Submerged Forests/Chapter 3
CHAPTER III
THE EAST COAST
It is not our purpose to describe in detail the many exposures of submerged land-surfaces which have been seen on the shores of the North Sea. This would serve no useful purpose and would be merely tedious. We need only say that the floor of Eocene or Cretaceous strata on which these ancient subaerial deposits rest is constantly found at depths of 50 or 60 feet below the level of the existing salt-marsh. But where, as in the estuary of the Thames and Humber, an older channel underlies a modern channel, the floor sinks about 30 feet lower. From present marsh-level to ancient marsh-level is about 60 feet; from present river-bottom to old river-bottom is also about 60 feet. This, therefore, is the extent of the former elevation, unless we can prove that the sea was then so far away that the river once had many miles to flow before reaching it. This is the point we have now to consider as we trace the submerged forests northward and towards the deeper seas.
Before we leave the southern part of the North Sea basin it will be well to draw attention to a few of the half-tide exposures which for one reason or another may tend to mislead the observer. The mere occurrence of roots below tide marks is not sufficient to prove that the land-surfaces seen are all of one date.
Not far from Tilbury is found the well-known geological hunting ground of Grays, where the brickyards have yielded numerous extinct mammalia and several land and freshwater shells now extinct in Britain. These deposits lie in an old channel of the Thames, cut to below mean-tide level, but here not coinciding exactly in position either with the channel of the existing river, or with the channel in which the submerged forests lie.
It is fortunate that the channels do not coincide, for this enables us to distinguish the more ancient deposits. A glance at a geological map shows, however, that they must coincide elsewhere, and where the Thames has re-occupied its old channel it is clear that the destruction of the earlier deposits may have led to a mixture of fossils and implements belonging to three different dates. Mammoth teeth and Palaeolithic implements, Irish elk and polished stone implements, may all be dredged up in the modern river gravel, associated with bits of iron chain, old shoes, and pottery. Such a mixture does actually occur in the Thames estuary, and it makes us hesitate to accept the teeth of mammoth which were dredged in the Thames as really belonging to so late a period as that of the submerged forests.
At Clacton a similar difficulty is met with, for there again an ancient channel contains alternating estuarine and freshwater deposits with layers of peat, and is full of bones belonging to rhinoceros, hippopotamus, elephant and other extinct mammalia. Of course the peat-beds in this channel are just as much entitled to the name "submerged forest" as the more modern deposits to which recent usage restricts it. They belong, however, to another and more ancient chapter of the geological record than that with which we are now dealing. I do not say a less interesting one, for they are of the greatest importance when we study the times when Palaeolithic man flourished; but at present we have as much as we can do to understand the later deposits and to realize the great changes to which they point. We must not turn aside for everything of interest that we come across in this study; these earlier strata are worthy of a book to themselves.
As we travel northward along the coast, again and again we meet with evidence of a submerged nearly level platform, "basal plane," or ancient "plane of marine denudation," lying about 50 feet below the sea. We find it at Langer Fort, which lies opposite to Harwich on a spit of sand and shingle stretching across Harwich Harbour. Here the floor of London Clay was met with in a boring at 54 feet below the surface.
The Suffolk coast north of Southwold yields yet another complication, for between Southwold and Sherringham in Norfolk there appears at the sea-level a land-surface considerably more ancient than anything we have yet been dealing with. This is the so-called "Cromer Forest-bed," which consists of alternating freshwater and estuarine beds, with ancient land-surfaces and masses of peat. It contains numerous extinct mammals, mainly of species older than and different from those of Clacton and Grays.
The mammalian remains differentiate these deposits at once; but if no determinable mammals are found, the crushing of the bones and the greater compression and alteration of the peaty beds serves to distinguish them, for this Forest-bed dates back to Pliocene times, passes under a considerable thickness of glacial beds, and has been over-ridden by the ice-sheet during the Glacial epoch.
The Cromer Forest-bed has been exposed particularly well of late years at Kessingland, near Lowestoft, where the sea has encroached greatly. It is well worth while to make a comparative study of this deposit, of the Grays and Clacton Cyrena-bed, of the submerged forests of the Thames docks, and of the strata now being formed in and around the Norfolk Broads. By such a comparison we can trace the effects of similar conditions occurring again and again. The fauna and flora slowly change, species come and go, man appears and races change: though the same physical conditions may recur life ever changes.
