THE ENGLISH CHANNEL
The English Channel, like our other enclosed seas, is bordered on
either side by a fringe of ancient alluvia and submerged forests,
which however are fast disappearing through the attacks of the waves.
The destruction is so rapid, and in many parts has been so complete,
that we are apt to forget how altered is the appearance of the
English coast. Even so recently as the time of Caesar’s invasion flat
muddy shores or low gravelly plains occupied many parts of the coast
where we now see cliffs and rocky ledges.
We will not labour this point, which must be obvious to anyone who
has noticed how little the low terrace which still fringes great part
of the Sussex coast can resist the waves, and how quickly it is eaten
away during storms. Any restoration of our coast-line for the time of
Caesar must take these changes into account.
The material thus being removed by the attacks of the sea is partly
Pleistocene gravel, partly alluvium of later date; and the alluvial
strata with their accompanying buried land-surfaces resemble so
closely those already described that we need not linger long over
their description.
If we commence at the Strait of Dover we are immediately confronted
with clear evidence of the change of sea-level. Submerged forests are
well seen between tide-marks in Pegwell Bay, and valleys with their
seaward ends submerged and forming harbours are conspicuous in Kent.
Owing to local conditions, the valleys are mostly narrow and steep,
and the small harbours therefore soon filled up, or were lost through
the cutting back of the cliffs on either side. Possibly in Caesar’s
day good natural harbours were still in existence here.
Unfortunately on this part of the coast the study of coastal changes
has been involved in a good deal of needless obscurity. Many
writers, even geologists, make no clear distinction between loss by
submergence and loss by marine erosion. We are told, for instance,
that the Goodwin Sands were land about 900 years ago, and that this
land disappeared during an exceptional storm. We are sometimes even
told that here and elsewhere walls are still visible beneath the
sea. Popular writers, to add to the confusion, have some hazy notion
that these changes are connected with the existence of submerged
forests or “Noah’s Woods,” and that these again are evidence of a
universal deluge. The whole of the arguments are strangely tangled,
and we must try and make things a little clearer before passing on.
An understanding of the changes which have taken place on this part
of the coast is needed for historical purposes, and still more needed
if we make a study of the origin of the existing fauna and flora of
Britain.
One of the crucial questions, both for the naturalist and
archaeologist, is the date at which Britain was finally severed from
the Continent. Did this happen within the range of written history,
or tradition? Or if earlier, did it take place after or before
climatic conditions had become such as we now experience? For the
proper understanding of many different problems it is essential to
settle this point.
It is scarcely satisfactory to read history backwards, though
geologists are often compelled thus to work from the known to the
unknown. We will therefore not in this case ask our readers to follow
us through the detailed evidence and arguments which have enabled
geologists stage by stage to reconstruct the physical geography
of this part of Britain as it was in days before written history.
They must take this preliminary work for granted, and allow the
description of the changes to be taken in their correct historical
order.
We need not go back far geologically. In late Tertiary (probably
Newer Pliocene) times there was a ridge of chalk joining the range of
the North Downs to the corresponding hills of France; but the divide
between the North Sea and the English Channel was low at this point.
Afterwards, during the Glacial Epoch, when an ice-sheet accumulated
and blocked the northern outlet of the North Sea, the water was
ponded back in the southern part. There was no easy outlet northward
for the water of the Rhine and other great rivers, so the level of
the North Sea rose slightly till it overflowed this low col and cut
an outlet where lies the present Strait of Dover.
The general sea-level during this period of glaciation seems to have
been a few feet higher than that of the present day, for glacially
transported erratics are found strewn over the flat coastal plain of
Sussex. One erratic block, probably derived from the Channel Islands,
was discovered under the loess as far east as Sangatte cliff, close
to Calais. The icy English Channel must therefore have met the icy
North Sea some time during the Glacial Epoch.
Some time after the cold had passed away there came in the period
with which this book deals—when the lowest submerged forest
flourished, on land now 50 or 60 feet below the sea. This elevation
of the land, as already shown, converted a great part of the North
Sea into a wide alluvial plain. At the same time it raised above the
sea-level and obliterated the newly-formed strait, leaving it in all
probability as a shallow valley sloping both ways and filled up with
alluvium. The Strait of Dover was again a watershed, or perhaps its
position was occupied by a small stream, which may have flowed in
either direction.
