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.
[Illustration: Fig. 4.—Showing approximate Coast-line at the period
of the lowest Submerged Forest.]
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 from the nearest land. This shoal forms a wide
irregular plateau having an area nearly as big as Denmark. Over
it for the most part the sea has a depth of only 50 or 60 feet;
all round its edge it slopes somewhat abruptly into deeper water,
about 150 feet in the south, east, and west, but much deeper on
the north. This peculiar bank has been explained as an eastward
submerged continuation of the Oolite escarpment of Yorkshire; or,
alternatively, as a mere shoal accumulated through the effects of
some tidal eddy; but neither of these explanations will hold, for
Oolitic rocks do not occur there, and the bank has a core quite
unlike the sand of the North Sea.
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:—
Ursus (bear)
Canis lupus (wolf)
Hyaena spelaea (hyaena)
Cervus megaceros (Irish elk)
” tarandus (reindeer)
” elaphus (red-deer)
” a fourth species
Bos primigenius (wild ox)
Bison priscus (bison)
Equus caballus (horse)
Rhinoceros tichorhinus (woolly rhinoceros)
Elephas primigenius (mammoth)
Castor fiber (beaver)
Trichechus rosmarus (walrus)
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 even 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:—
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
Betula alba
” nana
Corylus Avellana
Salix repens
” aurita
Sparganium simplex
Alisma Plantago
Potamogeton natans
Ruppia rostellata
Scirpus sp.
Carex sp.
Phragmites communis
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 to 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.