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

Font size: 10 px 15 px 20 px 25 px 30 px 35 px

Background color: BlackWhiteGrayLight GrayDark GrayDim Gray

Text color: BlackWhiteGrayLight GrayDark GrayDim Gray

Apply/Save settings

Ebook has 1277 lines and 79981 words, and 26 pages

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 56 feet below the marsh-level. The section recorded by Mr Blake is as follows:--

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.

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.

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 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.

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 subaerial 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.

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.

THE DOGGER BANK

The other locality is far more extraordinary. In the middle of the North Sea lies the extensive shoal known as the Dogger Bank, about 60 or 70 miles from the nearest land. This shoal forms a wide irregular plateau having an area nearly as big as Denmark. Over it for the most part the sea has a depth of only 50 or 60 feet; all round its edge it slopes somewhat abruptly into deeper water, about 150 feet in the south, east, and west, but much deeper on the north. This peculiar bank has been explained as an eastward submerged continuation of the Oolite escarpment of Yorkshire; or, alternatively, as a mere shoal accumulated through the effects of some tidal eddy; but neither of these explanations will hold, for Oolitic rocks do not occur there, and the bank has a core quite unlike the sand of the North Sea.

When trawlers first visited the Dogger Bank its surface seems to have been strewn with large bones of land animals and loose masses of peat, known to the fishermen as "moorlog," and there were also many erratic blocks in the neighbourhood. As all this refuse did much damage to the trawls, and bruised the fish, the erratics and bones were thrown into deeper water, and the large cakes of moorlog were broken in pieces. A few of the erratics and some of the bones were however brought to Yarmouth as curiosities. Now the whole surface of the Dogger Bank has been gone over again and again by the trawlers, and very few of the fossil bones are found; unfortunately no record seems to have been kept as to the exact place where these bones were trawled.

The species found were:--

Ursus Canis lupus Hyaena spelaea Cervus megaceros " tarandus " elaphus " a fourth species Bos primigenius Bison priscus Equus caballus Rhinoceros tichorhinus Elephas primigenius Castor fiber Trichechus rosmarus

Though mammalian bones are now so seldom found, whenever the sand-banks shift slightly, as they tend to do under the influence of tides and currents, the edges of the submerged plateau are laid bare, exposing submarine ledges of moorlog, which still yield a continuous supply of this material. Messrs. Whitehead and Goodchild have recently published an excellent account of it, having obtained from the trawlers numerous slabs of the peculiar peaty deposit, with particulars as to the latitude and longitude in which the specimens were dredged. Mrs Reid and I have to thank the authors for an opportunity of examining samples of the material, which has yielded most interesting evidence as to the physical history, botany, and climatic conditions of this sunken land. The following account is mainly taken from their paper and our appendix to it.

Though we are still unable to locate exactly these submarine ledges or fix their depth below the sea, the blocks of moorlog are so widely distributed around the Dogger Bank, and have been dredged in such large masses, that it seems clear that a "submerged forest" forms part of the core of the bank. As nothing else approaching to a solid stratum appears to be dredged over this shoal, we may assume that the moorlog forms a sort of cap or cornice at a depth of about 10 fathoms, overlying loose sandy strata, and perhaps boulder clay, which extend downward to another 10 fathoms, or 120 feet altogether. Unfortunately we cannot say from what deposit the large bones of extinct animals were washed; they may come from the sands below the moorlog, but it is quite as probable that the Pleistocene deposits formed islands in the ancient fen--as they do now in East Anglia, Holderness, and Holland.

More than one submerged forest may be present on the Dogger Bank. The masses of moorlog are usually dredged on the slopes at a depth of 22 or 23 fathoms; but at the south-west end it occurs on the top as well as on the slope, the sea-bottom on which the moorlog is found consisting of fine grey sand, probably an estuarine silt connected with the submerged forest, for the North Sea sand is commonly coarse and gritty.