Thus the work done during the Glacial Epoch was almost cancelled and
had to be done again; but now there was merely a low narrow divide
of chalk and a strip of marsh between the two basins, and the chalk
ridge was steadily being attacked by the waves of the sea from the
west.
When subsidence again set in the strip of alluvium was soon submerged
and the two seas again met; but in all probability for a long time
the Strait was only a narrow one, over which animals could easily
swim. Then tidal scour, deeper submergence, and the action of the
waves did the rest, so that ever since that time the Strait of Dover
has been getting steadily wider and wider, and also deeper. Its
bottom is to a large extent composed of bare chalk with patches of
gravel; and the movement of this gravel during storms, combined with
the action of boring molluscs must slowly eat away the chalk far
below ordinary wave-action.
The above explanation is needed, for it will not do to take existing
soundings, and say that all the sea-bottom below a certain level,
corresponding with a particular submerged forest, was then sea and
all above was then land. This is an easy way of reconstructing the
physical geography; but it may be a very misleading one. A little
consideration will show that whilst in large areas sand-banks have
accumulated to a great thickness, in other areas, of which we know
the Strait of Dover is one and the Dogger Bank a second, there has
been much submarine erosion, which is still going on. In neither case
is it safe entirely to reconstruct the ancient contours from the
present-day soundings.
Even such a gigantic feature as the continental platform, which
ceases suddenly at a depth of 100 fathoms, is in all probability in
the main a feature formed by the deposition of sediment during long
ages. Its outer edge marks, not the limit of some ancient continent,
but the limiting depth at which gentle wave-action has been felt, and
beyond which the sediment cannot be carried.
After this necessary digression we must return to our study of the
actual evidence for such changes of sea-level in the English Channel.
It has been pointed out already that for this purpose the present
depth below sea-level of the rocky floor of the Strait cannot in
itself be accepted as sufficient evidence. Nor can the depth at which
rock was met with under the Goodwin Sands; though here a cylinder was
sunk 75 feet before it reached the chalk. Unfortunately no record of
the strata passed through seems to have been preserved, though it is
perhaps implied that nothing but sea-sand was penetrated.
Romney Marsh is a wide alluvial flat occupying a silted-up bay, the
floor of which in places lies at least 70 feet below sea-level. There
are here unfortunately no extensive excavations for docks, and all we
can say is that the few borings which have penetrated the alluvial
strata prove the existence of a slightly undulating rock-surface
below. In short Romney Marsh appears to be a submerged flat-bottomed
open valley, like that which we have already seen underlies the marsh
deposits of the Fenland.
In the case of Romney Marsh, however, it is doubtful whether
submerged land-surfaces would be found at any great distance from the
rising ground. There is a striking peculiarity about this marsh; it
only lies partly in a bay, the greater part of the area consisting of
alluvial flats which have accumulated during recent centuries behind
the projecting shingle beaches of Dunge Ness. In short, the marsh
steadily gains on the sea, is advancing into fairly deep water, and
the parts near the Ness may be underlain by marine strata right down
to the Wealden rocks below. The rock floor was met with at 58 feet
below the marsh at Holmston Range, not far from the Ness; but we
have no information as to the character of the strata passed through
before this floor was reached. In all probability this floor at 58
feet would be proved to be part of a true land-surface, could we
examine it.
Near Hastings the submerged forests have long been known, and are
often exposed on the foreshore between tide-marks. They contain
antlers of deer, leaves, hazel nuts, acorns, and oak wood.
Then we come to Pevensey Level, which is another of the submerged and
silted up wide flat-bottomed valleys, such as we have so often met
with. But as we have no details as to strata underlying this marsh we
must pass on.
Along the Sussex coast west of Beachy Head a series of south-flowing
rivers reaches the sea, each cutting through the high chalk-hills
of the South Downs. We need not discuss the origin of these peculiar
courses, which date back to the period when the central axis of the
Weald was uplifted; that discussion would take too much time, and is
here unnecessary. We are now only concerned with the later stages of
the evolution of these river-valleys, each of which yields striking
confirmation of the view that a sinking of the land has taken place
in comparatively modern times.
