Chalky Limestone

Chalky limestone, or chalk, is a soft, fine-grained, porous variety of limestone composed predominantly of calcium carbonate. Much of it is built from the microscopic calcite plates (coccoliths) of marine algae together with tiny foraminifera shells.

It is famously white to cream, friable enough to crumble and mark surfaces, and lightweight because of its high porosity. The white cliffs of Dover and the chalk downs of England and northern France are made of this rock.

Chalk is a relatively pure carbonate that accumulated slowly on quiet sea floors, and it remains one of the most recognizable and culturally familiar sedimentary rocks.

Chalky limestone forms from the slow accumulation of calcareous plankton skeletons on the floor of warm, clear, relatively deep shelf seas. Coccolithophores and foraminifera living in the surface waters die and rain down their calcite tests, building up fine carbonate ooze.

Over millions of years this ooze compacts into soft, porous chalk. The great chalk deposits of Europe formed during the Cretaceous Period, a time of high sea levels and warm oceans, the very name Cretaceous deriving from the Latin for chalk.

Low input of mud or sand was essential, so chalk records quiet, sediment-starved marine conditions far from major rivers.

Chalky limestone is recognized by being very soft (it can be scratched with a fingernail and leaves a white mark on surfaces), highly porous, lightweight, and white to cream in color.

It fizzes strongly in dilute hydrochloric acid because it is nearly pure calcium carbonate. The texture is fine and earthy, often crumbly, and it may contain flint nodules and fossils such as sea urchins, belemnites, and bivalves.

Look-alikes include diatomite and kaolin (which do not fizz in acid) and harder, denser micritic limestone (which resists fingernail scratching). The combination of softness, white color, and strong acid reaction identifies chalk.

Chalk has long been used to make writing chalk (though blackboard chalk today is often gypsum), whiting for paints and putty, and as a filler in plastics, rubber, paper, and toothpaste.

It is a key raw material for cement and agricultural lime, used to neutralize acidic soils. Chalk landscapes also form important aquifers, storing and supplying groundwater.

Geologically and historically, chalk is significant as a marker of Cretaceous seas and as the iconic stone of the white cliffs and chalk downland. Pure precipitated calcium carbonate is also used in antacids and dietary calcium supplements.