Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. It forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. It is common to find flint and chert nodules embedded in chalk.
Chalk is relatively resistant to erosion and slumping compared to the clays with which it is usually associated, thus forming tall steep cliffs where chalk ridges meet the sea. Chalk hills, known as chalk downland, usually form where bands of chalk reach the surface at an angle, so forming a scarp slope. Because chalk is porous it can hold a large volume of ground water, providing a natural reservoir that releases water slowly through dry seasons.
Chalk has been quarried since prehistory, providing building material and marl for fields. In southeast England, deneholes are a notable example of ancient chalk pits.
The Chalk Formation is a European stratigraphic unit deposited during the late Cretaceous Period. It forms the famous White Cliffs of Dover in Kent, England. The Champagne region of France is mostly underlain by chalk deposits, which contain artificial caves used for wine storage.
The traditional uses of chalk have in many cases been replaced by other substances, although the word "chalk" is often still applied to the replacements.
- Blackboard chalk is a substance used for drawing on rough surfaces, as it readily crumbles leaving particles that stick loosely to these surfaces. Although traditionally composed of natural chalk, modern blackboard chalk is generally made from the mineral gypsum (calcium sulfate), often supplied in sticks of compressed powder about 10 cm long.
- In agriculture chalk is used for raising pH in soils with high acidity. The most common forms are CaCO3 (Calcium carbonate) and CaO (Calcium Oxide).
- In lawn tennis, powdered chalk was used to mark the boundary lines of the court. This gives the advantage that, if the ball hits the line, a cloud of chalk or pigment dust can be seen. Nowadays the substance used is mostly titanium dioxide.
- In gymnastics, rock-climbing, bouldering, weight-lifting and tug of war, chalk—now usually magnesium carbonate—is applied to the hands to remove perspiration and prevent slipping.
- Tailor's chalk is traditionally a hard chalk used to make temporary markings on cloth, mainly by tailors. Nowadays it is usually made from talc (magnesium silicate).
- Sidewalk chalk is made of sticks of colored (and sometimes white) chalk (now mostly gypsum) used to draw on sidewalks, streets, and driveways. It is often done by children, but in many cities, talented adult artists create masterpieces on the walkways.
- Toothpaste also commonly contains small amounts of chalk.