Geology if Norfolk

If a bicyclist were to cross England from Holyhead to Yarmouth he might, in the course of a few days, pass successively and almost in the chronological order of their deposition over strata comprising a representative series of the principal formations known to geological science, from the Archaean schists of Anglesea, to the modern estuarine deposits of the valley of the Yare. As he approached the confines of Norfolk, however, he would find he had left behind him the Palaeozoic, and all but the later portion of the Secondary or Mesozoic rocks. The prehistoric history of East Anglia deals only with the more recent chapters of the geological record.

The Mesozoic deposits of England east of the Pennine chain dip towards the east, resting against and upon each other as a row of books on a table might do, if they were made to lean over in one direction. Hence it is that, as we pass eastward from the midland counties to Norfolk, we cross successively over the upturned edges of the different secondary strata, from the Trias to the Chalk. The section will show the way in which this movement of upheaval towards the west and subsidence towards the east has affected the Oolitic and Cretaceous rocks of the county. The former, represented by the Kimeridge Clay, appear only in the west of Norfolk, occupying a strip of low country bordering the fens, from whence they dip eastwards, disappearing beneath the Lower Greensand. The Kimeridge Clay is not well exposed in Norfolk, but may be studied in a large pit near the Great Eastern Railway, a mile or so east of Ely Station, where it is shown to consist of bluish-grey fossiliferous clay and shale, with septaria, large nodular concretions of argillaceous limestone. In deposits of this period, in other parts of England, there have been found remains of the great and extinct marine reptiles Icthyosaurut, Plesiosaurus, Pliosaurus, and Teleosaurus, and of the Pterodactyls, enormous flying lizards.

The Lower Greensand is believed to be represented in the Hunstanton cliff section, and at Snettisham, where it is quarried for building, by a fine ferruginous conglomerate, known as Carstone, and at the latter place by clay and white " silver sand," used for glass-making.

A bed of blue clay, probably the Gault, which is absent from the Hunstanton section, was met with at the bottom of well borings at Carrow Works, Norwich, and Holkham, and it comes to the surface near Downham Market. The Gault is largely used near Cambridge for brick-making. At Folkestone, in the south of England, it has been divided into a number of zones, each characterised by a different species of Ammonite.

The Chalk is by far the most important formation present in Norfolk; it underlies the greater part of the county, and attains a thickness at Norwith of about 1200 feet. Closely resembling in appearance and composition the globigerina ooze of the bed of the Atlantic, it must have originated under similar conditions, and in a sea of considerable depth which then extended in an easterly direction across north-western Europe from Normandy to Denmark. Except in the west of the county, it is covered by newer beds, and is exposed only along the sides of river valleys, as in that of the Wensum. For the reason before given, the lowest beds of the Chalk do not come to the surface except in the west of Norfolk, the upper portion of the formation having there been planed off and removed by denudation. It is probable that the Chalk was originally as thick toward Hunstanton as it is at Norwich, and that it extended westward over England far beyond its present limits.

The oldest part of the Chalk, the well-known Red rock of Hunstanton, is a bed of limestone about 4 feet in thickness, stained by peroxide of iron. It has been regarded by some geologists as equivalent to the Gault, by others to the Upper Greensand; it represents an early stage of the subsidence by which the deep sea conditions of the Chalk period were introduced over the East of England. Higher in the Cretaceous series occurs the Hard Chalk without flint. From it have been obtained, near Stoke Ferry, the bones of Icthyosaurus, and some large Ammonites 2 feet in diameter. The Middle and Upper Chalk contain flint, derived from the decomposition of the siliceous organisms of the Chalk sea; flint occurs in the former in mid Norfolk, as at Wells, in the form of tabular sheets; and in the latter, further to the East, as nodules, in horizontal layers, and more rarely as paramoudras, large pear-shaped masses, 2 feet or more in length, arranged vertically one upon another. Among the characteristic fossils of the Upper Chalk, which at Norwich is 500 feet in thickness, may be mentioned Belemnitella mucronata, Terebratula carnea, Rhynchonella plicatilis, and Ananchytes ovatus. The entire skeleton of a large marine reptile, Mosasaurus, was found forty years ago at that place in a quarry near Bishops bridge.

No beds of Eocene age have been observed at the surface in Norfolk, but 51 feet of the Woolwich and Reading Beds, and 305 feet of the London Clay, were met with in a well-boring at Messrs. Laeon's brewery at Yarmouth (see section); the latter is a marine deposit of fine muddy sediment, brought down by a river which probably flowed from the west.

