Read Ebook: Pour la patrie: Roman du XXe siècle by Tardivel Jules Paul

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Ebook has 1277 lines and 79981 words, and 26 pages

CHAP. PAGE

Preface v

Bibliography 122

Index 125

FIG. PAGE

INTRODUCTORY

These submerged forests or "Noah's Woods" as they are called locally, have attracted attention from early times, all the more so owing to the existence of an uneasy feeling that, though like most other geological phenomena they were popularly explained by Noah's deluge, it was difficult thus to account for trees rooted in their original soil, and yet now found well below the level of high tide.

It may be thought that these flats of black peaty soil though curious have no particular bearing on scientific questions. They show that certain plants and trees then lived in this country, as they do now; and that certain animals now extinct in Britain once flourished here, for bones and teeth of wild-boar, wolf, bear, and beaver are often found. Beyond this, however, the submerged forests seem to be of little interest. They are particularly dirty to handle or walk upon; so that the archaeologist is inclined to say that they belong to the province of geology, and the geologist remarks that they are too modern to be worth his attention; and both pass on.

Should we conquer our natural repugnance for such soft and messy deposits, and examine more closely into these submerged forests, they turn out to be full of interest. It is largely their extremely inconvenient position, always either wet or submerged, that has made them so little studied. It is necessary to get at things more satisfactorily than can be done by kneeling down on a wet muddy foreshore, with the feeling that one may be caught at any time by the advancing tide, if the study is allowed to become too engrossing. But before leaving for a time the old land-surface exposed between tide marks, it will be well to note that we have already gained one piece of valuable information from this hasty traverse. We have learnt that the relative level of land and sea has changed somewhat, even since this geologically modern deposit was formed.

Geologists, however, sometimes speak of the submerged forests as owing their present position to various accidental causes. Landslips, compression of the underlying strata, or the removal of some protecting shingle-beach or chain of sand-dunes are all called into play, in order to avoid the conclusion that the sea-level has in truth changed so recently. The causes above mentioned have undoubtedly all of them affected certain localities, and it behoves us to be extremely careful not to be misled. Landslips cannot happen without causing some disturbance, and a careful examination commonly shows no sign of disturbance, the roots descending unbroken into the rock below. It is also evident in most cases that no landslip is possible, for the "forest" occupies a large area and lies nearly level.

Compression of the underlying strata, and consequent sinking of the land-surface above, is however a more difficult matter to deal with. Such compression undoubtedly takes place, and some of the appearances of subsidence since the Roman invasion are really cases of this sort. Where the trees of the submerged forest can be seen rooted into hard rock, or into firm undisturbed strata of ancient date, there can, however, be no question that their position below sea-level is due to subsidence of the land or to a rise of the sea, and not to compression. But in certain cases it is found that our submerged land-surface rests on a considerable thickness of soft alluvial strata, consisting of alternate beds of silt and vegetable matter. Here it is perfectly obvious that in course of time the vegetable matter will decay, and the silt will pack more closely, thus causing the land-surface above slowly to sink. Subsidence of this character is well known in the Fenland and in Holland, and we must be careful not to be misled by it into thinking that a change of sea-level has happened within the last few centuries. The sinking of the Fenland due to this cause amounts to several feet.

The third cause of uncertainty above mentioned, destruction of some bank which formerly protected the forest, needs a few words. It is a real difficulty in some cases, and is very liable to mislead the archaeologist. We shall see, however, that it can apply only to a very limited range of level.

Extensive areas of marsh or meadow, protected by a high shingle-beach or chain of sand-dunes, are not uncommon, especially along our eastern coast. These marshes may be quite fresh, and even have trees growing on them, below the level of high tide, as long as the barrier remains unbroken. The reason of this is obvious. The rise and fall of the tide allows sea water to percolate landward and the fresh water to percolate seaward; but the friction is so great as to obliterate most of the tidal wave. Thus the sea at high tide is kept out, the fresh water behind the barrier remaining at a level slightly above that of mean tide, and just above that level we may find a wet soil on which trees can grow. But, and here is the important point, a protected land-surface behind such a barrier can never lie below the level of mean tide; if it sinks below that level it must immediately be flooded, either by fresh water or by sea water. This rule applies everywhere, except to countries where evaporation exceeds precipitation; only in such countries, Palestine for instance, can one find sunk or Dead Sea depressions below mean-tide level of the open sea.

The submerged forest that we have already examined stretched far below the level of mean tide, in fact we followed it down to the level of the lowest spring tides. Nothing but a change of sea-level will account for its present position. In short, the three objections above referred to, while teaching us to be careful to examine the evidence in doubtful cases, cannot be accepted as any explanation of the constant and widespread occurrence of ancient land-surfaces passing beneath the sea.

