Chalk is a soft, white, porous sedimentary rock composed of calcium carbonate. It is a form of LIMESTONE and is composed mainly of the mineral calcite. Chalk has been used since prehistoric times as a writing material and for drawing. MARK is a four-letter word for a visible trace or impression left by something that has passed. LIMESTONE is a nine-letter word for a sedimentary rock composed mainly of calcium carbonate, which is formed from the remains of marine organisms. TALC is a four-letter word for a soft, silky mineral composed of hydrated magnesium silicate. Creta is a five-letter word for a type of chalk found in the Mediterranean region. CRAYON is a six-letter word for a stick of colored wax, charcoal, or other material used for drawing or coloring.