A generally soft, white, very fine-grained, extremely pure, porous limestone. It forms
under marine conditions from the gradual accumulation of skeletal elements from minute
planktonic green algae (cocoliths), associated with varying proportions of larger
microscopic fragments of bivalves, foraminifera and ostracods. It is common to find
flint and chert nodules embedded in chalk.
http://en.wikipedia.org/wiki/Chalk; C.S. Harris, 2009, unpublished web page, http://www.geologyshop.co.uk/chalk.htm