Chalk is a soft, white coloured sedimentary rock. It is porous and is made of Calcite. It was formed when trillions of microscopic planktonic creatures called Coccoliths died and sank to the bottom of the ocean. Compression caused the layers to thin and the calcite shells to be turned into stone.
Chalk is known all around the UK due to the prominence and popularity in British Culture and Landscapes. The Isle of Wight's Needles are made of Chalk as well as the cliff of Dover. Chalk has been used for hundreds of years as a writing implement on blackboards.