We have thus traced the submerged forest down to low-water mark, and have seen it pass out of our reach below the sea. We naturally ask next, what happens at still lower levels? It is usually difficult to examine deposits below the sea-level; but fortunately most of our docks are excavated just in such places as those in which the submerged forests are likely to occur. Docks are usually placed in the wide, open, estuaries, and it is often necessary nowadays to carry the excavations fully fifty feet below the marsh-level. Such excavations should be carefully watched, for they throw a flood of light on the deposits we wish to examine.

Every dock excavation, however, does not necessarily cut through the submerged forests, for channels in an estuary are constantly shifting, and many of our docks happen to be so placed as to coincide with comparatively modern silted-up channels. Thus at King's Lynn they hit on an old and forgotten channel of the Ouse, and the bottom of the dock showed a layer of ancient shoes, mediaeval pottery, and such-like--interesting to the archaeologist, but not what we are now in search of. At Devonport also the recent dock extension coincided with a modern silted-up channel. In various other cases, however, the excavations have cut through a most curious alternation of deposits, though the details vary from place to place.

This estuarine silt may continue downward to a level below mean tide, or perhaps even to low-water level; but if the sequence is complete we notice below it a sudden change to a black peaty soil , full of vegetable matter, showing sallows, alder, and hazel rooted in their position of growth. In this soil we may also find seams of shell-marl, or chara-marl, such as would form in shallow pools or channels in a freshwater marsh. This black peaty soil is obviously the same "submerged forest" that we have already examined on the foreshore at the mouth of the estuary; the only difference being that in the more exposed situation the waves of the sea have cleared away the overlying silt, thus laying bare the land surface beneath. In the dock excavations, therefore, the submerged forest can be seen in section and examined at leisure.

The next deposit , lying beneath the submerged forest, is commonly another bed of estuarine silt, extending to a depth of several feet and carrying our observations well below the level of low-water. Then comes a second land-surface , perhaps with trees differing from those of the one above; or it may be a thick layer of marsh peat. More silt follows; another submerged forest ; then more estuarine deposits ; and finally at the base of the channel, fully 50 feet below the level of high-water, we may find stools of oak still rooted in the undisturbed rock below.

As each of these deposits commonly extends continuously across the dock, except where it happens to abut against the rising ground, it is obvious that it is absolutely cut off from each of the others. The lowest land-surface is covered by laminated silts, and that again is sealed up by the matted vegetation of the next growth. Thus nothing can work its way down from layer to layer, unless it be a pile forcibly driven down by repeated blows. Materials from the older deposits in other parts of the estuary may occasionally be scoured out and re-deposited in a newer layer; but no object of a later period will find its way into older beds.

Thus we have in these strongly marked alternations of peat and warp an ideal series of deposits for the study of successive stages. In them the geologist should be able to study ancient changes of sea-level, under such favourable conditions as to leave no doubt as to the reality and exact amount of these changes. The antiquary should find the remains of ancient races of man, sealed up with his weapons and tools. Here he will be troubled by no complications from rifled tombs, burials in older graves, false inscriptions, or accidental mixture. He ought here to find also implements of wood, basket-work, or objects in leather, such as are so rarely preserved in deposits above the water-level, except in a very dry country.

To the zoologist and botanist the study of each successive layer should yield evidence of the gradual changes and fluctuations in our fauna and flora, during early periods when man, except as hunter, had little influence on the face of nature. If I can persuade observers to pay more attention to these modern deposits my object is secured, and we shall soon know more about some very obscure branches of geology and archaeology.

I do not wish to imply that excellent work has not already been done in the examination of these deposits. Much has been done; but it has usually been done unsystematically, or else from the point of view of the geologist alone. What is wanted is something more than this--the deposits should be examined bed by bed, and nothing should be overlooked, whether it belong to geology, archaeology, or natural history. We desire to know not merely what was the sea-level at each successive stage, but what were the climatic conditions. We must enquire also what the fauna and flora were like, what race of man then inhabited the country, how he lived, what weapons and boats he used, and how he and all these animals and plants were able to cross to this country after the passing away of the cold of the Glacial period.

To certain of the above questions we can already make some answer; but before dealing with conclusions, it will be advisable to give some account of the submerged land-surfaces known in various parts of Britain. This we will do in the next chapters.

THE THAMES VALLEY

In the last chapter an attempt was made to give a general idea of the nature of the deposits; we will now give actual examples of what has been seen. Unfortunately we cannot say "what can be seen," for the lower submerged forests are only visible in dock excavations. As these works are carried well below the sea-level and have to be kept dry by pumping, it is impossible for them to remain open long, and though new excavations are constantly being made, the old ones are nearly always hidden within a few weeks of their becoming visible. Of course these remarks do not apply to the highest of these submerged land-surfaces, which can be examined again and again between tide-marks, whenever the tide is favourable and the sand of the foreshore has been swept away.

