Cretaceous System

Cretaceous System, the highest division of the Mesozoic or Secondary strata, rests conformably upon the Jurassic System (q.v.), and is overlaid unconformably by the oldest deposits of the Eocene System (q.v.). The cretaceous strata of Britain are confined chiefly to the east and south-east of England. They form the Yorkshire Wolds, extend over large parts of Norfolk, Suffolk, and Hertford, and compose the Chiltern Hills, Salisbury Plain, the Downs, and the south part of the Isle of Wight. On the Continent the cretaceous rocks have a considerable development. They form a broad basin in the north of France, and stretch eastward from Belgium, Holland, Denmark, and the south of Sweden, through the great plains of northern Europe to the south end of the Ural Mountains. But over extensive regions within that wide area they lie more or less concealed under younger formations. There is another extensive development of cretaceous strata in southern Europe, where they enter largely into the composition of many of the Mediterranean coast-lands. The chief petrological feature of the cretaceous strata of western and northern Europe is the great development of white chalk in the Anglo-French area, and its gradual replacement, when followed eastwards into Germany, &c., by earthy limestones, shales, sandstones, &c. The most marked characteristic of the cretaceous system in southern Europe is the great development in that region of massive marine limestone (hippurite limestone).

In North America cretaceous strata likewise occur in force, especially in the western states and territories. They also occupy wide tracts in the Gulf states, whence they extend up the Mississippi valley to the Ohio; they put in an appearance at intervals on the Atlantic borders between South Carolina and New Jersey; and we meet with them again on the Pacific border and in the coast-range. Strata of the same age occur also in the far west of British America, at the mouth of the Mackenzie River, and in Greenland. In India the system is marked in the Deccan by a massive series of basalt-rocks, 4000 to 6000 feet thick, and covering an area of 200,000 sq. m. In Australia and New Zealand there is a considerable development of cretaceous rocks, such as the 'desert sandstones' of Queensland, and a small coal-bearing group of beds. In New Zealand the system likewise contains coals, some of which are lignites, while others are bituminous coals of fair quality. The following is the succession of cretaceous strata in England:

The Wealden Beds consist largely of clay and sand, and are almost entirely of fresh-water origin. In Yorkshire, however, the strata which occur on the same horizon as the Wealden beds of the south are of marine origin, as seen in the Speeton clay near Bridlington. The Lower Greensand, consisting of sand, clay, &c., are marine. The Gault (q.v.), a tough blue clay, is likewise marine, and so also are the shallow-water sands of the Upper Greensand, and the thin layer of chalky marl called Chloritic Marl (q.v.), which is characterised by the presence of glauconitic grains and phosphatic nodules. The most characteristic rocks of the system, however, are the chalk beds. The basement of these beds is the argillaceous chalk known as Chalk Marl. The lower chalk is a grayish-white chalk, while the middle chalk is a pure white chalk, containing in its upper portions layers of flint nodules. A hard layer of yellowish limestone called Chalk-rock lies at the top of this division. The upper white chalk is a thick massive white chalk, containing numerous layers of nodules and occasional tabular sheets of flint. All these chalks are of marine origin. The following are the divisions adopted by continental geologists, and now largely used by geologists in Britain:

