CHALK, the name given to any soft, pulverulent, pure white limestone. The word is an old one, having its origin in the Saxon cealc, and the hard form “kalk” is still in use amongst the country folk of Lincolnshire. The German Kalk comprehends all forms of limestone; therefore a special term, Kreide, is employed for chalk—French craie. From being used as a common name, denoting a particular material, the word was subsequently utilized by geologists as an appellation for the Chalk formation; and so prominent was this formation in the eyes of the earlier workers that it imposed its name upon a whole system of rocks, the Cretaceous (Lat. creta, chalk), although this rock itself is by no means generally characteristic of the system as a whole.
The Chalk formation, in addition to the typical chalk material—creta scriptoria—comprises several variations; argillaceous kinds—creta marga of Linnaeus—known locally as malm, marl, clunch, &c.; and harder, more stony kinds, called rag, freestone, rock, hurlock or harrock in different districts. In certain parts of the formation layers of nodular flints (q.v.) abound; in parts, it is inclined to be sandy, or to contain grains of glauconite which was originally confounded with another green mineral, chlorite, hence the name “chloritic marl” applied to one of the subdivisions of the chalk. In its purest form chalk consists of from 95 to 99% of calcium carbonate (carbonate of lime); in this condition it is composed of a mass of fine granular particles held together by a somewhat feeble calcareous cement. The particles are mostly the broken tests of foraminifera, along with the débris of echinoderm and molluscan shells, and many minute bodies, like coccoliths, of somewhat obscure nature.
The earliest attempts at subdivision of the Chalk formation initiated by Wm. Phillips were based upon lithological characters, and such a classification as “Upper Chalk with Flints,” “Lower Chalk without Flints,” “Chalk marl or Grey chalk,” was generally in use in England until W. Whitaker established the following order in 1865:—
| Upper Chalk, with flints | |
| Lower Chalk | chalk rock chalk with few flints chalk without flints |
| Chalk Marl | Totternhoe stone Totternhoe marl |
In France, a similar system of classification was in vogue, the subdivisions being craie blanche, craie tufan, craie chloritée, until 1843 when d’Orbigny proposed the term Senonien for the Upper Chalk and Turonien for the Lower; later he divided the Turonien, giving the name Cénomanien to the lower portion. The subdivisions of d’Orbigny were based upon the fossil contents and not upon the lithological characters of the rocks. In 1876 Prof. Ch. Barrois showed how d’Orbigny’s classification might be applied to the British chalk rocks; and this scheme has been generally adopted by geologists, although there is some divergence of opinion as to the exact position of the base line of the Cenomanian.
The accompanying table shows the classification now adopted in England, with the zonal fossils and the continental names of the substages:—
| Zonal fossils used in Britain. | Stages. | N. France and Belgium.* | S.E. and S. France. | |
| A. | Ostrea lunata (Norfolk) Belemnitella mucronata Actinocamax quadratus = Inoceramus lingua in Yorkshire Marsupites testudinarium {Marsupites, Uintacrinus} | Danian? (Trimingham) Upper Chalk Senonian Craie blanche | Flint- bearing chalk. | Marls, sandstones and limestones (not chalky) with Hippurites. |
| B. | Micraster cor-anguinum Micraster cor-testudinarium Holaster planus, Chalk rock | |||
| Terebratulina gracilis Rhynchonella Cuvieri, Melbourne rock | Middle Chalk Turonian Craie marneuse | |||
| Lower Chalk, Chalk Marl and Cambridge Greensand | Marly chalk. | |||
| Actinocamax plenus Holaster subglobosus, Totternhoe stone. Schloenbachia varians. | Cenomanian Craie glauconieuse | |||
| *(See table in article [Cretaceous System].) | ||||
Since Prof. Barrois introduced the zonal system of subdivision (C. Evans had used a similar scheme six years earlier), our knowledge of the English chalk has been greatly increased by the work of Jukes-Browne and William Hill, and particularly by the laborious studies of Dr A.W. Rowe. Instead of employing the mixed assemblage of animals indicated as zone fossils in the table, A. de Grossouvre proposed a scheme for the north of France based upon ammonite faunas alone, which he contended would be of more general applicability (Recherches sur la Craie Supérieure, Paris, 1901).
The Upper Chalk has a maximum thickness in England of about 1000 ft., but post-cretaceous erosion has removed much of it in many districts. It is more constant in character, and more typically chalky than the lower stages; flints are abundant, and harder nodular beds are limited to the lower portions, where some of the compact limestones are known as “chalk rock.” The thickness of the Middle Chalk varies from about 100 to 240 ft.; flints become scarcer in descending from the upper to the lower portions. The whole is more compact than the upper stage, and nodular layers are more frequent—the “chalk rock” of Dorset and the Isle of Wight belong to this stage. At the base is the hard “Melbourne rock.” The thickness of the Lower Chalk in England varies from 60 to 240 ft. This stage includes part of the “white chalk without flints,” the “chalk marl,” and the “grey chalk.” The Totternhoe stone is a hard freestone found locally in this stage. The basement bed in Norfolk is a pure limestone, but very frequently it is marly with grains of sand and glauconite, and often contains phosphatic nodules; this facies is equivalent to the “Cambridge Greensand” of some districts and the “chloritic marl” of others. In Devonshire the Lower Chalk has become thin sandy calcareous series.