The Antiquity of Man Part 20

TABLE 12/2. DISTRIBUTION OF THE ABOVE MARINE TESTACEA.

COLUMN 1: NAME.

COLUMN 2: NUMBER.

Norwich Crag: 81.

Red Crag: 225.

Coralline Crag: 327.

Species common to the Norwich and Red Crag (not in Coralline): 33.

Species common to the Norwich and Coralline (not in Red): 4.

Species common to the Red and Coralline (not in Norwich): 116.

Species common to the Norwich, Red, and Coralline: 19.*

(* These 19 species must be added to the numbers 33, 4, and 116 respectively, in order to obtain the full amount of common species in each of those cases.)

TABLE 12/3. PROPORTION OF RECENT TO EXTINCT SPECIES.

COLUMN 1: NAME.

COLUMN 2: NUMBER OF RECENT.

COLUMN 3: NUMBER OF EXTINCT.

COLUMN 4: PERCENTAGE OF RECENT.

Norwich Crag: 69: 12: 85%.

Red Crag: 130: 95: 57%.

Coralline Crag: 168: 159: 51%.

TABLE 12/4. RECENT SPECIES NOT LIVING NOW IN BRITISH SEAS.

COLUMN 1: NAME.

COLUMN 2: NUMBER OF NORTHERN.

COLUMN 3: NUMBER OF SOUTHERN.

Norwich Crag: 12: 0.

Red Crag: 8: 16.

Coralline Crag: 2: 27.

In the above list I have not included the sh.e.l.ls of the glacial beds of the Clyde and of several other British deposits of newer origin than the Norwich Crag, in which nearly all--perhaps all--the species are Recent.

The land and freshwater sh.e.l.ls, thirty-two in number, have also been purposely omitted, as well as three species of London Clay sh.e.l.ls, suspected by Mr. Wood himself to be spurious.

By far the greater number of the living marine species included in these tables are still inhabitants of the British seas; but even these differ considerably in their relative abundance, some of the commonest of the Crag sh.e.l.ls being now extremely scarce; as, for example, Buccinopsis Dalei; and others, rarely met with in a fossil state, being now very common, as Murex erinaceus and Cardium echinatum.

The last table throws light on a marked alteration in the climate of the three successive periods. It will be seen that in the Coralline Crag there are twenty-seven southern sh.e.l.ls, including twenty-six Mediterranean, and one West Indian species (Erato Maugeriae). Of these only thirteen occur in the Red Crag, a.s.sociated with three new southern species, while the whole of them disappear from the Norwich beds. On the other hand, the Coralline Crag contains only two sh.e.l.ls closely related to arctic forms of the genera Admete and Limopsis. The Red Crag contains, as stated in the table, eight northern species, all of which recur in the Norwich Crag, with the addition of four others, also inhabitants of the arctic regions; so that there is good evidence of a continual refrigeration of climate during the Pliocene period in Britain. The presence of these northern sh.e.l.ls cannot be explained away by supposing that they were inhabitants of the deep parts of the sea; for some of them, such as Tellina calcarea and Astarte borealis, occur plentifully, and sometimes, with the valves united by their ligament, in company with other littoral sh.e.l.ls, such as Mya arenaria and Littorina rudis, and evidently not thrown up from deep water. Yet the northern character of the Norwich Crag is not fully shown by simply saying that it contains twelve northern species. It is the predominance of certain genera and species, such as Tellina calcarea, Astarte borealis, Scalaria groenlandica, and Fusus carinatus, which satisfies the mind of a conchologist as to the arctic character of the Norwich Crag. In like manner, it is the presence of such genera as Pyrula, Columbella, Terebra, Ca.s.sidaria, Pholadomya, Lingula, Discina, and others which give a southern aspect to the Coralline Crag sh.e.l.ls.

