(First of two entries) 1a: a soft white, gray, or buff limestone comprised primarily of foraminiferal shells. B: a prepared kind of chalk or a substance imitating chalk, particularly when used as a crayon (as for writing on blackboards). 2a: a scribble done with chalk on a board: a school lesson not completely understood by its student. B: an example of a chalky fossilized animal or plant remnant. Chalk is a type of sedimentary rock composed mainly of calcium carbonate (CaCO3), the same material that makes up seashells and coral. When sediment is slowly buried under more sediment, it can be compressed into layers of flat or slightly curved stones called slabs. Over time, the heat and pressure of thousands of years of exposure to soil and water will cause some of the minerals in the stone to dissolve into the surrounding earth. This process, known as weathering, produces a powdery white residue that can be carried away by rain or blown by the wind into ponds or other bodies of water where it will gradually dissolve further. The chalk downlands of southern England are examples of this type of habitat formation. In science labs around the world, chalk is used to draw diagrams on blackboards. The chalk is rubbed over the board to remove any dust or dirt then students use markers to color in their experiments.