The Norfolk Broads, just referred to, deserve study from another point of view: their origin is directly connected with the submergence which forms the theme of this book. These broads are shallow lakes, always occupying part of the widest alluvial flats which border the rivers; but they are usually out of the direct course of the present river; they therefore receive little of the sediment brought down in flood-time. On the other hand they are steadily being filled up with growing vegetation and turned into peat mosses.
The origin of these shallow freshwater lakes, which form a characteristic feature in the scenery of East Anglia, has been much debated; but with the knowledge obtained from a study of the submerged forests the explanation is perfectly simple. During this period of slow submergence each of the shallow valleys in which the broads now lie was turned into a wide and deep navigable estuary, which extended inland for many miles. When the subsidence stopped the sea and tides soon formed bars and sand-banks at the mouths of the estuaries, and lateral tributaries pushed their deltas across. The Norfolk rivers, being small and sluggish, were driven to one side, and could neither cut away the sand-banks nor fill up with sediment such wide expanses. These estuaries therefore were silted up with tidal mud and turned into irregular chains of lakes, separated by irregular bars and sand-banks. The lakes, instead of becoming rapidly obliterated and filled up by deltas which crept gradually seaward, remained as freshwater broads; for as soon as a bank became high enough for the growth of reeds and sedges the river mud was strained out and only nearly clean water reached the lagoon behind. Thus a depression once left, provided it was out of the direct course of the river, tended to remain as a freshwater lake until vegetable growth could fill it, and the river mud was spread out over the salt-marshes or went to raise the sand-banks till they became alluvial flats, and thus still more thoroughly isolated the broad.
A few centuries will see the disappearance of the last of the broads, which have silted up to an enormous extent within historic times; but the fact that so many of these broads still exist may be taken as clear evidence of the recent date of the depression which led to their formation.
When we look at ancient records, and notice the rapidity with which the broads and navigable estuaries are becoming obliterated, we cannot help wondering whether the measure of this silting up may not give us the date of the last change of sea-level. It should do so if we could obtain accurate measurements of the amount of sediment deposited annually, of the rate at which the sea is now washing it in, and of the rate at which the rivers are bringing it down. All these factors, however, are uncertain, and it is particularly difficult to ascertain the part played by the muddy tidal stream which flows in after storms and spreads far and wide over the marsh.
Though all the factors are so uncertain, we can form some idea of the date of the submergence. Many years ago I made a series of calculations, founded on the silting up of our east coast estuaries, the growth of the shingle-spits, and the accumulation of sand-dunes. The results were only roughly concordant, but they seemed to show that the subsidence stopped about 2500 years ago and was probably still in progress at a date 500 years earlier. This question of dates will be again referred to in a later chapter.
Before leaving the Broad district we must refer to a boring made at Yarmouth, which, according to Prof. Prestwich, showed that the recent estuarine deposits are there 120 feet thick, and consequently that the ancient valley was far deeper than any recorded in the foregoing pages. There is no doubt, however, that this interpretation is founded on a mistake, for other borings at Yarmouth, Lowestoft, and Beccles came to muddy sands and clays belonging to the upper part of the Crag, now known to thicken greatly eastward. The recent deposits descend only to a depth of about 50 feet at Yarmouth, and consist of sand and shingle; the beds below contain Pliocene mollusca. This emendation is also borne out by the entirely different character of the recent estuarine deposits at Potter Heigham, where we again find a submerged forest at about 50 feet below the marsh-level. The section recorded by Mr Blake is as follows:—
| feet. | ||
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24 | |
|
12 to 2 | |
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13 | |
|
17 | |
|
2 | |
| 58 |
It will be noticed that here only one peat bed was found, and was at the usual depth of the lowest submerged forest. Possibly the white sand below was the bleached top of the Crag; but this point was not cleared up.
