What is chalk?

Chalk cliff and the sea
The White Cliffs of Dover a place where beauty, nature and history combine National Trust / Chris Tapley

Ever since the days of early 19th-century interest in geology, the White Cliffs of Dover have offered one of the most accessible and complete records of the story of chalk formation.

How is chalk formed?

The cliffs are made from chalk, a soft white, very finely grained pure limestone, and are commonly 300-400m deep. The chalk layers built up gradually over millions of years.

They're formed from the skeletal remains of minute planktonic green algae that lived floating in the upper levels of the ocean. When the algae died, their remains sank to the bottom of the ocean and combined with the remains of other creatures to form the chalk that shapes the cliffs today.

Over millions of years, the seabed became exposed and is now above sea level. The resulting edge of chalk is the iconic White Cliffs of Dover.

Natural erosion

The cliffs only stay white because they're allowed to erode naturally. Where the cliffs are protected from erosion by man-made structures, like in the Port of Dover, plants will colonise the cliff-face making it appear green when viewed from the sea.

What’s in a name?

Chalk has many uses, but not all chalk is the same substance. For example, the chalk cliffs here are made of calcium carbonate. But the French chalk used to repair bicycle punctures is a natural magnesium silicate, and the chalk board chalk is actually gypsum or calcium sulphate.

Discover the history of the cliffs