THE IRISH SEA AND THE BRISTOL CHANNEL
On the west coast of Scotland, as on the east, the succession of
events seems to have been quite different from that which can be
proved further south. It looks as though we must seek for equivalents
of our submerged forests in certain very modern looking raised
beaches and estuarine deposits, such as those of the Clyde. Even when
we move southward to the Isle of Man deeply submerged post-glacial
land-surfaces appear to be unknown, though there is evidence of a
slight sinking, and roots of trees are found a few feet below the
sea-level. In the Isle of Man we still come across the modern-looking
raised beaches so prevalent in Scotland though unknown in England.
The Lancashire and Cheshire coasts, with their numerous deep
estuaries and extensive flats, are noted, however, for their
submerged forests, sometimes seen on the foreshore between
tide-marks, sometimes laid open in the extensive dock or harbour
works. The Heysham Harbour excavations, for instance, were carried
far below sea-level and a thin peat-bed was met with in a boring at
52 feet below Ordnance datum. Mellard Reade considered this peat
once to have been continuous with an ancient land-surface seen
between tide-marks. A boring is not altogether satisfactory evidence
for the occurrence of a land-surface at such a depth; but if it is
trustworthy it points to a subsidence of about 60 feet, an amount
identical with that observed in the Thames Valley.
The estuaries of the Ribble, Mersey, and Dee tell a similar story,
for on their shores and under their marshes are found some of the
most extensive submerged land-surfaces now traceable in Britain. Many
accounts of these have been published; but the alternations of marine
with freshwater strata and with land-surfaces are so like those
already described that a short account will suffice.
Carefully plotted engineer’s sections will be found in Mellard
Reade’s papers, and his account of the succession is so interesting
that it is worth quoting. He postulates two periods of elevation,
alternating with three periods of depression; but in this area, as
in the Thames Valley, it appears as though all the phenomena can be
accounted for by one long period of intermittent depression. His
generalised section of the deposits in these estuaries is as follows:—
3rd period { Blown sand
of depression {
{ Recent silts with beds of peat;
{ _Scrobicularia_, occasional
{ freshwater shells, red-deer, horse,
{ _Bos primigenius_, _Bos longifrons_,
{ and human skull
2nd period } Superior peat- and forest-bed
of elevation }
2nd period { Formby and Leasowe marine beds; human
of depression { skeleton, bones of horse and red-deer,
{ _Scrobicularia_, _Tellina baltica_,
{ _Turritella communis_, etc.
1st period } Inferior peat- and forest-bed
of elevation }
1st period { Washed drift-sand (apparently no
of depression { contemporaneous fossils)
{ Boulder clay
It may be an accidental coincidence; but it is noteworthy that both
the Mersey and Thames show two main peat-beds separated by marine
strata.
The forest exposed on the foreshore at Leasowe (frontispiece) is a
particularly good example of these old land-surfaces, and it is often
visible. It evidently once formed a wet, peaty flat on which grew
swamp plants, brushwood, and some large trees. Parts of it show a
perfect network of the rhizomes of _Osmunda_. This “superior peat-
and forest-bed” was forming when the sea was only a few feet below
its present level. The “inferior peat- and forest-bed” probably
indicates a drier soil; but it is difficult to get at and requires
fuller investigation.
The excavation for an extension of the Barry Docks, in
Glamorganshire, exposed in 1895 an interesting succession of
deposits, and fortunately a particularly competent observer, Dr
Strahan, was on the spot to note them and their exact levels. He
also obtained masses of material from each of the beds, and from an
examination of the contents of these I was able to gather a clear
idea of the changes of sea-level which had affected this part of
South Wales. The following sequence was met with:—
1. Blown sand. }
} Recent subaerial
2. _Scrobicularia_-clay. } and
1. Blown sand. } tidal deposits.
3. Sand and gravel with rolled }
shells (_Scrobicularia_, _Tellina_, }
_Cardium_, _Patella_, _Littorina_).}
Strong line of erosion.
4. Blue silt with many sedges, and at the bottom a few foraminifera.
5. _The Upper Peat Bed_, about four feet below Ordnance datum
and fairly constant in level. It ranges from one to two feet in
thickness, and where fully developed it presents the following
details:—
5 _a._ Laminated peat with logs of willow, fir and oak, passing
down into
5 _b._ Light-coloured flexible marl composed of ostracoda with much
vegetable matter.
