Submerged Forests/Chapter 4
CHAPTER IV
THE DOGGER BANK
For the last 50 years it has been known to geologists that the bed of the North Sea yields numerous bones of large land animals, belonging in great part to extinct species. These were first obtained by oyster-dredgers, and later by trawlers. Fortunately a good collection of them was secured by the British Museum, where it has been carefully studied by William Davies, The bones came from two localities. One of them, close to the Norfolk coast off Happisburgh, yielded mainly teeth of Elephas meridionalis, and its fossils were evidently derived from the Pliocene Cromer Forest bed, which in that neighbourhood is rapidly being destroyed by the sea. This need not now detain us.
The other locality is far more extraordinary. In the middle of the North Sea lies the extensive shoal known as the Dogger Bank, about 60 or 70 miles ,_054.png)
Fig. 4.—Showing approximate Coast-line at the period of the lowest Submerged Forest.
When trawlers first visited the Dogger Bank its surface seems to have been strewn with large bones of land animals and loose masses of peat, known to the fishermen as "moorlog," and there were also many erratic blocks in the neighbourhood. As all this refuse did much damage to the trawls, and bruised the fish, the erratics and bones were thrown into deeper water, and the large cakes of moorlog were broken in pieces. A few of the erratics and some of the bones were however brought to Yarmouth as curiosities. Now the whole surface of the Dogger Bank has been gone over again and again by the trawlers, and very few of the fossil bones are found; unfortunately no record seems to have been kept as to the exact place where these bones were trawled.
The species found were:—
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Ursus (bear) Canis lupus (wolf) Hyaena spelaea (hyaena) Cervus megaceros (Irish elk) „ tarandus (reindeer) „ elaphus (red-deer) „ a fourth species Bos primigenius (wild ox)
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Bison priscus (bison) Equus caballus (horse) Rhinoceros tichorhinus (woolly rhinoceros) Elephas primigenius (mammoth) Castor fiber (beaver) Trichechus rosmarus (walrus)
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Though mammalian bones are now so seldom found, whenever the sand-banks shift slightly, as they tend to do under the influence of tides and currents, the edges of the submerged plateau are laid bare, exposing submarine ledges of moorlog, which still yield a continuous supply of this material. Messrs. Whitehead and Goodchild have recently published an excellent account of it, having obtained from the trawlers numerous slabs of the peculiar peaty deposit, with particulars as to the latitude and longitude in which the specimens were dredged. Mrs Reid and I have to thank the authors for an opportunity of examining samples of the material, which has yielded most interesting evidence as to the physical history, botany, and climatic conditions of this sunken land. The following account is mainly taken from their paper and our appendix to it.
We are still without information as to the exact positions of the submarine ledges and cliffs of peat from which the masses have been torn; but there seems little doubt that some of them were actually torn off by the trawl. One block sent to me was full of recently dead half-grown Pholas parva, all of one age, and must evidently have been torn off the solid ledge. Pholas never makes its home in loose blocks. We unfortunately know very little about the natural history of the boring mollusca and their length of life. If, as I think, this species takes two years to reach full growth, then it is evident that the ledge of moorlog full of half-grown specimens must have been exposed to the sea continuously for one year, but not for longer. It ought also perhaps to tell us the depth of water from which the mass was torn; but nothing is known as to the depth to which Pholas may extend—it has the reputation of occurring between tide-marks or just below, but it may extend downwards wherever there is a submarine cliff.
Though we are still unable to locate exactly these submarine ledges or fix their depth below the sea, the blocks of moorlog are so widely distributed around the Dogger Bank, and have been dredged in such large masses, that it seems clear that a "submerged forest" forms part of the core of the bank. As nothing else approaching to a solid stratum appears to be dredged over this shoal, we may assume that the moorlog forms a sort of cap or cornice at a depth of about 10 fathoms, overlying loose sandy strata, and perhaps boulder clay, which extend downward to another 10 fathoms, or 120 feet altogether. Unfortunately we cannot say from what deposit the large bones of extinct animals were washed; they may come from the sands below the moorlog, but it is quite as probable that the Pleistocene deposits formed islands in the ancient fen—as they do now in East Anglia, Holderness, and Holland.
More than one submerged forest may be present on the Dogger Bank. The masses of moorlog are usually dredged on the slopes at a depth of 22 or 23 fathoms; but at the south-west end it occurs on the top as well as on the slope, the sea-bottom on which the moorlog is found consisting of fine grey sand, probably an estuarine silt connected with the submerged forest, for the North Sea sand is commonly coarse and gritty.
With regard to the moorlog itself and its contents, it is possible that some of the mammals in the list, such as the reindeer, beaver, and walrus, may belong to this upper deposit; but we have no means of distinguishing them, as the bones were all found loose and free from the matrix. The insects and plants were all obtained from slabs of this peat.
The dredged cakes of peat handed to us for examination came from different parts of the Bank; but they were all very similar in character, and showed only the slight differences found in different parts of the same fen. The bed is essentially a fen-deposit of purely organic origin, with little trace of inorganic mud. It is fissile and very hard when dry, and in it are scattered a certain number of fairly well-preserved seeds, principally belonging to the bog-bean. Other recognisable plant-remains are not abundant. They consist of rare willow-leaves, fragments of birch-wood and bark, pieces of the scalariform tissue and sporangia of a fern, and moss, and, curiously enough, of groups of stamens of willow-herb with well-preserved pollen-grains, though the whole of the rest of the plant to which they belonged had decayed.
