Submerged Forests/Chapter 6
CHAPTER VI
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 sandbanks 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 fi-om 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 water-ways must seem hopelessly confused and in- explicable. 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 | ||
|
20 | |
|
variable up to 17 | |
|
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.