The Remains of Tiny Shells

The material we call 'chalk' was formed during the time when dinosaurs lived on Earth. At that time the oceans were rising higher and higher until finally they covered most of the land.

Billions of tiny animals live in those oceans. They were so small you could not have seen them - they were even smaller than the full stop at the end of this sentence. These tiny creatures had shells made from the element calcium. When they died, their shells fell to the bottom of the sea. After thousands of years, there were many layers of shells on the ocean floor.

As more and more of the tiny shells pressed down from the top, those on the bottom became harder and began to stick together. Eventually the shells changed into a mineral called 'calcite', the main ingredient of the rock known as 'limestone'.

Many millions of years passed after the first chalk was made. The Earth's surface changed its shape, and the land and sea developed new coastlines. This left many chalk layers on dry land, both in the middle of continents and by the sea. In the south of England there are chalk cliffs 244 metres high. These are the famous White Cliffs of Dover, and they are almost solid chalk!

If you had a piece of chalk from of those cliffs, you could use it to write on a chalkboard. But the chalk that is used in classrooms is not dug from the cliffs or the ground. It is made in factories by mixing several different materials.