Cretaceous System
the highest strata, rests conformably upon the division of the Mesozoic or Secondary formably by the oldest deposits of the EOCENE SYSTEM. The Cretaceous strata of Great Britain are confined chiefly to the E. and S. E. of England. They form of Great Britain are confined chiefly to parts of Norfolk, Suffolk, and Hertford, the Yorkshire Wolds, extend over large bury Plain, the Downs, and the S. part of the Isle of Wight. On the Continent the Cretaceous rocks form a broad basin in the N. of France, and stretch E. from Belgium, Holland, D Denmark, and the S. of Sweden, through the great plains of Belgium, Holland, Denmark, and the S.
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 CRETACEOUS SYSTEM Mediterranean coast-lands. The chief is the great development of white chalk strata of western and northern Europe is the great development of white chalk in the Anglo-French area, and its gradual replacement, when following E. into Germany, etc., by earthy limestones, in southern Europe is the great decharacteristic of the Cretaceous system in southern Europe is the great development in that region of massive marine limestone (hippurite limestone). occur also In North America Cretaceous strata likewise occur, 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 appearance at intervals on the Atlantic border between South Carolina and New Jersey, and are met with again on the Pacific border and in the coast-range.
Strata of the same age in the far W. of Canada, at the mouth of the Mackenzie river, and in Greenland. In India the system is marked in land. In India the system is marked in the Deccan by a massive series of basalt-rocks 4,000 to 6,000 feet thick, and covering an area of 200,000 square miles.
In Australia and New Zealand there is considerable development of these rocks, such as the "desert sandstones" of Queensland, and a small_coal-bearing group of beds. In New Zealand, the system contains coals, some of which are lignites while others are bituminous of fair quality. 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 S. are of marine origin, as seen in the Speeton clay near Bridlington. The Lower Greensand, consisting of a of sand, clay, etc., is marine. The gault, a tough blue clay, is likewise marine, and SO also are the shallow-water , and so also are the shallow-water sands of the Upper Greensand, and the thin layer of chalky marl called Chlo- The most characteristic ritic Marl. rocks of the system are the chalk beds.
The Cretaceous strata of Great Britain being almost exclusively of marine ain being almost exclusively of marine plants seldom occur, and that they are met with chiefly in the fresh-water beds near the base of the system. They connear the base of the system. They cona flora resembling that of the preceding Jurassic period. The Upper Cretaceous rocks of Germany, however, have furnished many plant remains. Among nished many plant remains. Among such as extinct species of maple, oak, several proteaceous plants. A similar CRETACEOUS SYSTEM admixture of forms occurs in the Cretaceous strata of North America. Among 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. Ordinary bivalves were also very numerous. 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. Among 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 the ichthyosaurus and plesiosaurus. Winged reptiles were also present, such as pterodactylus.
The American Cretaceous system is likewise characterized 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 is the occurrence in them of the toothed birds, ichthyornis and hesperornis.
No break separates the Jurassic from the Cretaceous system; there is a gradual passage from the upper beds of the one into the lower beds of the other. At the beginning of Cretaceous times most of the British and Irish area existed as dry land. Over the S. E. of England lay the estuary of a large river, flowing probably from the N. The Wealden beds are the delta-deposits of that river; the English and French beds of this division covering an area of 20,000 square miles. The sea into which that river flowed occupied a considerable area in the N. of France, spread over the Low Countries into into Hanover, filled the basin of the North Sea, and overflowed a portion of eastern England. Wealden beds occur in northwest Germany, and indicate the delta of a river, like that of the British area, flowing from the N.
While land-conditions predominated in northern and middle Europe, an open sea covered vast areas in southern Europe. Gradual subsidence of the seabottom 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 E. submerged vast regions of middle Europe up to the slopes of the Ural Mountains. The depression was CRETE greatest in the W. areas where in the deep clear waters there gradually accumulated the calcareous matter which subsequently formed our white chalk.
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 the 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.