Chalk, a very soft, powdery variety of limestone. Like all limestone, it consists mainly of calcium carbonate. Chalk is used in the manufacture of some putties, cements, and scouring powders. In stick form, chalk is used for drawing and writing. Finely ground, purified chalk is used as a tooth powder and as an antacid, a drug for treating excess stomach acidity.

Chalk deposits were formed on the floor of the ocean and consist almost entirely of the shells of tiny aquatic organisms called foraminifera. These deposits accumulated continuously for thousands of years. The greatest chalk beds were formed during the Cretaceous Period, which ended 65 million years ago.

Deep-sea dredgings show that a fine white ooze is being formed at the bottom of the ocean in a way that is probably similar to the process that formed chalk. Chalk formations usually contain fossils of seaweeds, sponges, corals, mollusks, fishes, and reptiles.

A chalk formation extends from northern France into the southern parts of England. Chalk cliffs border both shores of the English Channel. These white cliffs, which include the famous White Cliffs of Dover, gave Britain its ancient name of Albion (from the Latin word for white). A notable chalk deposit in the United States runs from Austin, Texas, southwestward into Mexico. Other large United States deposits are the Niobrara formation in the central Great Plains, and the Selma Chalk in the Black Belt of Alabama and Mississippi.