The cold, which had gone on increasing from the time of the Coralline to that of the Norwich Crag, continued, though not perhaps without some oscillations of temperature, to become more and more severe after the acc.u.mulation of the Norwich Crag, until it reached its maximum in what has been called the glacial epoch. The marine fauna of this last period contains, both in Ireland and Scotland, Recent species of mollusca now living in Greenland and other seas far north of the areas where we find their remains in a fossil state.

The refrigeration of climate from the time of the older to that of the newer Pliocene strata is not now announced for the first time, as it was inferred from a study of the Crag sh.e.l.ls in 1846 by the late Edward Forbes.*

(* "Memoirs of the Geological Survey" London 1846 page 391.)

The most southern point to which the marine beds of the Norwich Crag have yet been traced is at Chillesford, near Woodbridge, in Suffolk, about 80 miles north-east of London, where, as Messrs. Prestwich and Searles Wood have pointed out,* they exhibit decided marks of having been deposited in a sea of a much lower temperature than that now prevailing in the same lat.i.tude. (* "Quarterly Journal of the Geological Society" volume 5 1849 page 345.) Out of twenty-three sh.e.l.ls obtained in that locality from argillaceous strata 20 feet thick, two only, namely, Nucula Cobboldiae and Tellina obliqua, are extinct, and not a few of the other species, such as Leda lanceolata, Cardium groenlandic.u.m, Lucina borealis, Cyprina islandica, Panopaea norvegica, and Mya truncata, betray a northern, and some of them an arctic character.

These Chillesford beds are supposed to be somewhat more modern than any of the purely marine strata of the Norwich Crag exhibited by the sections of the Norfolk cliffs north-west of Cromer, which I am about to describe. Yet they probably preceded in date the "Forest Bed" and fluvio-marine deposits of those same cliffs. They are, therefore, of no small importance in reference to the chronology of the glacial period, since they afford evidence of an a.s.semblage of fossil sh.e.l.ls with a proportion of between eight and nine in a hundred of extinct species occurring so far south as lat.i.tude 53 degrees north, and indicating so cold a climate as to imply that the glacial period commenced before the close of the Pliocene era.

[Ill.u.s.tration: Figure 27. Succession of Strata]

(FIGURE 27. DIAGRAM TO ILl.u.s.tRATE THE GENERAL SUCCESSION OF THE STRATA IN THE NORFOLK CLIFFS, EXTENDING SEVERAL MILES NORTH-WEST AND SOUTH-EAST OF CROMER.

A. Site of Cromer Jetty.

1. Upper Chalk with flints in regular stratification.

2. Norwich Crag, rising from low water at Cromer to the top of the cliffs at Weybourn, seven miles distant.

3. "Forest Bed," with stumps of trees in situ and remains of Elephas meridionalis, E. primigenius, E. antiquus, Rhinoceros etruscus, etc. This bed increases in depth and thickness eastward. No Crag (Number 2) known east of Cromer Jetty.

3 prime. Fluvio-marine series. At Cromer and eastward, with abundant lignite beds and mammalian remains, and with cones of the Scotch and spruce firs and wood. At Runton, north-west of Cromer, expanding into a thick freshwater deposit, with overlying marine strata, elsewhere consisting of alternating sands and clays, tranquilly deposited, some with marine, others with freshwater sh.e.l.ls.

4. Boulder clay of glacial period, with far transported erratics, some of them polished and scratched, 20 to 80 feet in thickness.