In Eocene times Great Britain enjoyed a warmer climate than it does at present, evidenced by the fact that fossil remains of palms, crocodiles, and turtles, have been found in the London Clay in the Isle of Sheppey. Up to and during the Eocene epoch, the physiography of the Euro- Asiatic continent differed widely from that of our own day, an ocean then extending from the Atlantic into Northern India, while such lofty mountain ranges as the Alps and the Himalayas, had not as yet come into existence.

Before the deposition of the Pliocene deposits of East Anglia, however, separated longo intervallo from those of the Eocene period, the distribution of land and water in Europe had to a great extent assumed its present form. The North Sea received then, as now, the drainage of the Rhine and its affluents, but it extended both to the east and the west somewhat beyond its present limits. The Crag beds of East Anglia (famed for the beauty and variety of their fossil mollusca), which in the form of an almost uninterrupted sheet of sand, always crowded with shells, reach from Essex to Norfolk, are the littoral deposits of the Pliocene sea, having been accumulated against the shore as beaches, or near to it, as banks or shoals in shallow water. Two great changes were taking place during this epoch - one climatic, the other tectonic. A considerable percentage of the species of mollusca found in the oldest Crag beds, some of them survivors from Miocene times, are not known living; as to the rest, the general character of the fauna is similar to that of the Mediterranean at the present day, the presumption being that the climate of the Eastern counties of England was somewhat warmer at that period than it now is. As the glacial period approached, colder conditions prevailed, the southern and the older shells gradually died out, while the North Sea was invaded by Scandinavian and arctic forms - at first in small, but afterwards in increasing numbers. Contemporaneously, an earth movement was in progress, by which the southern part of the Crag area was elevated, and the northern part depressed, the result being that, during the deposition of the Crag, the German Ocean gradually retreated towards the north. As these deposits were the marginal deposits of this sea, accumulating as it retired northwards, it follows that the oldest Crag beds, those containing a molluscan fauna of a southern character, are to be found in the southern part of the Crag area, and the newest, in which northern shells predominate, towards the north. The shelly sands of Norfolk belong, therefore, to the newest portion of the series, to the period when boreal conditions were establishing themselves in these latitudes.

The best known exposures of the Crag sands in Norfolk are at Bramerton, and at Thorpe, near Norwich. Among the more characteristic species of mollusca to be found at those places may be mentioned: -

Purpura lapillus; *Astarte compressa; Cerithium tricinctum; *Tellina iata; Littorina littorea; Tellina obliqua; Nucula Cobboldiae; Tellina prastenuis; Cardium edule; Mactra ovalis; *Astarte borealis; Mactra subtruncata.

Those marked * are now confined to northern seas, as are Scalaria groenlandica, Natlca clansa, N. Helicoides, Leda oblongoides, and Cardium groenlandicum, which occur, but less abundantly, in the Norwich Crag.

At Surlingham, 4 miles to the east of Norwich, and at Wroxham and elsewhere in the valley of the Bure, are some festuarine deposits, known as the Chillesford Clay. They always contain, generally abundantly, minute flakes of mica, which it is suggested were derived from micaceous rocks either in the Ardennes, or in the highlands further to the south. These estuarine beds represent a still further stage of the retreat of the German Ocean northwards, when the part of Norfolk previously covered by the sea had emerged, and was traversed by an estuary, which, if this view is correct, must have been one of the channels through which the Rhine or the Meuse reached the sea.

Of later date than the Chillesford Clay are some beds of Crag, occurring at Weybourn near Cromer, and at Belaugh, Wroxham, and elsewhere, a few miles north of Norwich, which contain very abundantly Tellina balthica, a shell still living on the Norfolk coast, but unknown from any older horizon of the Pliocene epoch. The sudden appearance of this mollusc in such extraordinary profusion at this stage points to an opening up of communication, by a continuance of the earth-movements before alluded to, between the North Sea and some other marine basin, possibly the Baltic, where it had previously established itself.