With regard to the moorlog itself and its contents, it is possible that some of the mammals in the list, such as the reindeer, beaver, and walrus, may belong to this upper deposit; but we have no means of distinguishing them, as the bones were all found loose and free from the matrix. The insects and plants were all obtained from slabs of this peat.

The dredged cakes of peat handed to us for examination came from different parts of the Bank; but they were all very similar in character, and showed only the slight differences found in different parts of the same fen. The bed is essentially a fen-deposit of purely organic origin, with little trace of inorganic mud. It is fissile and very hard when dry, and in it are scattered a certain number of fairly well-preserved seeds, principally belonging to the bog-bean. Other recognisable plant-remains are not abundant. They consist of rare willow-leaves, fragments of birch-wood and bark, pieces of the scalariform tissue and sporangia of a fern, and moss, and, curiously enough, of groups of stamens of willow-herb with well-preserved pollen-grains, though the whole of the rest of the plant to which they belonged had decayed.

The material is exceptionally tough, and is very difficult to disintegrate. In order to remove the structureless humus which composed the greater part of the peat, we found it necessary to break it into thin flakes and boil it in a strong soda solution for three or four days. Afterwards the material was passed through a sieve, the fine flocculent parts being washed away by a stream of water, the undecomposed plant remains being left behind in a state for examination. These remains were mixed with a large amount of shreds of cuticle, etc., but recognisable leaves were not found in the washed material.

The plants already found are:--

Ranunculus Lingua Castalia alba Cochlearia sp. Lychnis Flos-cuculi Arenaria trinervia Spiraea Ulmaria Rubus fruticosus Epilobium sp. Galium sp. Valeriana officinalis Menyanthes trifoliata Lycopus europaeus Atriplex patula Betula alba " nana Corylus Avellana Salix repens " aurita Sparganium simplex Alisma Plantago Potamogeton natans Ruppia rostellata Scirpus sp. Carex sp. Phragmites communis

Among the nine species of beetle determined by Mr G. C. Champion it is noticeable that two belong to sandy places. This suggests that the fen may have had its seaward edge protected by a belt of sand-dunes, just as the coast of Holland is at the present day.

This submerged forest in the middle of the North Sea has been described fully, for it raises a host of interesting questions, that require much more research before we can answer them. A sunken land-surface 60 feet and more below the sea at high-tide corresponds very closely with the lowest of the submerged forests met with in our dock-excavations. But if another bed of peat occurs at a depth of 130 or 140 feet at the Dogger Bank, this would be far below the level of any recently sunk land-surface yet recognised in Britain. Also, if the slabs of very modern-looking peat, containing only plants and insects still living in Britain, come from such a depth, out of what older deposit can the Pleistocene mammals, such as elephant, rhinoceros, and hyaena, have been washed?

These questions cannot be answered conclusively without scientific dredging, to fix the exact positions and depths of the outcrops of moorlog. When we remember also that beneath a submerged forest at about the depth of the Dogger Bank there was found at Tilbury, in the Thames Valley, a human skeleton; and that both human remains and stone implements have been discovered in similar deposits elsewhere, we can point to the Dogger Bank as an excellent field for scientific exploration.

The Dogger Bank once formed the northern edge of a great alluvial plain, occupying what is now the southern half of the North Sea, and stretching across to Holland and Denmark. If we go beyond the Dogger Bank and seek for answers to these questions on the further shore, we find moorlog washed up abundantly on the coasts of both Holland and Denmark, and it has evidently been torn off submerged ledges like those of the Bank. Numerous borings in Holland give us still further information, for they show that beneath the wide alluvial plain, which lies close to the level of the sea, there exists a considerable thickness of modern strata. At Amsterdam, for instance, two beds of peat are met with well below the sea-level, the upper occurring at about the level of low-tide, the lower at a depth of about 50 or 60 feet below mean-tide. That is to say, the lowest submerged land-surface is found in Holland at just about the same depth as it occurs in England, and probably on the Dogger Bank also.