At the present day the tidal part of each of these rivers extends
right through the Downs into the lower Wealden area, and it is
obvious that their valleys tend to silt up, not to deepen, and
scarcely anywhere to become wider. When we examine further we find
that the true valley-bottom lies far below the present alluvial flat;
though the scarcity of borings and the uncertainty of many of the
records make it difficult to say exactly how deep it lies.
If we follow these rivers upwards we find that in each case the
alluvial flat widens out greatly after we have passed the chalk-hills
and reached the clay lands beyond. These wide flats, according to old
ideas, were formed by the swinging from side to side of the stream,
which thus gradually widened its valley in the softer strata. If
this were the case in these instances, we should find a solid floor
beneath each marsh at a depth not exceeding that of the present
river-channels. The rivers, however, are not now cutting into rocky
banks or flowing over beds of Secondary strata; they are flowing
sluggishly in the middle of alluvial flats, which tend to silt up
with every flood or exceptionally high tide.
Thus all the evidence seems to show that marshes like those near
Lewes and Amberley Wild Brook have originated through the submergence
of flat-bottomed valleys cut in soft strata. The ponding back of the
muddy tidal water would soon lead to the silting-up of any shallow
lakes left after this submergence.
When the land stood 70 or 80 feet higher than it does now, the
country must have looked very different. The rivers then traversed
the chalk downs through V-shaped comparatively narrow valleys; but
these valleys opened out in their upper reaches, where they crossed
the Gault and Weald Clay. If we could lay bare the true floor of
the valley, we should see however that there is always a steady and
fairly regular fall seaward, just as there is in the part of its
course which lies above the influence of the submergence, which is
felt for some 10 or 12 miles from the sea. Except on this theory of
recent submergence it seems impossible to account for these curious
marshes, with tributary valleys obviously plunging sharply beneath
them on either side; they are quite unlike the undulating flats
which occur higher up.
The flat of Selsey Bill yields evidence of submerged land-surfaces
opposite each of the shallow valleys; but here we meet with the same
difficulty which confronted us in the Thames Valley and on the east
coast. Pleistocene land-surfaces and alluvial deposits of early date
are seen on the foreshore side by side with the more modern Neolithic
alluvium and submerged forests. Unless great care is taken it may be
thought that the well-preserved bones of rhinoceros and elephant, and
the shells of _Corbicula fluminalis_, come from the same alluvium
that yields Neolithic flint-flakes, or that plants such as the South
European _Cotoneaster Pyracantha_ flourished in Britain up to this
recent date. Except for the sake of warning against these sources of
error the submerged forests of Selsey Bill need not detain us.
Still travelling westward we next arrive at the series of tidal
harbours opening into Spithead, Southampton Water and the Solent. All
of these are obviously continuations of the valleys which lengthen
them inland; and this is amply proved by dock excavations and borings.
Even Southampton Water and the Solent themselves are nothing but
submerged valleys. A well at the Horse Sand Fort—one of the iron
forts which rises out of the sea at Spithead—showed a band of
compressed vegetable matter, probably an old land-surface, more
than 50 feet below high-water level, the floor of Eocene strata not
being met with till 98 feet below high water was reached. In this
case, however, the strata below 50 feet seem, from the published
description, to be of marine origin.
The well at Norman Fort is stated to have penetrated to a depth of
127 feet below the sea before Eocene strata were reached; but in this
case the lower strata were of marine origin, and the only land animal
recorded was a jaw of red deer, found apparently between 80 and 90
feet down. These deep channels may be relics of the very ancient
(Tertiary) Solent River, and were probably arms of the sea till they
were sufficiently silted up for the lowest submerged forest to grow.
We have not yet sufficient data, nor is it necessary to our purpose,
to give a detailed reconstruction of this interesting area during
the successive stages of elevation and depression. During the time
when the lowest of the submerged forests flourished the Isle of Wight
was connected with the mainland where the Solent now narrows about
Yarmouth, and probably for some distance westward. This connexion was
kept up till comparatively recent times, only breaking down finally a
short time before Caesar’s invasion.
In early Neolithic times the ancient Solent Valley had already been
decapitated by the inroads of the sea west of the Needles, and the
remains of this big valley were occupied by a small river flowing
eastward through the middle of the present Solent. In its course it
received numerous tributaries on either side. It probably opened out
into an estuary where it joined Southampton Water, and so continued
to and beyond Spithead, receiving other tributaries from the valleys
now occupied by Portsmouth Harbour, Langston Harbour and Chichester
Harbour.
