Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. It forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. It is common to find flint and chert nodules embedded in chalk. Chalk is relatively resistant to erosion and slumping compared to the clays with which it's usually associated, thus forming tall steep cliffs where chalk ridges mee… (More on Chalk)