The most convenient way of dealing with the evidence will perhaps be to describe first what has been seen in the estuary of the Thames. Then in later chapters we will take the localities on our east coast and connected with the North Sea basin. Next we will speak of those on the Irish Sea and English Channel. Lastly, the numerous exposures on the west or Atlantic coast will require notice, and with them may be taken the corresponding deposits on the French coast. Each of these groups will require a separate chapter.

The Thames near London forms a convenient starting point, for the numerous dock-excavations, tunnels, deep drains and dredgings have laid open the structure of this valley and its deposits in an exceptionally complete way. The published accounts of the excavations in the Thames Valley are so voluminous that it is impossible here to deal with them in any detail; we must therefore confine ourselves to those which best illustrate the points we have in view, choosing modern excavations which have been carefully watched, noted, and collected from rather than ancient ones.

We cannot do better than take as an illustration of the mode of occurrence and levels of the submerged land-surfaces the section seen in the excavation of Tilbury Docks, for this was most carefully noted by the engineers, and was visited by two competent observers, Messrs. W. Whitaker and F. C. J. Spurrell. This excavation is of great scientific importance, for it led to the discovery of a human skeleton beneath three distinct layers of submerged peat, and these remains have been most carefully studied by Owen and Huxley, and more recently by Professor Keith.

The section communicated to Sir Richard Owen by Mr Donald Baynes, the engineer superintending the excavation at the time of the discovery, is shown in the diagram on p. 14. As Mr Baynes himself saw part of the skeleton in the deposit, his measured section is most important as showing its exact relation to the submerged forests. It is also well supplemented by the careful study of the different

layers made by Mr Spurrell, for though his specimens did not come from exactly this part of the docks, the various beds are traceable over so large an area that there is no doubt as to their continuity.

Owen thought that this skeleton belonged to a man of the Palaeolithic period, considering it contemporaneous with the mammoth and rhinoceros found elsewhere in the neighbourhood. Other geological writers showed however that these deposits were much more modern, and some of them spoke somewhat contemptuously of their extremely recent date. But Huxley saw the importance of this "river-drift man" as an ancient and peculiar race, and Professor Keith has more recently drawn especial attention to the well-marked characteristics of the type. The skeleton is not of Palaeolithic date, but neither is it truly modern; other examples have turned up in similar deposits elsewhere.

We will now describe more fully the successive layers met with in Tilbury Docks, condensing the account from that given by Messrs. Spurrell and Whitaker, and using where possible the numbers attached by the engineer to the successive beds.

It will be noticed that the marsh-level lies several feet below Trinity high water. Below the sod of the marsh came a bed of fine grey tidal clay , in which at a depth of seven feet below the surface Mr Spurrell noted, in one part of the docks, an old grass-grown surface strewn with Roman refuse, such as tiles, pottery, and oyster shells. This fixes the date of the layer above as post-Roman; but the low position of the Roman land-surface, now at about mean-tide level, is due in great part to shrinkage since the marsh was embanked and drained--it is unconnected with any general post-Roman subsidence of the land.

The main peat rests on another bed of estuarine silt , which seems to vary considerably in thickness, from 5 to 12 feet. It is not quite clear from the descriptions whether the "thin woody peat" of Mr Whitaker and the "sand with decayed wood" of the engineer represent a true growth in place, like the main peat; it is somewhat irregular and tends to abut against banks of sand rising from below. In one of these banks, according to Mr Spurrell, the human skeleton was found.

The floor of chalk beneath these alluvial deposits lies about 60 or 70 feet below the Ordnance datum in the neighbourhood of Tilbury and Gravesend, and in the middle of the ancient channel of the Thames it may be 10 feet lower; but there is no evidence of a greater depth than this. We may take it therefore that here the Thames once cut a channel about 60 feet below its modern bed. We cannot say, however, from this evidence alone that the sea-level then was only 60 feet below Ordnance datum, for it is obvious that it may have been considerably lower. If, as we believe, the southern part of the North Sea was then a wide marsh, the Thames may have followed a winding course of many miles before reaching the sea, then probably far away, in the latitude of the Dogger Bank. This must be borne in mind: we know the minimum extent of the change of level; but its full amount has to be ascertained from other localities.

This difficulty has seemed of far greater importance than it really is, and some geologists have suggested that at this period of maximum elevation, England stood several hundred feet higher above the sea than it does now. I doubt if such can have been the case. Granting that the sea may have been some 300 miles away from Tilbury, measured along the course of the winding river, this 300 miles would need a very small fall per mile, probably not more than an inch or two. The Thames was rapidly growing in volume, from the access of tributaries, and was therefore flowing in a deeper and wider channel, which was cut through soft alluvial strata; it therefore required less and less fall per mile. Long before it reached the Dogger it probably flowed into the Rhine, then containing an enormous volume of water and draining twice its present catchment area.

As far as the Tilbury evidence goes it suggests a maximum elevation of the land of about 80 feet above its present level; but we will return to this question when we have dealt with the other rivers flowing more directly into deep sea. The animals and plants found at Tilbury were all living species.

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 brick-yards 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 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.

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