If we resume our journey northward along the Norfolk coast we come to the well-known locality of Eccles, where the old church tower described and figured by Lyell in his Principles of Geology long stood on the foreshore, washed by every spring tide. The position of this church formed a striking illustration of the protection afforded by a chain of sand-dunes. The church was originally built on the marshes inside these dunes, at a level just below that of high-water spring tides. But as the dunes were driven inland they gradually overwhelmed the church, till only the top of its tower appeared above the sand. In this state it was pictured by Lyell in the year 1839. Later on (in 1862) it was again sketched by the Rev. S. W. King, and stood on the seaward side of the dune and almost free from sand. For a series of years, from 1877 onward, I watched the advance of the sea, and as the church tower was more and more often reached by the tides, its foundations were laid bare and attacked by the waves, till at last the tower fell.
Not only were the foundations of Eccles church exposed on the foreshore, but an old road across the marshes also appeared on the seaward side of the dunes, giving a still more exact idea of the former great influence of the chain of dunes in damping the oscillations of the tidal wave. The tide outside now rises and falls some 12 or 15 feet; on the marsh within its influence is only felt under exceptional circumstances. A road across the marsh at a level four or five feet below high-water, as this one stood, would still be passable, except during unusual floods.
Eccles Church is an excellent example of the way in which an ancient land-surface may now be found below the level of high-water, and yet no subsidence of the land has taken place. But this coast can give even more curious examples. It does not need a sand-dune to deaden the rise and fall of the tides; even a submerged bank will have much the same effect. Extensive submerged sand-banks extend parallel with the coast, protecting the anchorage known as Yarmouth Roads. These banks rise so nearly to the surface of the sea that not only do they protect the town and anchorage against the waves, they deaden the tidal oscillation to such an extent that its range is much greater outside the bank than within.
If these submerged outer banks were to be swept away by some change in the set of the currents, large areas now cultivated and inhabited would be flooded by salt water at every spring tide, and the turf of the meadows would be covered by a layer of marine silt, such as we see alternating with the submerged forests in the docks of the Thames. Such alternations, if thin, do not necessarily prove a change in the level of the sea; they may only point to the alternate accumulation and removal of sand-banks in a distant part of the estuary.
The Norfolk coast trends westward soon after leaving Cromer, and where the cliff seems to pass inland at Weybourn we enter an ancient valley, one side of which has been entirely cut away by the sea, except for a few relics of the further bank, now included in the shingle beach which runs out to sea nearly parallel with the coast and protects Blakeney Harbour. Here again we find that in the bottom of the valley there must be a submerged forest, for slabs of peat are often thrown up at Weybourn, and by the use of a grapnel the peat was found in place off Weybourn at a depth of several fathoms.
When the coast turns southward again, and the wide bay of the Wash is entered, we find an extensive development of submerged land-surfaces and peat beds, extending over great part of the Fenland. In fact the whole Fenland and Wash was once a slightly undulating plain, cut into by numerous shallow open valleys. The effect of the submergence of this area has been to cause the greater part of it to silt up to a uniform level, through the accumulation of warp and growth of peat; so that now the Fenland has become a dead level, out of which a few low hills rise abruptly. The islands of the Fenland, such as those on which Ely and March are built, are merely almost submerged hill-tops; they were not isolated by marine action.
It is obvious that a wide sheltered bay of this sort forms an ideal area in which to study the gradual filling up and obliteration of the valleys, as the land sank; and it may enable us to learn the maximum amount of the change of sea-level. The Fenland unfortunately does not contain very deep dock excavations, and we have only various shallower engineering works to depend on, though numerous borings reach the old floor.
A preliminary difficulty, however, meets us in the study of the Fen deposits; it is the same difficulty that we have already referred to when describing Clacton and Grays, and we shall meet with it again. In certain parts of the Fenland, particularly about March and Chatteris, a sheet of shoal-water marine gravelly sand caps some of the low hills, which rise a few feet above the fen-level. The gravel for long was taken to be the same bed that passes under the marshes. Later work showed however that these gravels, with their sub-arctic marine fauna and containing also Corbicula fluminalis, were of much earlier date than the true fen-deposits. Just as we saw happen in the Thames Valley, a wide plain and estuary existed long before the deeper channels containing the submerged forests were cut; and the deposits of this older estuary and its tributaries are still to be found in patches here and there. Sometimes, as at March, they cap hills a few feet above the fen-level; but as often they fill channels not quite coinciding with the later channels; just as they do at Grays. Or two deposits of quite different date may lie side by side, as they do in the Nar Valley, or at Clacton, or on the Sussex coast.