5 _c._ Shell-marl composed principally of _Limnaea_, _Bythinia_,
etc., with ostracoda and much vegetable matter. This seam must have
been formed in a nearly freshwater tidal marsh; it yielded _Najas
marina_, a plant now confined to Norfolk.
5 _d._ Peat with logs of oak, etc. A Neolithic worked flint
was found by Mr Storrie in this seam, three inches below the
shell-marl. This implement is a fragment of a polished flint celt,
which seems to have been used subsequently as a strike-a-light. Two
bone needles are said to have been found in this peat-bed during
the construction of the first Barry Dock.
6. Blue silty clay with many sedges. From five to seven feet in
thickness.
7. _The Second Peat_ is an impersistent brown band, a few inches in
thickness, composed mainly of _Scirpus maritimus_. It suggests merely
that for the time plant-remains were accumulating more rapidly than
mud.
8. Blue silty clay, like Nos. 6 and 4. In its upper part, immediately
under the peat bed No. 7, it contains land and salt-marsh shells,
_Helix arbustorum_, _Pupa_, _Melampus myosotis_, _Hydrobia ventrosa_.
Upright stems of a sedge, probably _Scirpus maritimus_, occur
throughout this bed as through all the other silts.
9. _The Third Peat_ occurs at or close to the bottom of the dock,
at 20 feet below Ordnance datum. It rarely exceeds eight inches in
thickness, but is persistent. In several places it is made up almost
entirely of large timber, both trunks and stools of trees, while in
one section roots and rootlets extended downward from the peat into a
soil composed of disintegrated Keuper Marl. Mr Storrie identified oak
and roots of a conifer. On washing a sample collected at a few yards’
distance, I found it to consist of a tough mass of vegetable matter,
principally sallow and reed, both roots and stems. It also contained
seeds of _Valeriana officinalis_ and _Carex_, and elytra of beetles.
There was no evidence of salt water.
At this point it will be observed that the floor of Keuper Marl
rises, and Bed 9 abuts against it. Beds 10, 11 and 12 lay below
the dock bottom, and were exposed only in the excavation for the
foundations of walls, etc. Fortunately, Dr Strahan was able to
examine a good exposure of the important part of them.
10. The section commenced at the dock bottom—that is, at the peat
last described (No. 9); in the upper part it was timbered up, but at
a depth of about nine feet, blue silty clay of the usual character
could be seen and dug out through the timbers. This was followed
by two feet of greenish sandy silt full of reeds, and containing
leaves of willow, and land and freshwater shells, such as _Limnaea
auricularia_, _Planorbis albus_, _P. nautileus_, _Hydrobia ventrosa_,
_Valvata piscinalis_, _V. cristata_. The plants were _Salix caprea_
and _Phragmites_.
11. Peat with much broken oak-wood, mixed with seeds and freshwater
shells. The plants obtained were oak, hazel, cornel, hawthorn,
bur-reed and sallow.
12. Reddish clayey gravel with land shells and penetrated by roots,
passing down into red and green grits, limestone and marls. This
gravel is undoubtedly an old land-surface, lying at a depth of 35
feet below Ordnance datum. This old soil contains:—
Carychium minimum
Helix arbustorum
” rotundata
” hispida
Hyalinia
Succinea
Limnaea truncatula
Pupa
Valvata piscinalis
Cardium edule (two fragments—probably brought by gulls)
Crataegus Oxyacantha (seed)
Cornus sanguinea (seed)
Quercus Robur (wood)
The examination of these deposits made it perfectly clear that the
lowest land-surface represents a true forest-growth, such as could
only live at an elevation clear of the highest tides; one tide of
brackish water in the year would have sufficed to alter markedly the
character of the fauna and flora of the deposit. Dr Strahan, assuming
that the range of the tides was the same as at the present day, and
noting the present highest level to which the salt-marshes reach,
comes to the conclusion that 55 feet at least is the amount of the
subsidence. I should be inclined to add a few feet more, in order to
keep the oak-roots well clear of the highest tide during a westerly
gale. An exceptional gale occurring only once during the lifetime of
an oak might bank up the sea water sufficiently to kill the tree, if
it grew at a lower elevation.
