The chalk was formed by countless microscopic plankton, which lived floating in the upper levels of the ancient ocean. When the plankton died, their skeletons, called coccoliths, sank to the sea bed, mixing with the remains of bottom-dwelling shellfish and crustaceans. These remains broke down to form white lime mud. This vast plankton graveyard grew very slowly, at a rate of about half a millimetre a year - the height of 180 coccoliths piled on top of each other.

Within the white chalk there are horizontal layers of a dark stone called flint, formed of silica from the skeletons of sea sponges and other marine creatures. The silica dissolved, forming a gel, which flowed through the soft lime mud until it found hard objects, such as shells, to collect around. It then hardened to become flint. Some flints have strange branching shapes, like fingers. These were probably created when the silica flowed into burrows left by lobsters or crabs.

At its height, the ancient sea was 200m higher than present-day levels. The weight of this vast body of water compressed and hardened the lime mud. The heat of the earth's crust also played a role in turning the mud into chalk.

The age of chalk

The period when the White Cliffs formed is called the ''Cretaceous'' (chalk bearing) - a name suggested in 1822 by the Belgian geologist, D'Omalius D'Halloy, who charted chalk laid down in the same periods in France and Belgium. He could date the stone thanks to the types of fossils found within it. Different fossils are found at different levels, as a result of changes in evolution and the marine environment over the millions of years when the chalk was forming.

The Cretaceous age ended around 65 million years ago, when most life on earth, including the dinosaurs, was wiped out during a mass extinction. This was probably due to the impact of an asteroid. It was during the following Cenozoic ("new life") age that mammals replaced the dinosaurs as the most successful land animals. Now movements of the earth's crust, as the great African plate smashed into the European plate, caused the chalk to rise above the sea and become dry land. This same collision led to the creation of the Alps.

Shaped by ice

Two million years ago saw the beginning of a series of Ice Ages, when the whole of northern Europe was covered with ice. Now glaciers - slowly flowing rivers of ice - eroded the soft chalk to produce valleys between rolling hills. Until the end of the last Ice Age, around 10,000 years ago, Britain was joined onto Europe, with chalk hills stretching continuously across what would later be the Channel. As the ice melted, a vast lake rose in what is now the North Sea. At some point, this overflowed to the west, joining up with the Atlantic to create the Channel. Now the faces of the White Cliffs were created, eroded by sea and rain.

The White Cliffs are still eroding today, at a rate of about a centimetre a year.