Chalk

Chalk is 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.

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. Sidewalk chalk is similar to blackboard chalk, except that it is formed into larger sticks and often colored. It is used to draw on sidewalks, streets, and driveways, mostly by children, but also in many cities used by talented adult artists create temporary masterpieces on the walkways.

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.

Chalk comprises of a sequence of mainly soft, white, very fine-grained extremely pure limestones which are commonly 300-400 m thick. These rocks consist mainly of the remains of skeletal bits of planktonic green algae, associated with varing proportions of larger microscopic fragments of bivalves, foraminifera and ostracods. Flints are a well known component of Chalk and they are present as nodular seams, tabular beds ann linings to fractures. The silica was derived from the dissolution of the siliceous skeletons of sponges and other organisms and has been redistributed in the form of nodules during several stages of crystallisation.