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Origin of Limestone / Chalk

Limestone is a very common sedimentary rock of biochemical origin.

Limestone is a sedimentary rock made up of calcite (CaCO3) as its main mineral. Some limestones were formed by chemical deposition and others by the accumulation of shells from minute sea creatures. Many invertebrate animals (animals with no backbones) take calcite from sea water to construct their shells. When they die the shells fall to the sea bed. Areas where there is little deposition of mud or sand will be ideal for the formation of limestone. One type of limestone which is very pure is called chalk, but most other limestones contain variable amounts of mud or sand or other material.


The images above show a typical section through limestone that has ‘clay impurities’ (above) and ‘pure chalk’ (below) which is typical of East Coast British Chalk. The East Coast British chalk is an apparently featureless white limestone with repeated seams of flints. Chalk deposition spanned 30 million years, both Britain and the globe were subject to changes in climate, sea-level, ocean chemistry and marine life during this period. There was virtually no input of land-sourced clays or sands, and the planktonic ooze which accumulated was very uniform in grain-size, composition and colouration as a result.

Sedimentary Limestone

In the chalk deposits found mainly on the east of the UK, the rock is formed from the skeletons of billions of microscopic marine algae called coccolithophorids, which used sunlight to synthesise food. They died and settled on to the sea bed in the Upper Cretaceous period (between 65 – 100 million years ago). Chalk or lime was discovered early in the history of civilised man and there are references to lime in both Egyptian and Roman times.

In the Peak District, limestone was formed during the Carboniferous geological period, some 340 million years ago. At this time, Britain was part of a large continental landmass close to the equator. In these tropical conditions rivers flowed into shallow warm seas teeming with primitive fish, molluscs, and coral reefs. Their calcium shells combined with silt and or clay to form layer upon layer of calcium carbonate rich sediments several hundred metres thick. This limestone is nor as pure as the chalk found on the east side of the UK.

At Unity Lime we make our products from pure chalk found in the east of the UK.

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