In time we may be able to make a more complete reconstruction of the
physical geography of this area for definite dates; but the point now
to be insisted on is that the Isle of Wight was part of the mainland
up to quite recent times, so that its fauna and flora could readily
pass backwards and forwards without crossing the sea.
Perhaps to the geographer or geologist one of the most striking
confirmations of a recent submergence affecting this part of England
will be found in the strange series of enclosed harbours extending
from Chichester westward to Fareham. These harbours are not each
distinct and separate; all of them have cross connexions in the form
of shallower channels some four or five miles inland from Spithead.
I have often been asked what is the meaning and origin of these
peculiar harbours, which are not forming or widening now, but rather
tend to silt up.
The origin of these harbours is quite easy to understand, if we
admit the recent sinking of the land, and for this we will presently
give abundant evidence. On any other hypothesis these inosculating
waterways must seem hopelessly confused and inexplicable. Sea and
waves do not erode enclosed basins such as these.
Granting the submergence, we see that each of these harbours must
once have been a shallow valley; but this does not account for their
basin-like shape and their cross connexions. For the reason of
these peculiar features we must look at the map by the Geological
Survey showing the superficial deposits. It will then be seen that
all this part of Hampshire shows a widespread sheet of gravel and
gravelly loam which slopes gently seaward and passes below the sea
at Spithead. Northward the gravel rises, and the soft Eocene and
Cretaceous strata appear beneath the gravel between tide-marks at
various places toward the northern ends of these harbours.
The waves of the sea can remove loose gravel as readily as clay,
and we see that on this coast wave-action is practically confined
to the low cliff facing the sea and does not affect the interior
of the harbour. But it is well known to geologists that a sheet of
coarse angular gravel such as this, notwithstanding its looseness,
is much less readily attacked by a small stream than is a surface of
hard clay or even chalk. Thus plains of Tertiary deposits capped by
gravel, under the action of rain or rivers develop into gravel-capped
plateaus and hills, which fall abruptly into open flat-bottomed
valleys. The denudation takes place at the edge, where the gravel
rests on the Tertiary strata and numerous springs are given out;
there is scarcely any denudation in the gravel flat, and unless
the height of the land is considerable there is no great amount of
denudation in the flat bottom of the valley.
Thus there is a tendency for the valley to widen out on every side,
wherever the gravel rests on impervious or soft strata. But where
the gravel plunges below the water-level, as it did at the entrance
to each of these harbours, the valley narrowed, for there were no
landslips, the drainage was subterranean, and the stream could not
readily remove the large flints.
The widening of the valleys, where they were cut in soft strata, led
to the development of small lateral valleys to the right and left,
leaving only narrow divides between their head waters and those of
the next valley. When the land sank these divides were flooded, and
so were developed the shallow cross connexions, much as we now see
them.
In order that it may not be imagined that this reconstruction is
merely hypothetical, it will be as well to give some evidence that
such an elevation and submergence did take place in this district
as in others. We cannot in this little book deal with the whole
of the evidence, so we will take the Southampton Dock excavations
as sufficient to prove our point, condensing the account from the
_Geology of Southampton_, published by the Geological Survey.
The general section at Southampton Docks is as follows, though the
thickness varies considerably at different points, and the greatest
depth of the old valley has not yet been proved:—
Feet
Estuarine silt 20
Peat, old vegetable soil, or tufaceous marl;
ox, pig, horse, pine, beech, birch, oak,
and hazel variable up to 17
Gravel, with reindeer 10 or more
The bottom gravel is apparently of Pleistocene date, though it
may include also a basement bed belonging to the newer deposits.
T. W. Shore recorded from the peat above the gravel a fine stone
hammer-head of Neolithic date and worked articles of bone, but no
instruments of metal were found. The associated marl was full of
freshwater shells.
Poole Harbour tells a similar story, and evidence of this submergence
is seen in the various submerged forests found along the Dorset and
Devon coasts, opposite the mouths of the valleys. These rocky coasts
are, however, so different from those we have just been describing,
that they will more conveniently be treated of in a separate chapter.