Chalk

Chalk is a soft form 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. As they are slowly eroded a steep scarp slope forms where the rock has been removed, and a shallow dip slope remains where the rock has not been eroded. Chalk is permeable and can hold large quantities of water, removing the need for large reservoirs in areas with chalk hills. The height of the water table in chalk hills rises in winter and falls in summer, and chalk areas often contain bournes - rivers which are dry in the summer, and in England many villages named Winterbourne can be found in the South Downs, Dorset Downs and Salisbury Plain.

Chalk can be found in many places, including:

In the United Kingdom:

Dorset Downs
Isle of Wight
On the Jurassic Coast (Dorset):
- Lulworth Cove
- Old Harry Rocks
- White Nothe
Salisbury Plain
South Downs
On the Sussex coast:
- At Beachy Head
- At Eastbourne
- At the famous white cliffs of Dover

In the United States:

In the extensive Austin Chalk formation, which underlies much of central Texas

The substance known as "blackboard chalk", often supplied in sticks about 2 inches (or 5 cm) long and used for drawing, especially on blackboards and sidewalks, is not actually chalk: it is made from gypsum, calcium sulfate. Similarly, the "chalk" used by tailors is usually made from talc, magnesium silicate.