2.2 Cretaceous
2.2.1 Chalk
2.2.2 Upper Greensand/Gault
2.2.3 Lower Greensand
The Cretaceous beds form a broad band extending in an open curve from Flamborough Head to the Dorset coast, and, like the Jurassic beds they parallel, they have in general an easterly or south-easterly dip. But in the south of England the structure is complicated by a series of folds and faults which run from west to east. In consequence of-these a broad arm of Cretaceous rocks spreads from the main band at Salisbury Plain to the shores of Sussex and Kent, and a narrower strip forms the southern rim of the Hampshire basin in the Isle of Purbeck and the Isle of Wight. Cretaceous rocks are also brought to the surface at one or two places in the midst of the Tertiary beds, both in the London and the Hampshire basins.
Except where it is broken by the sea the main outcrop is practically continuous, and it is only in the south that there are neighbouring outliers of any considerable extent. But far away to the north-west, around the basaltic plateau of Antrim and beneath the lavas of Mull and Morvern, Cretaceous beds are again met with. They contain marine fossils, and prove that at one time the Cretaceous sea must have covered the greater part of the British Isles. Both in Ireland and in Scotland, however, much of the deposit is littoral in character, and some of the Scottish beds are estuarine.
In the main outcrop of the Cretaceous rocks the Chalk is always the dominant member of the system, and everywhere it forms an undulating plateau which terminates westward in a well-defined escarpment overlooking the Jurassic or Triassic plains, while eastward it sinks gradually beneath the Tertiary or later deposits. This escarpment is one of the most marked of the physical features of Eastern England, and is known under various names in different parts of the country. It forms the Yorkshire and Lincolnshire Wolds in the north, and the Gogmagog and Chiltern Hills farther to the south.
In the easterly arm the lower beds play a more important part than in the main outcrop. The hilly region in the middle of the Weald consists of Lower Cretaceous rocks ; but the North Downs and the South Downs are formed by the Chalk.
2.2.1 Chalk
The Cretaceous system falls naturally into two divisions, which differ widely in character and in distribution.
The Lower Cretaceous consists chiefly of sands and clays, in part of fresh-water origin and in part marine.
The Upper Cretaceous consists mainly of Chalk, but at its base is a variable series of clays and sands. Except in the west of Scotland, it is altogether marine.
It is not, however, the difference in lithological character that constitutes the principal reason for separating the Lower from the Upper Cretaceous, but rather the difference in their distribution. The Lower Cretaceous is comparatively limited in extent. It is fully developed in the Weald, the Isle of Wight and the Isle of Purbeck; but westward it is completely overlapped by the Upper Cretaceous, which here extends almost to the Carboniferous syncline of Devonshire. Towards the north, also, the overlap is almost equally marked, and throughout the greater part of the Cretaceous outcrop the Upper Cretaceous rests directly upon Jurassic rocks. The Lower Cretaceous, however, appears at intervals, but generally only the upper beds of that division. It is only in the north and south of England that the whole of the Lower Cretaceous is present.
In all directions, the Upper Cretaceous sea spreads far beyond the margins of the Lower Cretaceous waters. And this is true not only of the British Isles, but also of a large part of the globe. Nearly everywhere the Upper Cretaceous extends beyond the limits of the Lower. Almost everywhere about this period the sea overflowed the shores of the ancient continents, and the ocean grew at the expense of the land. The overflow appears to have taken place almost simultaneously throughout the globe, and it is one of the most striking and widespread events in geological history. It is often called the ' Cenomanian transgression/ but the transgression or overflow began before the Cenomanian epoch.
To the unaided eye the Chalk differs from other calcareous rocks in the purity of its colour, the absence of any visible crystalline structure and the extreme fineness of the particles which compose it. Chemically it is distinguished by the very small proportion of insoluble material which it contains. Frequently the residue which remains after treatment with hydrochloric acid amounts to less than 2 per cent. But in the more marly beds it may rise to 15 or 20 per cent. Towards the base the quantity increases, but it becomes obvious even to the eye that the Chalk is no longer pure. The insoluble residue consists chiefly of detrital material derived from the land; and in the south of England it is noticeable that the proportion increases gradually from Kent to Dorset.
Microscopically, the Chalk consists chiefly of the tests of foraminifera, minute fragments of shells, especially of Inocemmus, and a matrix of very finely divided calcareous material. The proportion of these constituents varies greatly. In some cases the foraminifera or the fragments of shells predominate, but in general the finer matrix forms the bulk of the rock. Sponge spicules are sometimes very numerous, and sometimes little globules of colloid silica abound. Often small round calcareous discs are common in the matrix.
