Cretaceous System

Cretaceous System, though taking its name from what is to us its most familiar rock, the Chalk (Latin, creta), varies very much petrographically. In Europe, when it was laid down, there were two areas of deposit: the southern, an open-sea area through the Mediterranean region into Hindostan, represented by massive limestone containing a remarkable group of pelecypod mollusks, the Hippuritidae (q.v.); the north-western, a shallower water area, from Bohemia into Britain, represented first - in its lower half - by sands and clays containing phosphatic nodules (q.v.), and largely green from the presence of glauconite (q.v.), and subsequently - in the upper part of the series - by white chalk (q.v.). In the lower part the plant-remains resemble those of the subjacent Jurassic (q.v.) rocks; but in the upper, dicotyledons (q.v.) occur in considerable variety at Aix-la-Chapelle, in Dakota, and even in the north of Greenland. The chalk (q.v.) itself is largely composed of foraminifera (q.v.), of which Globigerina is one of the most abundant. Siliceous sponges were numerous, including Siphonia and Ventriculites, forming the nuclei of many of the flint (q.v.) nodules, bands of which characterise the Upper Chalk. Corals and crinoids were not abundant; but echinoids were especially so, including Cidaris, Ananchytes, Micraster, and Echinoconus. The brachiopods Terebratula and Rhynchonella, and the pelecypods Ostrea, Exogyra, Pecten, and Inoceramus were numerous, and in addition to the last of the belemnites and ammonites we have a remarkable variety of unrolled ammonitids, including Turrilites, Hamites, Baculites, etc. Among fishes, in addition to elasmobranchs, such as sharks and rays, there were during the later half of the period the earliest teleostean or true bony fishes. The chief reptiles were the huge terrestrial herbivorous dinosaur Iguanodon (q.v.) and the marine serpent-like Mosasaurus, besides the last pterodactyls and ichthyosaurs. No mammals are as yet known from Cretaceous rocks of the Old World, though some have been described in the United States; but in Kansas both ratite and carinate birds are represented by the toothed forms Hesperornis and Ichthyornis. In northern Europe the system is thus divided into series, named mostly from French localities: -

Upper. Danian or Maestrichtian. Absent in England. Faxoe in Denmark.
Senonian, or Upper Chalk, with flints, named from Sens.
Turonian, or Lower Chalk, without flints, named from Touraine.
Cenomanian, marly chalk and Upper Greensand, named from Mans (Coenomanum), and
Albian, or Gault clay (q.v.), named from Aube.

Lower. Neocomian, named from Neuchatel (Neocomum), represented in England by the upper part of the marine Speeton clay and the freshwater Wealden and marine Lower Greensand.

A slight unconformity and a marked palaeontological break usually separates the Neocomian from the Upper Cretaceous. In India during this period the Deccan traps, 4,000 to 5,000 feet thick, over 200,000 square miles, were erupted. In the western United States Cretaceous rocks reach a thickness of 11,000 to 13,000 feet, and there and in New Zealand there seems to be no great break between them and overlying Tertiary rocks. In Britain and over most of Europe, on the other hand, the close of the Secondary period is indicated by a marked break at the top of the Cretaceous.