It may be argued that when the land stood at the higher level the
range of the tides was less, and that consequently the amount of
the proved subsidence may not be so great as 55 feet. The old
land-surface on which the oaks grew lies, however, 35 feet below
_mean tide_, so that any supposed lesser tidal range in ancient times
could not make any great difference in the amount of subsidence here
proved—it cannot be less than 45 feet. When, however, we notice
the rapid increase in the range of the tides at the present day as
the channel narrows towards Chepstow, and think what would be the
probable effect of raising the whole country 50 or 60 feet, we are
compelled to think that any narrowing and shoaling of the channel
would have the effect of increasing, not decreasing, the tidal range
at Barry Docks. In short it looks as if when the lowest submerged
forest grew, the abnormal tides of Bristol may have extended further
west, to near Cardiff.
Whatever may have been the exact range of the tides in these early
days, it seems that the Bristol Channel points to a subsidence in
post-glacial times of about 60 feet—or just the same amount as the
Thames, Humber, and Mersey. The amount may have been more; but the
Barry Dock sections show that it cannot have been less; we will
return to the question of its maximum extent later on.
Before leaving this locality it may be well to enquire what further
light it sheds on the movement of submergence, and on its continuous
or intermittent character. The succession of the strata above the
lowest land-surface, and the nature of their enclosed fossils,
suggest long-continued but intermittent subsidence; I can see,
however, no indication of a reversal of the process. The land-surface
is carried beneath the water, the estuary then silts up, becomes
fresh water, marsh-plants grow, and even trees may flourish on this
marsh before it subsides again. But there is no sign that the strata
were ever raised above the level to which ordinary floods could build
up an alluvial flat. The land-surfaces seem always to have been
swampy, and bed succeeds bed in fairly regular sequence, without the
deep channelling we might expect to find when an alluvial flat was
raised to a noticeable extent above the level of high water.
The width of the Bristol Channel makes it clear that this gulf must
occupy a submerged valley of great antiquity. It becomes therefore of
interest to enquire whether the wide valley is correspondingly deep,
or whether its rocky floor is found at the same shallow depth as in
the case of the other river-valleys which we have been considering.
The wide valley may have been formed in either of two ways. It may
have been excavated as a deep valley with its bottom many hundred
feet below the present sea-level. Or it may have commenced as the
shallow valley of a big river with exceptionally powerful tides, and
as this river swung from side to side it greatly widened its valley
without making it any deeper.
Possibly a deep channel may exist towards the Atlantic; but we know
that none extends as far up as Bristol. Near Bristol the Severn
Tunnel was carried through Carboniferous and Triassic rock, and
showed that no buried channel is found much below the present one,
which here happens to be scoured by the tides to an exceptional
depth. The bottom of the old channel cannot be more than 40 feet
below the bottom of the present channel known as The Shoots.
It may be that the Severn was once prolonged seaward as a swift river
falling in a series of rapids over hard ledges of Palaeozoic rocks;
but of this there is no evidence. It also does not seem probable, for
all the geological indications go to suggest that west of Bristol
the Channel coincides in the main with a wide area once occupied by
comparatively soft Secondary or even Tertiary rocks. However this
may be, we can only trace an ancient post-glacial channel cutting to
about the same depth as the channels of the other rivers, and the
lowest submerged land-surface of Barry Docks corresponds quite well
with an alluvial flat formed when the river ran at that level. Here
again we seem to find the river cutting to an ancient base-level
which was about 60 feet below the present sea.
The reader may perhaps think that this point, the limited range of
the upward and downward movements in post-glacial times, is being
insisted on with wearisome iteration. But the insistence is necessary
when we remember how constantly both geologists and naturalists, in
order to account for anomalies in the geographical distribution of
animals and plants, bring into play such movements. The argument
is constantly used, that a certain species cannot cross the sea:
therefore if it is found in an island, that island must once have
been connected with the mainland. Nature is more full of resource
than we imagine, and does not thus neglect her children. The
cumulative effect of rare accidents spread over many thousand years
is also far greater than may be thought by those who only consider
what has been noted since means of dispersal have been studied
scientifically.
An examination of the south side of the Bristol Channel need not
long delay us, except for two pieces of evidence which should not be
passed over. In Somerset there are wide expanses of marsh land known
as the Bridgwater and Glastonbury Levels. These greatly resemble the
Fenland, and like it are underlain by a submerged rock-platform which
has sunk in post-glacial times. But in this case we are able to fix a
definite historical date by which all movement had ceased—it may have
ceased much earlier, but we can prove that at any rate there has been
no change of the sea-level subsequent to a certain date.