Below this submerged land-surface at Amsterdam are found marine clays and sands, which seem to show that the lowest "continental deposit," as it is called by Dutch geologists, spread seaward over the silted-up bed of the North Sea; but no buried land-surfaces have yet been found below the 60-foot level anywhere in Holland.

This appearance of two distinct and thick peat-beds, underlain, separated, and overlaid by marine deposits, seems to characterise great part of the Dutch plain. It points to a long period of subsidence, broken by two intervals of stationary sea-level, when peat-mosses flourished and spread far and wide over the flat, interspersed with shallow lakes, like the Norfolk broads.

The enclosed and almost tideless Baltic apparently tells the same story, for at Rostock at its southern end, a submerged peat-bed has been met with at a depth of 46 feet.

On passing northward into Scandinavia we enter an area in which, as in Scotland, recent changes in sea-level have been complicated by tilting, so that ancient beach-lines no longer correspond in elevation at different places. The deformation has been so great that it is impossible to trace the submerged forests; they may be represented in the north by the raised beaches, which in Norway and Sweden, as in Scotland and the north of Ireland, seem to belong to a far more recent period than the raised beaches of the south of England. It seems useless to attempt to continue our researches on submerged forests further in this direction, especially as during the latest stages, when we know England was sinking, Gothland appears to have been slowly rising. Those who wish to learn about the changes that took place in the south of Sweden should refer to the recent monograph by Dr Munthe.

THE IRISH SEA AND THE BRISTOL CHANNEL

On the west coast of Scotland, as on the east, the succession of events seems to have been quite different from that which can be proved further south. It looks as though we must seek for equivalents of our submerged forests in certain very modern looking raised beaches and estuarine deposits, such as those of the Clyde. Even when we move southward to the Isle of Man deeply submerged post-glacial land-surfaces appear to be unknown, though there is evidence of a slight sinking, and roots of trees are found a few feet below the sea-level. In the Isle of Man we still come across the modern-looking raised beaches so prevalent in Scotland though unknown in England.

The Lancashire and Cheshire coasts, with their numerous deep estuaries and extensive flats, are noted, however, for their submerged forests, sometimes seen on the foreshore between tide-marks, sometimes laid open in the extensive dock or harbour works. The Heysham Harbour excavations, for instance, were carried far below sea-level and a thin peat-bed was met with in a boring at 52 feet below Ordnance datum. Mellard Reade considered this peat once to have been continuous with an ancient land-surface seen between tide-marks. A boring is not altogether satisfactory evidence for the occurrence of a land-surface at such a depth; but if it is trustworthy it points to a subsidence of about 60 feet, an amount identical with that observed in the Thames Valley.

The estuaries of the Ribble, Mersey, and Dee tell a similar story, for on their shores and under their marshes are found some of the most extensive submerged land-surfaces now traceable in Britain. Many accounts of these have been published; but the alternations of marine with freshwater strata and with land-surfaces are so like those already described that a short account will suffice.

Carefully plotted engineer's sections will be found in Mellard Reade's papers, and his account of the succession is so interesting that it is worth quoting. He postulates two periods of elevation, alternating with three periods of depression; but in this area, as in the Thames Valley, it appears as though all the phenomena can be accounted for by one long period of intermittent depression. His generalised section of the deposits in these estuaries is as follows:--

It may be an accidental coincidence; but it is noteworthy that both the Mersey and Thames show two main peat-beds separated by marine strata.

The forest exposed on the foreshore at Leasowe is a particularly good example of these old laoit-on l'admirer quand il se consacre au mal?

--Le talent, l'intelligence, cher monsieur, c'est toujours chose digne d'admiration, parce que c'est un don de l'?tre Supr?me, une parcelle de l'?me universelle.

--Dans l'intelligence humaine il faut, ce me semble, consid?rer deux choses: l'oeuvre de Dieu qui est toujours belle et l'oeuvre de l'homme, c'est-?-dire l'usage que l'homme fait de ses facult?s. Malheureusement, cette derni?re oeuvre est souvent mauvaise et laide.

Share on: WhatsApp @Hash Facebook Tweet