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

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.