What is limestone?

Limestone landscape around Llangollen, North Wales.

Limestone is a sedimentary rock, most of which originally formed by the accumulation of sediments rich in carbonate minerals, like calcite, underwater. These sediments, which were turned to limestone rock over millions of years after they were laid down, are composed of over 50 per cent carbonate minerals. There are several different types of limestone, which can be classified according to how they formed.

Formation of limestone

Wenlock Limestone, showing fossils of marine creatures like crinoids and brachiopods.

Limestone forms predominantly on the sea floor, although some forms in fresh water too. The material rich in calcium carbonate ('calcareous' material) accumulates there, and this calcareous material may be organic, chemical or detrital in origin.

More about the chemical composition of limestone and how limestone is formed.

Weathering and erosion

The breakdown of rocks via weathering or the erosion processes, e.g. rivers, sea, glaciers or wind, can create very distinctive landscapes and features in limestone areas.

More about weathering and erosion of limestone.

Movement of water through limestone

The porosity and the permeability of limestone is important because these properties control how water moves over and through the rock.

There are several routes that water might take to pass through limestone:

  • pore spaces
  • cracks, joints or fissures
  • caves

More about the movement of water through limestone.