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    • 2 images: microscopic shells of foraminifera & coccolithophores
    Rights: Scanning electron micrograph of a single coccolithophore cell from Alison R. Taylor (University of North Carolina Wilmington Microscopy Facility) Creative Commons Attribution 2.5 Generic Published 1 October 2012 Size: 230 KB Referencing Hub media

    Chalk is a special form of limestone made up of the shells of microscopic marine organisms such as coccolithophores (left) and foraminifera (on the right). It is uncommon in the southern hemisphere.

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      Limestone origins

      Limestone is a very common sedimentary rock consisting of more than 50% calcium carbonate. Although it occurs in many different forms, its origins can be traced back to either chemical or ...

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