The material is exceptionally tough, and is very difficult to disintegrate. In order to remove the structureless humus which composed the greater part of the peat, we found it necessary to break it into thin flakes and boil it in a strong soda solution for three or four days. Afterwards the material was passed through a sieve, the fine flocculent parts being washed away by a stream of water, the undecomposed plant remains being left behind in a state for examination. These remains were mixed with a large amount of shreds of cuticle, etc., but recognisable leaves were not found in the washed material.
The general result of our examination is to suggest that the deposit comes from the middle of some vast fen, so far from rising land that all terrigenous material has been strained out of the peaty water. The vegetation, as far as we have yet seen, consists exclusively of swamp species, with no admixture of hard-seeded edible fruits, usually so widely distributed by birds, and no wind-borne composites. The sea was probably some distance away, as there is little sign of brackish-water plants, or eveu of plants which usually occur within reach of an occasional tide; one piece however yielded seeds of Ruppia, The climate to which the plants point may be described as northern. The white-birch, sallow and hazel were the only trees; the alder is absent. All the plants have a high northern range, and one, the dwarf Arctic-birch, is never found at sea-level in latitudes as far south as the Dogger Bank (except very rarely in the Baltic provinces of Germany).
The plants already found are:—
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Ranunculus Lingua Castalia alba Cochlearia sp. Lychnis Flos-cuculi Arenaria trinervia Spiraea Ulmaria Rubus fruticosus Epilobium sp. Galium sp. Valeriana officinalis Menyanthes trifoliata Lycopus europaeus Atriplex patula
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Betula alba „ nana Corylus Avellana Salix repens „ aurita Sparganium simplex Alisma Plantago Potamogeton natans Ruppia rostellata Scirpus sp. Carex sp. Phragmites communis
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Among the nine species of beetle determined by Mr G. C. Champion it is noticeable that two belong to sandy places. This suggests that the fen may have had its seaward edge protected by a belt of sand-dunes, just as the coast of Holland is at the present day.
This submerged forest in the middle of the North Sea has been described fully, for it raises a host of interesting questions, that require much more research before we can answer them. A sunken land-surface 60 feet and more below the sea at high-tide corresponds very closely with the lowest of the submerged forests met with in our dock-excavations. But if another bed of peat occurs at a depth of 130 or 140 feet at the Dogger Bank, this would be far below the level of any recently sunk land-surface yet recognised in Britain. Also, if the slabs of very modern-looking peat, containing only plants and insects still living in Britain, come from such a depth, out of what older deposit can the Pleistocene mammals, such as elephant, rhinoceros, and hyaena, have been washed?
These questions cannot be answered conclusively without scientific dredging, to fix the exact positions and depths of the outcrops of moorlog. When we remember also that beneath a submerged forest at about the depth of the Dogger Bank there was found at Tilbury, in the Thames Valley, a human skeleton; and that both human remains and stone implements have been discovered in similar deposits elsewhere, we can point to the Dogger Bank as an excellent field for scientific exploration.
The Dogger Bank once formed the northern edge of a great alluvial plain, occupying what is now the southern half of the North Sea, and stretching across to Holland and Denmark. If we go beyond the Dogger Bank and seek for answers to these questions on the further shore, we find moorlog washed up abundantly on the coasts of both Holland and Denmark, and it has evidently been torn off submerged ledges like those of the Bank. Numerous borings in Holland give us still further information, for they show that beneath the wide alluvial plain, which lies close to the level of the sea, there exists a considerable thickness of modern strata. At Amsterdam, for instance, two beds of peat are met with well below the sea-level, the upper occurring at about the level of low-tide, the lower at a depth of about 50 or 60 feet below mean-tide. That is to say, the lowest submerged land-surface is found in Holland at just about the same depth as it occurs in England, and probably on the Dogger Bank also.
Below this submerged land-surface at Amsterdam are found marine clays and sands, which seem tp show that the lowest "continental deposit," as it is called by Dutch geologists, spread seaward over the silted-up bed of the North Sea; but no buried land- surfaces have yet been found below the 60-foot level anywhere in Holland.
This appearance of two distinct and thick peat-beds, underlain, separated, and overlaid by marine deposits, seems to characterise great part of the Dutch plain. It points to a long period of subsidence, broken by two intervals of stationary sea-level, when peat-mosses flourished and spread far and wide over the flat, interspersed with shallow lakes, like the Norfolk broads.
The enclosed and almost tideless Baltic apparently tells the same story, for at Rostock at its southern end, a submerged peat-bed has been met with at a depth of 46 feet.
On passing northward into Scandinavia we enter an area in which, as in Scotland, recent changes in sea-level have been complicated by tilting, so that ancient beach-lines no longer correspond in elevation at different places. The deformation has been so great that it is impossible to trace the submerged forests; they may be represented in the north by the raised beaches, which in Norway and Sweden, as in Scotland and the north of Ireland, seem to belong to a far more recent period than the raised beaches of the south of England. It seems useless to attempt to continue our researches on submerged forests further in this direction, especially as during the latest stages, when we know England was sinking, Gothland appears to have been slowly rising. Those who wish to learn about the changes that took place in the south of Sweden should refer to the recent monograph by Dr Munthe.