Chalk

Chalk, soft to very soft (powdery or crumbly), pure or nearly pure, porous, fine-textured form of marine limestone. White, light-grey, or sometimes buff in colour, it consists largely of the mineral calcite, or calcium carbonate (CaCO₃), that has formed by the accumulation, usually in shallow waters, of the tests (the hard parts) of dead, once-floating micro-organisms such as foraminifera and/or of tiny hard fragments of algae, such as coccoliths and rhabdoliths. It may also include the remains of such organisms as ammonites and echinoderms that lived on the sea floor.

Chalk occurs in various parts of the world but is particularly associated with north-western Europe. It is especially conspicuous in Britain, where it can be found both inland (for example, the South Downs) and along coasts (the White Cliffs of Dover). Its formation coincided largely with the Upper Cretaceous Period (Latin creta, “chalk”) and is so distinctive that in Europe, where the period is usually known as the Chalk, they are almost synonymous—in other words, “Chalk” is used as a stratigraphic term. In Britain, the Chalk is divided into Lower, Middle, and Upper coinciding (respectively) with the Cenomanian (97.5 to 91 million years ago), the Turonian (91 to 88.5 million years ago), and the Senonian (88.5 to 65 million years ago) divisions of the Upper Cretaceous; that is, the entire Upper Cretaceous Period. In other parts of Europe, however, the Chalk extended upwards into the Danian (65 to 60.2 million years ago), which is the lower part of the Tertiary Period.

Small marine fossils (such as clams, small plants, sharks’ teeth, and seashells) are frequently found in chalk; somewhat rarer are fossils of large fish, giant swimming and flying reptiles, and swimming birds. The stone flint is often found in chalk deposits. In England chalk is quarried in Kent near Dartford, while in the United States large deposits are found in Iowa, Texas, and Arkansas. Chalky soils are often poor, lacking good drainage, and are most often used for grazing, although some cropping is undertaken. Processed chalk is used as a writing implement and a medium for drawing.