The true fen-deposits were carefully examined by Messrs. Marshall, Fisher, and Skertchly, as far as the shallow sections would allow, and the following account is mainly condensed from that given by Mr Skertchly in his Geology of the Fenland.
During the excavation of certain deep dykes for the purpose of draining the fens there was discovered at a depth of about 10 feet below the surface a forest of oaks, with their roots imbedded in the underlying Kimmeridge Clay. The trunks were broken off at a height of about three feet. Some of the fallen trees were of fine proportion, measuring three feet in diameter, quite straight and seldom forked. At an average height of two feet above this "forest No. 1" the remains of another were found (in the peat) consisting of oaks and yews. Three feet above "forest No. 2 " lay the remains of another, in which the trees are all Scotch firs, some of which were three feet in diameter. Above this and near to the surface was seen a still newer forest of small firs. The peat close to the surface contains remains of sallow and alder, and was formed with the sea at its present level.
It will be noticed that the greatest depth at which these rooted trees were found was only about ten feet below the sea-level. At this high level we must expect to find that the growth of the peat was practically continuous, and that the different submerged forests run together. In adjoining depressions the different forests would occur at lower levels and would be separated by beds of marine silt. It does not follow from the position that a low-level submerged land-surface is older than one at a higher elevation, for above the present sea-level all these stages are represented by a few inches of soil, on which forest after forest has grown and decayed. Anyone who has collected antiquities on fields knows what a curious jumble of Palaeolithic, Neolithic, bronze age, Roman, mediaeval and recent things may be found mixed in these few inches of soil, or may be thrown up by an uprooted tree. The great advantage of studying the deeply submerged forests is that in them the successive stages are separated and isolated, instead of being mingled in so confusing a fashion.
For further information as to the more deeply submerged land-surfaces we may turn to the numerous records of borings made in the Fenland and collected by the Geological Survey. These show that the thickness of the fen-deposits varies considerably from place to place, that the floor below undulates and is by no means so flat as the surface of the fen above. Most of these borings, however, were not continued through the gravels which lie at the base of the deposit, and thus we can only be certain of the total depth to the Jurassic clay or boulder clay in a few places. The maximum thickness of the fen-beds yet penetrated is less than 60 feet, and a submerged forest was found at Eaubrink at about 40 feet. It is possible however that none of these scattered borings has happened to hit upon one of the buried river-channels, which formerly wandered through this clayey lowland; if one were found it would probably show that the alluvial deposits are somewhat thicker than these measurements, and that they descend to a depth about equal to that reached in the valleys of the Thames or Humber.
It is useless to discuss in more detail the lower submerged forests of the Fenland, for we cannot get at them to examine them properly. They have been as effectually overwhelmed and hidden as the remains of King John's baggage train, which has never been seen again since it wandered off the flooded causeway during the disastrous spring-tide of October 11, 1216, and sank into the soft clay and quicksands.
The higher submerged forests of the Fenland are however of great interest, and as already pointed out they have been exposed to view in cutting the fen dykes, especially near Ely. Perhaps a closer study of these might enable us to arrive at some idea of the time taken for the growth of a series of forests of this sort, and for the accompanying mass of peat. The variations in the flora also need more exact analysis before we can say what they betoken. The oak-forest at the bottom is what we should expect on a clay soil; but the reason for the succession of trees above is not obvious. It need not necessarily point to climatic change, though it may do so; but it certainly looks as if the peaty bogs were alternately wetter and drier, so that sometimes moss grew, and sometimes fir-trees. Neither need this change imply an up-and-down movement of the land, though it may be due to such a cause.
Subsidence would destroy the oaks and allow a peat-moss to form; but if the subsidence were intermittent the moss would increase in thickness, become more compact, and its surface rise, till it was dry enough for pines. Another subsidence would cause spongy peat again to spread and kill the pine, and so on. Intermittent subsidence seems sufficient to account for all the changes of vegetation we have yet noticed in connexion with these submerged land-surfaces.
