How were the white cliffs of Dover, England, formed?

The white cliffs of Dover located on the southeastern coast of England, are composed of a variety of calcite shells, including single-celled plants called coccolithophores (a type of algae) that formed on the seafloor.  The conditions in the ocean were perfect to form these coccolithophores because the nitrite levels, low silicate concentrations, and the right amount of iron enabled them to survive.  They were formed over a 100 million years ago after layers of pulverized skeletons of tiny sea plants were deposited on the bottom of the ocean in a fine gray mud.  It isn’t really clear how these skeletons formed to cover the 300-foot cliffs being that they were only a fraction of an inch in diameter.  Over time the mud was deposited and compressed to form layers and layers of this sediment, then it hardened to make the limestone that we now call chalk.   Researchers believe that eventually this might happen again in the Calcite Belt in about another 100 million years.