Resting on these beds, and exposed on the coast only, are some fresh water and estuarine deposits, generally known by the somewhat misleading name of the Forest-bed. It is from them that the larger part of the magnificent collection of mammalian remains in the Norwich Museum have been obtained. These fossils do not occur as complete skeletons, preserved on the site of an ancient forest, as was formerly supposed, but are the drifted and fragmentary remains of animals which were surprised and carried away in time of flood by a river, probably the Rhine, flowing from the south, being afterwards, on their way to the sea, stranded and buried in its flood gravel, or in the muddy sediment of its estuary. The most characteristic forms of the so-called Forest-bed are the extinct and gigantic pachyderms, Elephas meridlonalis and E. Antiquits, but the remains of many species of deer occur in it, with those of Machalrodus (the sabre-toothed tiger), hyena, cave bear, glutton, musk ox, elk, hippopotamus, and two species of rhinoceros.

These deposits are overlaid by strata originating during the Great Ice Age, when Scandinavia and a considerable part of Great Britain and of Northern Europe was covered' by ice, as Greenland is now. The accumulations of the Glacial period are better represented in East Anglia than in any other part of England. They have been studied by many observers, and have given rise to a voluminous literature and to much controversy. The arrangement given previously, and the views here expressed are, however, in the main, those proposed by the late Searles V. Wood, Jr., and the present writer (British Association Reports (1868), Palsontographical Soc. (1872), etc. Reference is suggested to the geological map accompanying the latter memoir, a copy of which is in the Norwich Free Library.).

The initiatory stage of the Glacial period is represented on the Cromer coast by sands containing Leda myalis, and other northern shells, and by a freshwater bed in which the leaves of Betula nana (the Arctic birch) and Salix polaris (the Arctic willow) are common, and in other parts of Norfolk by beds of pebbly gravel, (possibly equivalent to the Westleton Shingle of Prestwich) overlying which occur near Norwich, especially to the north and east of that city, deposits of sandy clay, largely used for brickmaking, and these pass further north into the Till and Contorted Drift of the Cromer coast. Enormous ice-borne masses of marly chalk may be seen there in the Contorted Drift, as it was well called by Lyell, associated with beds of clay, sand, and gravel, folded and disturbed by the action of ice. Towards the north-west, as the chalk country is approached from which these masses of altered chalk may probably have been derived, the Lower Glacial deposits assume a more marly character, as between Holt and Wells. In places, large boulders of igneous rock, sometimes scratched and striated by the action of ice, occur in the lower Glacial, some of them being, it is believed, of Scandinavian origin. It is thought that at this period an enormous glacier from the Baltic invaded the North Sea, and it may even have approached the coast of Norfolk.

To the south-west of an irregular line drawn from Burnham Market near Wells, to Lowestoft, the lower Glacial beds disappear more or less completely, their place being taken by the Great Chalky Boulder Clay, a stiff tenacious deposit, which forms the heavy wheat-growing clay land of the Eastern Counties. It stretches in almost unbroken mass over central Norfolk and Suffolk, from Fakenham to the south of Essex, and can be traced into Lincolnshire in one direction, and into Buckinghamshire in another, representing, it is believed, the moraine profonde of a large ice-stream coming from the N.N.W. The chalky boulder clay is composed of material derived from the destruction of Jurassic and Cretaceous rocks, but it invariably contains, in great abundance, flint and small fragments of chalk, which give it a character of its own, by which it can be always and easily recognised.

The Lower Glacial beds are overlaid in many places by sands (Middle Glacial), which to the north ofNorwich form heaths, as at Horsford and Cawston. These sands pass under the chalky boulder clay (Upper Glacial), and near Yarmouth contain fossil mollusca, some of them being southern species. It seems that during the Middle Glacial period communication, was re-opened between the German Ocean and seas to the south, and it is possible that the currents by which the southern shells were reintroduced into the former, may have melted back to some extent and, for a time, the northern ice. Overlying the chalky boulder clay, and generally occupying high ground, as at Strumpshaw Hill, six miles to the west of Norwich, and at Wymondham and elsewhere in central Norfolk, are some sporadic masses of coarse flood Plateau gravel, composed principally of flint, accumulated by currents during the melting of the East Anglican ice-sheet, when the excessive cold of the Glacial period was beginning to abate. These gravels often occur, as at Mousehold Heath near Norwich, at the very edge of the high ground overlooking the valleys, and could not have originated in their present position under existing circumstances. From the fact, however, that patches of chalky boulder clay may be occasionally observed at the bottom of the valley, as near Thorpe Asylum, it is clear that the latter must at that time have assumed, more or less, its present form. The probable explanation seems to be that the ice melted first on the higher ground, eventually shrinking into the valleys, and that the valley of the river Wensum at Norwich, for example, was filled to the brim with ice when the flood gravels of Mousehold Heath were deposited.