The Glastonbury Levels lie at about the height of ordinary high
tides, and the channels through them would still be tidal were it
not for the banks which keep out the sea. Some years ago there
were discovered on the surface of these marshes a number of low
mounds, which on excavation proved to be the remains of a village of
lake-dwellings, approached by a boat-channel, by the side of which
were the remains of a rough landing stage. The dwelling-places rested
on the old salt-marsh vegetation, brushwood and soil being used
to raise their floors above the level of the highest tides. It is
evident that when this village was inhabited the sea-level must have
been the same as now, or within a foot or two of its present height.
If the sea-level was then higher, the village could not be inhabited;
if it were lower the channel would not have been navigable and the
landing stage would have been useless. The archaeological remains
found in this village prove that it belongs to a period dating about
the first century B.C. or the first century A.D.
Another locality on the south side of the Bristol Channel which we
must not pass without notice is Westward Ho, in Bideford Bay. There
is nothing exceptional about the submerged forest at this place, but
it has been carefully studied and collected from by Mr Inkermann
Rogers, and it may be taken as a typical example of such deposits in
the south of England.
The peaty deposits and old land-surface here seen between tide-marks
are rapidly being destroyed by the sea and are now much thinner than
they were a few years since. The soil on which the trees, here mainly
oaks, are rooted consists of a blue clay full of small pebbles and
fragments of the Culm Measure grit. Among these stones are numerous
flint-flakes made by man; but metal implements and pottery, so
common in the later deposits at Glastonbury, have not been found.
This ancient land-surface lies several feet below high water; it
shows therefore that the latest movement of depression dates from a
period between this Neolithic deposit and the Celtic lake-dwelling of
Glastonbury.
The possibility of fixing an approximate date for this submerged
forest, through its numerous flint-flakes and the accompanying bones
of domesticated animals, makes its contents of great interest, for
it shows how recently the movement has ceased—probably not more than
3500 years ago. It will be worth while therefore to give a fuller
account of the contents of this soil and its overlying peat-bed.
As regards articles of human workmanship, I have seen nothing
but waste flakes of flint and perhaps flint knives; and though
good implements may at any time be discovered, neither chipped
nor polished tools seem yet to have been found. Human remains are
represented by a clavicle.
The accompanying mammals are the stag, Celtic shorthorn, horse, dog
(a very slender breed), sheep, goat, and pig, all of which, except
the stag, seem to be domestic animals. Dr Chas. Andrews remarks that
the ox seems to be certainly the Celtic shorthorn (_Bos longifrons_),
while the small sheep is a characteristic Romano-British form, which
has been described from many places, where it has been found with
Roman and earlier remains.
A number of seeds were obtained from the peat which rests on this
old land-surface, and it is noticeable that several of them belong
to brackish water or sea-coast plants. No cultivated species have
yet been found, either here or elsewhere, in even the newest of the
submerged forests. The list of plants is still a small one; but it
may be worth giving, to show what species can be identified. It must
not be forgotten that in such deposits plants which do not possess
either deciduous leaves or hard seeds leave no recognisable traces,
though they may have been quite as abundant as the hazel, of which
everyone notices the nuts. The seeds belong to:—
Ranunculus Flammula
” repens
” sceleratus
Viola
Malachium aquaticum
Stellaria media
Lychnis Flos-cuculi
Rubus fruticosus
Callitriche
Cornus sanguinea
Sambucus nigra
Aster Tripolium
Solanum Dulcamara
Ajuga reptans
Sueda maritima
Atriplex patula
Rumex
Urtica dioica
Alnus glutinosa
Corylus Avellana
Quercus robur
Alisma Plantago
Ruppia maritima
Eleocharis palustris
Scirpus Tabernaemontani
Carex 3 sp.
In this list, as is usually the case with the newest submerged
forest, we find only plants that are still living in the immediate
neighbourhood. Also, only such plants as are widely distributed are
here found as fossils, the characteristic west-country flora being
unrepresented. The reason of this limitation will be discussed later.
For various reasons, which will be explained later, it will be well
before describing the submerged land-surfaces of Cornwall and the
Atlantic coast, to complete the account of those surrounding our
enclosed seas. We will therefore take next those bordering on the
English Channel.