The very fine texture of the whole deposit, the Chalk undoubtedly resembles the calcareous ooze of the present ocean-floors ; and accordingly many writers have concluded that it was laid down in deep waters. The abundance of hexactinellid sponges, similar to those which now live at considerable depths, adds support to this view. Many of the fishes, moreover, possess the delicate skeletons and slender fin-rays characteristic of deep-sea forms.
The Mollusca, on the other hand, point to a different conclusion. Deep- sea gastropods and lamellibranchs generally possess thin and fragile shells, while the shells which are found in the Chalk are often thick and strong like those of shallow-water molluscs. Moreover, many of the genera which are common in the Chalk are now characteristic of shallow water rather than of the deep sea.
No doubt the depth of the water varied from time to time. In some of the marly and nodular beds gastropods are very abundant, indicating a comparatively shallow sea; but generally they are rare. A study of the Mollusca of the bed known as the Chalk Rock has led to the conclusion that that deposit was formed at a depth not much exceeding 100 fathoms. But the Upper Chalk, with its abundant fauna of echinoids, was formed in a deeper sea.
At whatever depth the Chalk was formed, it was certainly laid down in a sea which was remarkably free from detrital material. Either the water was very deep, or the shore was far away, or the rivers brought but little sediment from the land.
It was, moreover, a sea of great extent, for the Upper Chalk spreads with little change of character from England to the east of Russia. But its width was comparatively small. In Scotland the deposits of the Chalk period are in part estuarine ; in Saxony they consist chiefly of sandstone. The northern border of the sea seems to have stretched from Scotland to the south of Scandinavia ; the southern border lay north of the Alps. The Chalk sea was therefore hardly comparable with the great oceans, but rather with the present Mediterranean.
Fauna and Flora.—So far as our islands are concerned, the flora of the Cretaceous is practically the flora of the fresh-water deposits at its base, for elsewhere remains of plants are rare. The forms which are found in these lower beds are very closely related to those of the Jurassic rocks. Ferns and cycads are still the dominant groups. Conifers, Equisetaceae and a few other forms also occur. On the Continent Dicotyledons appear in the Upper Cretaceous.
The fauna of the Cretaceous period includes many of the genera which lived during Jurassic times, but it differs in many important respects. The difference is greatest in the case of the Chalk, partly, no doubt, because in point of time the Chalk is farthest removed from the Jurassic deposits ; but partly also because it was laid down under very different conditions.
Sponges are more abundant than in any other system. Calcareous forms are common in the Lower Greensand, the highest division of the Lower Cretaceous. In the Upper Cretaceous, siliceous forms predominate, especially in the Chalk ; but calcareous forms are occasionally common in the Upper Greensand.
In the British Cretaceous, corals are nowhere abundant, and they form no reefs or beds comparable with those of the Middle Jurassic A few simple corals, however, occur, such as Trochocyathus, Parasmilia and Micrabacia, and one or two compound forms such as Holocystis.
Echinoderms are very common, especially in the Chalk. Echinoids predominate, and, indeed, this class attains its greatest development in the Upper Cretaceous. Amongst the most important genera are Galerites, Micraster, Hohster and Echinocorys. The crinoids are represented by Marsupites and a few other forms. Star-fish also occur.
Brachiopods are not so numerous as in the Jurassic deposits. But Terebraiula and Rhynchonella are still common, and other genera are also met with.
Polyzoa are often found, both cyclostomatous and cheilostomatous forms being abundant.
The Lamellibranchia are well represented. Fresh-water forms such as Unio and Cyrena occur in the Lower Cretaceous; but the special characteristic of the period is the abundance of Inoceramus, and (in Southern Europe) of the remarkable genus Hippurites and its allies. Gervillia and Spondylus are other genera which may be mentioned.
Gastropods are common in some beds. Amongst them are the marine genera Pleurotomaria and Aporrhais and the fresh-water form Viviparus.
Cephalopods are very abundant, especially ammonites and belemnites. The latter include the genus Belemnites itself, and also Actinocamax and Belemnitella. Amongst the ammonites are many of the normal type, but there is also a large variety of uncoiled or partly uncoiled forms such as Hamites, Baculites, Scaphites.
