1. Used as a proper noun chalk describes a rock unit of Cretaceous age, that consists predominately of relatively soft, white, porous limestone with beds of marl and bands or nodules of flint. The term is used without its initial capital to describe any rock with similar appearance and properties. Generally chalk has a relatively high primary permeability and so rarely develops caves of explorable size, though conduit-water flow does occur. Some harder chalks in northern France and south-eastern England hold explorable active and relict caves, which extend for many hundreds of meters [9]. 2. Soft poorly indurated limestone, generally light in color; commonly composed of the tests of floating microorganisms in a matrix of very finely crystalline calcite [10].