What is chalk?
"These cliffs are temporary. Reduced and solid, born from the warm, tropical broth of stockand stored in a stack above a solid blocknow broken off in chunks and re-dissolved in the cooler, less forgiving froth of the liquid finger from the North."
by Ros Barber from the "Chalk and Channel Way" (2000).
Chalk is world famous as the "white cliffs of Dover". There are many stretches of white cliffs in England, yet chalk is a rare type of rock in the world. The only other chalk cliffs in the world, are a short stretch in Northern Ireland, two stretches of white cliffs in France (in Pas de Calais and Normandy) and the white cliffs of the islands of Mon (Denmark) and Rugen (Germany).
Chalk is composed almost entirely of a chemical called calcium carbonate, which is why it is white in colour. Typically the upper chalk consists of 95% or more of calcium carbonate.
If you look closely at chalk (magnified 1,000 times) you will find that the rock is made up of large number of tiny skeletons of plants that floated in a warm, tropical sea 130-65 million years ago, in a geological period known as the Cretaceous. One of the principal features of the Cretaceous was the formation of the chalk; the name "Cretaceous" is derived from the Latin "creta" for chalk.
The plant skeletons are known as coccoliths and are made from calcium carbonate, which was extracted by the living plants from the sea water. When they died, the skeletons fell to the sea bed and was compacted over millions of years to form the chalk rock we see today.
Chalk is thought to have formed in the sea far from land, as there is very little other material present in the rock. The lower levels of chalk, known as chalk marl are not so white (they are a grey colour) and are softer because of the presence of small clay particles. The lower chalk is thought to have formed in a shallower sea close to land, where particles of clay were washed into the sea by rivers and fell to the bottom of the sea along with plant skeletons to form the muddy lower chalk. As time passed, the water became deeper and the land further away so there was less clay particles in the water and the pure white chalk rock formed.
The chalk downs were lifted up to form the North Downs because of crumpling of the earth's crust caused by the slow movement north of the African tectonic plate colliding with the European tectonic plate. The Alps have been formed as a result of this collision, which is still going on today and the Alps are still slowing rising.
The chalk marl was the route of the Channel Tunnel under the English Channel from Kent to France. The tunnel was bored through the chalk marl because it is softer than the higher levels of chalk, but also because it is waterproof due to its fine, clay structure. You can see the chalk marl at Samphire Hoe, where spoil from the construction of the Channel Tunnel has been landscaped as a new piece of land, which has been opened to the public.
A quick look at the white cliffs will reveal black lines in the rock. These are flints that form horizontal bands in the chalk. Flint is a hard black rock, usually with a grey surface layer, that is formed from silica.
Chalk is a member of the limestone family of rocks, that is they contain a large amount of calcium carbonate which means that the rock is alkaline, that is the opposite of acid. Chalk is a special form of limestone, which is composed almost entirely of calcium carbonate, which is why it is white in colour. Technically, limestone contains more than 50% impurities, which means that it can be a wide range of colours from pale grey to yellow and brown in colour. Limestone is usually much harder than chalk, and has been widely used for building. Chalk is much softer and has been little used for building, although flints can be widely seen used in building.