Fish remains are common, and the teeth of Ptyclwdus are well-known fossils from the Chalk. Reptiles are also found, Iguanodon being one of the most striking forms ; the other groups of reptiles resemble those of the Jurassic system. A single genus of birds, Enaliornis, has been found in England ; and other genera occur abroad. Many of them retain certain reptilian characters, such as the presence of teeth set in sockets. No mammalia are known in the English Cretaceous.
2.2.2 Upper Greensand/Gault
The Greensands always owe their colour to the presence of grains of glauconite, but they vary considerably in composition and structure. One variety, known in England as malmstone and in France as gaize, consists principally of colloid silica with only a small proportion of grains of quartz and a very variable amount of glauconite. Other varieties are true sandstones, made of grains of quartz and glauconite and flakes of mica, generally with a more or less calcareous cement. Still other beds are formed so largely of sponge spicules that they are really organic deposits.
At the present day glauconite may be formed at any depth down to 2,000 fathoms, or even beyond. Where the amount of terrigenous material is great, the proportion of glauconite is small; and the deposits in which the quantity of glauconite is sufficient to give a green colour to the whole occur mostly about the limit of wave and current action, especially at depths of 200 or 300 fathoms.
2.2.3 Lower Greensand
In the south of England the Lower Cretaceous passes downwards without a break into the Jurassic and upwards into the Gault. The lower part of the Lower Cretaceous is of fresh-water origin and is called the Wealden, the upper part is marine and is known as the Lower Greensand. Towards the north the higher beds overlap the lower, and consequently in the Vale of Wardour only the top of the Wealden series is present, and still farther to the north the Lower Greensand rests directly and unconformably upon the Jurassic.
In Yorkshire and Lincolnshire both the Wealden and the Lower Greensand are represented by marine deposits; but again the higher beds overlap the lower—in this case towards the south—and accordingly in Norfolk the lower beds are absent, and south of Ely the Lower Greensand rests directly upon the Jurassic.
Thus it appears that in the early part of the period the central district was land, separating a northern sea from a southern lagoon or estuary. Gradually the waters rose, and the deposits spread farther and farther over the central land until, in Lower Greensand times, the whole area was invaded by the sea. Even then, however, there remained a considerable difference between the fauna and deposits of the north and those of the south.
According to the manner in which the Lower Cretaceous was laid down, England may therefore be divided into three districts: (1) a southern district, in which the Wealden series consists of fresh-water deposits; (2) a central district, in which the Wealden is altogether absent; and (3) a northern district, in which the Wealden is represented by marine deposits.
Owing to the overlap of the Upper Cretaceous, the boundaries of these three areas cannot be determined with precision. But the southern district extends as far north as the Vale of Wardour, where the last exposures of the Wealden fresh- water beds are seen; the central district stretches from Devizes to Ely; and the northern district includes Norfolk, Lincolnshire, and Yorkshire. southern district
In the south of England the Lower Cretaceous forms the central portion of the dome of the Weald, occupying almost the whole of the area between the North and South Downs. Another anticlinal fold brings it again to the surface in the Isle of Wight, of which it forms the southern half; and owing to the same fold it crops out in a narrow band which runs from east to west across the Isle of Purbeck. A few small inliers occur along another anticlinal axis which passes about six miles north of Weymouth. Both towards the west and towards the north it is overlapped by the Upper Cretaceous ; but it reappears in the Vale of Wardour, where the Chalk escarpment has been cut back far to the east of its general direction. Here, however, the outcrop is narrow, and but little of the fresh-water series is visible.
In the middle of England the Lower Greensand appears at intervals along the Cretaceous outcrop from Devizes to Ely ; but it rests directly and unconformably upon the Jurassic beds below, and there is no trace of the Wealden unless the Shotover Sands belong to that series. Its base is often pebbly, and in Bedfordshire and Cambridgeshire is frequently full of coprolites or phosphatic nodules, which were formerly extensively worked for manure. The greater part of the series consists of white, yellow or brown sands, sometimes with highly ferruginous beds, sometimes with beds of clay ; but it is not possible to distinguish the subdivisions which have been recognised in the south of England. Near Faringdon, in Berkshire, is a remarkable deposit of gravel which is formed very largely of sponges.
On Shotover Hill, near Oxford, and at several other places between Oxford and Aylesbury, there are outliers of ferruginous sand containing Viviparus sussexensis and other fresh-water shells, and resting unconformably upon the Jurassic beds. The evidence of their age is not conclusive, but in one or two places they are associated with beds containing marine Lower Greensand fossils, and they are believed to be a fresh-water facies of the Lower Greensand.
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