Although a great deal of limestone is produced by biological processes, this origin is not always evident because shells and skeletons may undergo considerable change before becoming lithified into rock. However, one
easily identified biochemical limestone is coquina, a coarse rock composed of poorly cemented shells and shell fragments.
Another less obvious but nevertheless familiar example is chalk, a soft, porous rock made up almost entirely of the hard parts of microscopic marine organisms. Among the most famous chalk deposits are those exposed along the southeast coast of England.