Chillesford and Aldeby Beds.—It is in the counties of Norfolk, Suffolk, and Essex, that we obtain our most valuable information respecting the British Pliocene strata, whether newer or older. They have obtained in those counties the provincial name of “Crag,” applied particularly to masses of shelly sand which have long been used in agriculture to fertilise soils deficient in calcareous matter. At Chillesford, between Woodbridge and Aldborough in Suffolk, and Aldeby, near Beccles, in the same county, there occur stratified deposits, apparently older than any of the preceding drifts of Yorkshire, Norfolk, and Suffolk. They are composed at Chillesford of yellow sands and clays, with much mica, forming horizontal beds about twenty feet thick. Messrs. Prestwich and Searles Wood, senior, who first described these beds, point out that the shells indicate on the whole a colder climate than the Red Crag; two-thirds of them being characteristic of high latitudes. Among these are Cardium Grœnlandicum, Leda limatula, Tritonium carinatum, and Scalaria Grœnlandica. In the upper part of the laminated clays a skeleton of a whale was found associated with casts of the characteristic shells, Nucula Cobboldiæ and Tellina obliqua, already referred to as no longer inhabiting our seas, and as being extinct varieties if not species. The same shells occur in a perfect state in the lower part of the formation. Natica helicoides (Fig. 117) is an example of a species formerly known only as fossil, but which has now been found living in our seas.
At Aldeby, where beds occur decidedly similar in mineral character as well as fossil remains, Messrs. Crowfoot and Dowson have now obtained sixty-six species of mollusca, comprising the Chillesford species and some others. Of these about nine-tenths are recent. They are in a perfect state, clearly indicating a cold climate; as two-thirds of them are now met with in arctic regions. As a rule, the lamellibranchiate molluscs have both valves united, and many of them, such as Mya arenaria, stand with the siphonal end upward, as when in a living state. Tellina balthica, before mentioned (Fig. 116) as so characteristic of the glacial beds, including the drift of Bridlington, has not yet been found in deposits of Chillesford and Aldeby age, whether at Sudbourn, East Bavent, Horstead, Coltishall, Burgh, or in the highest beds overlying the Norwich Crag proper at Bramerton and Thorpe.
Norwich or Fluvio-marine Crag.—The beds above alluded to ought, perhaps, to be regarded as beds of passage between the glacial formations and those called from a provincial name “Crag,” the newest member of which has been commonly called the “Norwich Crag.” It is chiefly seen in the neighbourhood of Norwich, and consists of beds of incoherent sand, loam, and gravel, which are exposed to view on both banks of the Yare, as at Bramerton and Thorpe. As they contain a mixture of marine, land, and fresh-water shells, with bones of fish and mammalia, it is clear that these beds have been accumulated at the bottom of a sea near the mouth of a river. They form patches rarely exceeding twenty feet in thickness, resting on white chalk. At their junction with the chalk there invariably intervenes a bed called the “Stone-bed,” composed of unrolled chalk-flints, commonly of large size, mingled with the remains of a land fauna comprising Mastodon arvernensis, Elephas meridionalis, and an extinct species of deer. The mastodon, which is a species characteristic of the Pliocene strata of Italy and France, is the most abundant fossil, and one not found in the Cromer forest before mentioned. When these flints, probably long exposed in the atmosphere, became submerged, they were covered with barnacles, and the surface of the chalk became perforated by the Pholas crispata, each fossil shell still remaining at the bottom of its cylindrical cavity, now filled up with loose sand from the incumbent crag. This species of Pholas still exists, and drills the rocks between high and low water on the British coast. The name of “Fluvio-marine” has often been given to this formation, as no less than twenty species of land and fresh-water shells have been found in it. They are all of living species; at least only one univalve, Paludina lenta, has any, and that a very doubtful, claim to be regarded as extinct.
Of the marine shells, 124 in number, about 18 per cent are extinct, according to the latest estimate given me by Mr. Searles Wood; but, for reasons presently to be mentioned, this percentage must be only regarded as provisional. It must also be borne in mind that the proportion of recent shells would be augmented if the uppermost beds at Bramerton, near Norwich, which belong to the most modern or Chillesford division of the Crag, had been included, as they were formerly, by Mr. Woodward and myself, in the Norwich series. Arctic shells, which formed so large a proportion in the Chillesford and Aldeby beds, are more rare in the Norwich Crag, though many northern species—such as Rhynchonella psittacea, Scalaria Grœnlandica, Astarte borealis, Panopæa Norvegia, and others—still occur. The Nucula Cobboldiæ and Tellina obliqua, Figs. 119 and 120, before mentioned, p. 194, are frequent in these beds, as are also Littorina littorea, Cardium edule, and Turritella communis, of our seas, proving the littoral origin of the beds.
OLDER PLIOCENE STRATA.
Red Crag.—Among the English Pliocene beds the next in antiquity is the Red Crag, which often rests immediately on the London Clay, as in the county of Essex, illustrated in Fig. 121.
