Chalk is a soft, white, porous form of limestone composed of the mineral calcite. It is also a sedimentary rock. It is relatively resistant to erosion and slumping compared to the clays with which it is usually associated, so forms 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.

Chalk is formed in shallow waters by the gradual accumulation of the calcite mineral remains of micro-organisms, over millions of years. Embedded flint nodules are commonly found in chalk beds.

Because chalk is porous, chalk downland usually holds a large body of ground water, providing a natural reservoir that releases water slowly through dry seasons. The River Somme is an example of such water release from chalk.

Chalk has been quarried from 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 in the upper Cretaceous period. This includes the famous White cliffs of Dover of Kent in England, which are formed entirely of chalk deposits. The Champagne region of France is mostly on chalk formations, with the famous caves being carved out beneath the hills.