The cretaceous strata of Britain being almost exclusively of marine origin, it is not surprising that land-plants seldom occur, and that they are met with chiefly in the fresh-water beds near the base of the system. They consist chiefly of ferns, cycads, and conifers—a flora resembling that of the preceding Jurassic period. The upper cretaceous rocks of Germany, however, have furnished many plant remains. Amongst those are the oldest known dicotyledons, such as extinct species of maple, oak, walnut, beach, laurel, magnolia, &c., also several proteaceous plants. A similar admixture of forms occurs in the cretaceous strata of North America. Amongst animals the Protozoa played a very important part—the white chalks and earthy limestones being very largely composed of the minute shells of foraminifera, such as Globigerina, Rotalia, and Textularia, which still swarm in the ooze of the Atlantic. Sponges, such as Venticulites, Siphonia, &c., were very abundant, and sea-urchins also occurred in great numbers. Star-fishes and bryozoans were fairly common, as were also, amongst brachiopods, Terebratula and Rhynchonella. But the brachiopods as a class were feebly represented as compared with their abundance in the earlier stages of the world's history. Ordinary bivalves, however, were very numerous, such as Inoceramus, Hippurites, Spondylus, Lima, Pecten, &c. In the Danian beds carnivorous gasteropods begin to abound, and they include a number of existing genera. Cephalopods are not only the most abundant, but also the most characteristic fossils of the cretaceous rocks. Amongst them are a great variety of Ammonites, and many forms of Belemnitidae. Amongst the fishes were ganoids, and various kinds of the shark tribe, together with the earliest representatives of the Teleostei—which include most living genera of fishes. The waters of the period seem also to have swarmed with reptiles, such as Ichthyosaurus and Plesiosaurus. Winged reptiles were also present, such as Pterodactylus. Amongst dinosaurs were Cetiosaurus, Megalosaurus, and Iguanodon. Another remarkable reptile was the serpent-like Mosasaurus. Besides these, there were lizards, chelonians, and crocodiles. The American cretaceous system is likewise characterised by the presence of huge dinosaurs and other reptiles—some of them being European types, while others are peculiar. One of the most remarkable features of the American rocks, however, is the occurrence in them of the toothed birds—Ichthyornis and Hesperornis. (Separate articles deal with most of these various forms.)

No break separates the Jurassic from the cretaceous system—there is a gradual passage from the upper beds of the former into the lower beds of the latter. At the beginning of cretaceous times most of the British and Irish area existed as dry land. Over the south-east of England lay the estuary of a large river, flowing probably from the north. The Wealden beds are the delta-deposits of that river; the English and French beds of this division covering an area of 20,000 sq. m. The sea into which that river flowed occupied a considerable area in the north of France, spread over the Low Countries into Hanover, filled the basin of the North Sea, and overflowed a portion of eastern England. Wealden beds occur in north-west Germany, and indicate the delta of a river, like that of the British area, flowing from the north. While land-conditions predominated in northern and middle Europe, an open sea covered vast areas in southern Europe. Gradual subsidence of the sea-bottom took place during the deposition of the Wealden series, and eventually the great deltas became submerged, and a wide sea covered most of what are now the low grounds of the British area, and passing eastwards, submerged vast regions of middle Europe up to the slopes of the Ural Mountains. The depression was greatest in the western areas, where in the deep clear waters there gradually accumulated the calcareous matter which subsequently formed our white chalk. There is no deposit forming at present which is quite analogous to white chalk. The calcareous oozes of existing seas which most resemble it are of abyssal origin, but the sea in which the chalk accumulated probably did not exceed 1000 or 2000 feet in depth. The extreme purity of the chalk, consisting as that rock does of 95 per cent. and more of carbonate of lime, is difficult to account for on the supposition that the sea in which it formed was comparatively shallow.

The sea of western Europe may have been dotted with small islands—from none of which large rivers descended; and possibly the formation of the chalk was not so slow a process as many geologists suppose. Professor Prestwich even suggests that it may to some extent be of the nature of a chemical precipitate thrown down under special and peculiar conditions prevailing at the time. However that may be, the shells of foraminifera and other organic remains certainly enter very largely into its composition. In the Mediterranean basin, a deep open sea would seem to have persisted all through the cretaceous period. It was in this sea that the massive hippurite limestone was formed. Open water appears at this time to have extended through the Mediterranean area into Asia, covering there also vast tracts of what is now dry land, and communicating with the Indian Ocean. The conditions of climate seem to have been remarkably uniform over vast regions of the earth's surface. Ferns, cycads, and conifers flourished in the lands within the Arctic Circle, and the waters of the same region were tenanted by cuttlefish, ammonites, and huge reptiles.