5. Contorted drift.

6. Superficial gravel and sand with covering of vegetable soil.)

The annexed section (Figure 27) will give a general idea of the ordinary succession of the Pliocene and Pleistocene strata which rest upon the Chalk in the Norfolk and Suffolk cliffs. These cliffs vary in height from fifty to above three hundred feet. At the north-western extremity of the section at Weybourn (beyond the limits of the annexed diagram), and from thence to Cromer, a distance of 7 miles, the Norwich Crag, a marine deposit, reposes immediately upon the Chalk. A vast majority of its sh.e.l.ls are of living species such as Cardium edule, Cyprina islandica, Scalaria groenlandica, and Fusus antiquus, and some few extinct, as Tellina obliqua, and Nucula Cobboldiae. At Cromer jetty this formation thins out, as expressed in the diagram at A; and to the south we find Number 3, or what is commonly called the "Forest Bed," reposing immediately upon the Chalk, and occupying, as it were, the place previously held by the marine Crag Number 2. This buried forest has been traced for more than 40 miles, being exposed at certain seasons and states of the beach between high and low water mark. It extends from Cromer to near Kessingland, and consists of the stumps of numerous trees standing erect, with their roots attached to them, and penetrating in all directions into the loam or ancient vegetable soil on which they grew. They mark the site of a forest which existed there for a long time, since, besides the erect trunks of trees, some of them 2 and 3 feet in diameter, there is a vast acc.u.mulation of vegetable matter in the immediately overlying clays. Thirty years ago, when I first examined this bed, I saw many trees, with their roots in the old soil, laid open at the base of the cliff near Happisburgh; and long before my visit, other observers, and among them the late Mr. J.C. Taylor, had noticed the buried forest. Of late years it has been repeatedly seen at many points by Mr. Gunn, and, after the great storms of the autumn of 1861, by Mr. King. In order to expose the stumps to view, a vast body of sand and shingle must be cleared away by the force of the waves. [21]

As the sea is always gaining on the land, new sets of trees are brought to light from time to time, so that the breadth as well as length of the area of ancient forest land seems to have been considerable. Next above Number 3, we find a series of sands and clays with lignite (Number 3 prime), sometimes 10 feet thick, and containing alternations of fluviatile and marine strata, implying that the old forest land, which may at first have been considerably elevated above the level of the sea, had sunk down so as to be occasionally overflowed by a river, and at other times by the salt waters of an estuary. There were probably several oscillations of level which a.s.sisted in bringing about these changes, during which trees were often uprooted and laid prostrate, giving rise to layers of lignite. Occasionally marshes were formed and peaty matter acc.u.mulated, after which salt water again predominated, so that species of Mytilus, Mya, Leda, and other marine genera, lived in the same area where the Unio, Cyclas, and Paludina had flourished for a time. That the marine sh.e.l.ls lived and died on the spot, and were not thrown up by the waves during a storm, is proved, as Mr. King has remarked, by the fact that at West Runton, north-west of Cromer, the Mya truncata and Leda myalis are found with both valves united and erect in the loam, all with their posterior or siphuncular extremities uppermost.

This att.i.tude affords as good evidence to the conchologist that those mollusca lived and died on the spot as the upright position of the trees proves to the botanist that there was a forest over the Chalk east of Cromer.

Between the stumps of the buried forest, and in the lignite above them, are many well-preserved cones of the Scotch and spruce firs, Pinus sylvestris, and Pinus abies. The specific names of these fossils were determined for me in 1840, by a botanist of no less authority than the late Robert Brown; and Professor Heer has lately examined a large collection from the same stratum, and recognised among the cones of the spruce some which had only the central part or axis remaining, the rest having been bitten off, precisely in the same manner as when in our woods the squirrel has been feeding on the seeds. There is also in the forest-bed a great quant.i.ty of resin in lumps, resembling that gathered for use, according to Professor Heer, in Switzerland, from beneath spruce firs.

The following is a list of some of the plants and seeds which were collected by the Reverend S.W. King, in 1861, from the forest bed at Happisburgh, and named by Professor Heer:--

PLANTS AND SEEDS OF THE FOREST AND LIGNITE BEDS BELOW THE GLACIAL DRIFT OF THE NORFOLK CLIFFS.

Pinus sylvestris, Scotch fir.

Pinus abies, spruce fir.

Taxus baccata, yew.

Nuphar luteum, yellow water-lily.

Ceratophyllum demersum, hornwort.

Potamogeton, pondweed.

Prunus spinosus, common sloe.

Menyanthes trifoliata, buckbean.

Nymphaea alba, white water-lily.

Alnus, alder.

Quercus, oak.

Betula, birch.