Chalk 's 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's common to find flint and chert nodules embedded in chalk. Chalk can also refer to other compounds including magnesium silicate and calcium sulfate. Chalk's resistant to erosion and slumping compared to the clays with which i… (More on Chalk)