Of the fauna of the fen-silts and peats it is very difficult to give any satisfactory account. If we put aside the March and Chatteris marine gravels with Corbicula fluminalis, and the Nar Valley Clay with its northern marine mollusca as being of older date; and if we also reject the marginal gravels with hippopotamus and mammoth as being more ancient, there only remain a few mammals such as the beaver, wolf, wild boar, and certain cetacea, which we can be sure came out of the true fen-deposits. Implements made by man have only been found in the higher layers, and there seems to be no record in this area of a stone implement found below a submerged forest
Submerged forests of the ordinary type are often to be seen between tide-marks on the flat shores of Lincolnshire; but as they still await proper study they need not here detain us, and we will pass on to the next large indentation of the coast-line, the estuary of the Humber.
Here, owing to the excavation of extensive docks, and to a series of trial borings for a tunnel beneath the Humber, the structure of the valley has been clearly laid open. It is much the same as that of the Thames; but as we are in a glaciated area we find, as in the Fenland, that much of the erosion had taken place before or during the Glacial Epoch, for boulder clay occupies part of the valley.
Boulder clay or till not only occupies part of the valley, it descends far below the present river bottom and even below the lowest submerged forest. This we find always to be the case in the glaciated parts of Britain; but whether the deep trenching is due to the ploughing out of a trough by a tongue of the ice-sheet, to sub-glacial streams below sea-level, or to erosion by a true sub-aerial river is still a doubtful point. However, this question must not detain us; we are not now dealing with elevations and depressions of so ancient a date, and must confine our attention to post-glacial movements.
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Fig. 3.
The section shown in fig. 3 will explain better than any words the structure of the Humber Valley. It is drawn to scale from the engineer's section, and shows at a glance the three channels. The deepest and widest channel is that occupied by glacial deposits; an intermediate channel (shown in black) is occupied by silt and submerged forests; and a shallower channel is occupied by the present Humber and its alluvium. One interesting point, however, this section does not happen to illustrate. Somewhat lower down the Humber we come to gravels and silts full of sub-arctic marine mollusca and Corbicula fuminalis, exactly as in regions further south, and presumably of the same age as the deposits we have already mentioned as found at March in the Fenland and at Grays in Essex. The exact relation of these Corbicula-beds to the deep channel filled with glacial drift, below the marshes of the present Humber, is still somewhat uncertain, but the marine beds clearly rest on boulder clay, and seem also to be overlain by another glacial deposit.
The section leaves no doubt that in post-glacial times the Humber cut a channel about 60 feet below its present bed, or to just the same depth as did the Thames. This may possibly be an accidental coincidence; but it is very suggestive that both these rivers should have cut their beds to the same depth. Such coincidences suggest that we are dealing with a period when each of our great rivers was able to cut to a definite base-level, below which it could not go. This base-level must either have been the sea, or some vast alluvial plain then occupying the bed of the North Sea. In either case the plain must then have been fully 60 feet lower than the present sea-level. Not only did the ancient Humber cut to the same depth as the ancient Thames, but in each area the ancient river was flanked by a wide alluvial flat which now lies from 40 to 60 feet below the modern marsh level.
The flat coast of Holderness, which stretches from the Humber northward to Flamborough Head, shows also occasional submerged forests; but the want of excavations beneath the sea-level makes it impossible to say much about them. North of Flamborough Head it seems as though depression gave place to elevation, and when we pass into Scotland the Neolithic deposits seem to be raised beaches instead of submerged forests. We need not therefore devote more time to a consideration of the details connected with the submerged land-surfaces which border the lands facing the North Sea. They evidently once formed part of a wide alluvial flat stretching seaward and running up all our larger valleys. We must now consider how far seaward this plain formerly extended.
Here, fortunately, we meet with a most surprising piece of evidence, which adds enormously to the importance of this plain, and shows that the submergence is no local phenomenon, but a widespread movement of depression which must greatly have altered the physical geography of north-western Europe during times within the memory of man. This evidence deserves a separate chapter.