Chalk
Chalk is a soft form of limestone composed of the mineral calcium carbonate. Chalk is relatively resistant to erosion and slumping and 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.
The material was quarried from prehistory onwards to provide building material and marl for fields. In south east England deneholes are a notable example of ancient chalk pits.
The famous White cliffs of Dover, Kent, England are formed entirely of chalk deposits.
For more on chalk landscapes see: Downland.
The Chalk Formation is a European stratigraphic unit in the Upper Cretaceous Period.
Blackboard chalk is a substance used for drawing on rough surfaces as it readily crumbles leaving particles that stick loosely to these surfaces. Blackboard chalk, often supplied in sticks about 5 cm long, is not actually made from the mineral chalk but from gypsum, calcium sulfate. Similarly, the "chalk" used by tailors is usually made from talc, magnesium silicate.
A chalk in military terminology is a specific aircraft load, especially a group of airborne soldiers which deploy from a single aircraft. U.S. Army Ranger Chalk Four was the group which took the initial casualties in the Battle of Mogadishu.