found in England and Scotland are shown in the gallery below.