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Following on the lines laid down by Hutton, Lyell, and others, if we want to understand how ancient deposits were formed in the past we must investigate how deposition is going on now, and try to determine whether different deposits are characteristic of different conditions of deposition.
In most countries the and surface is being attacked by water, either on its surface by rain and streams or at its edges by the sea. In a few cases wind-action is of importance, as in the desert of the Sahara, and, where mountains rise up to the snow-line or the conditions are arctic, frost and ice produce important results. The removal of material from any spot is spoken of as denudation.
River-Action:
In the case of a river which rises in a mountain district and then flows over more level country to the sea very different results are to be seen at different parts of its course. In the first place, the swifter the stream the larger the stones which it can move, and consequently in the upper and steeper part of its stream-bed one sees large boulders which are only moved when the river is swollen after much rain; these, however, may get rounded by smaller blocks being banged against them when the stream is lower and less rapid.
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During a storm the waters of such streams become thick brown in colour owing to the amount of solid material being moved down stream, but on the next day when the storm is past the stream may be half the size that it was on the day before, and its waters may be quite clear. If the brown water be put into a cup a sediment falls to the bottom of the cup and clear water remains above it.
Such a torrent, therefore, washing against the sides of its course, removes material from one spot and carries it down stream to another. Wherever the pace of the stream gets checked there, material begins to settle, the coarser and heavier particles settling first and the finer ones being carried farther along.
Now, when a river passes from the steep mountain district to the plain its pace gets less, and in consequence, in the lower parts of the mountain region and in the upper parts of its plain region, we find gravel and sand deposits, while the lighter mud gets carried on.
The nature of the deposit must, however, depend on the rocks over which the river flows, and the harder the rock the less is the amount of material carried away from it, other things being equal.
Again, when the river enters a plain, its course is seldom a straight one, but owing to several causes it tends to wind about in curves. It may be that it meets some obstruction and so gets deflected, and when once deflected it will eat away the bank on the outside of its curve where its pace is great, and will deposit material on the inside where its pace becomes checked.
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One common cause of deflection is the entry of a tributary. If that tributary brings down with it such a quantity of solid material that the main stream cannot remove it, then a bank of deposit is pushed out into the main channel, and the river curves outwards from the side stream. If, however, such deposit is lacking, then the main stream bends towards the tributary, and a glance at a map which shows our rivers will quickly convince one of the constant occurrence of this cause of bending.
When these river-bends become very pronounced they are spoken of as meanders, and are commonly seen in small and large rivers, where they pass over a nearly flat part of their course.
In such a region the river is mainly concerned with carving laterally, and gradually removing the material carved out from the outside of one bend to the inside of another lower down its stream. Eventually a meander may be completely severed, and this is what has very nearly happened in the case shown in Fig. I.
Then the meander is left, at first as a marshy hollow only filled with water at flood-time, and eventually full of vegetation, and it then ceases to be used as an overflow channel. After a time meanders may cut down through a soft bed of, perhaps, clay, and reach a harder rock, and there they will be more permanent owing to the difficulty of removing the hard rock from the outside of their curves.
Ultimately the soft rock may be entirely removed, and the country left will be formed of the hard rock with the river meandering about in it and cutting its channel deeper and deeper. It is possibly this cause that has produced the marvellous loops in which the river Wye passes through the hard limestone of Monmouthshire.
It is to be noted that during a time when the river is swollen there may be a deposit of gravel at some spot, and that as the river goes down, first sand may be laid down on the gravel and then mud on the sand, and thus at one spot one may get alternations of different deposits or strata as they are called.
Amongst the deposits due to a river there may become entombed the remains of any animal, whether shell-fish, drowned dog, cat, or sheep, and these will tend to be more broken up amongst the coarser deposits of the upper parts of a river than in the finer sand and mud deposits lower down.
Since deposition is brought about by the checking of the pace of a river, it is bound to occur whenever a river flows either into a lake or into the sea (Fig. 2).
River-Deposits:
When a stream enters a lake the deposits, generally of sand and mud, gradually creep out into the lake and build up what is called a Delta. Flood- times build these deposits up to a level which is above the waters at other times, and a strip of marsh land gets formed which afterwards has vegetation on it and eventually becomes converted into dry land.
This alluvial deposit, as it is termed, gradually extends farther and farther into the lake and may ultimately completely fill it up. When a very big river, like the Nile or the Mississippi, comes out into the sea, similar deltas are formed, and alluvial deposits are built up which are being extended seawards year by year.
But round our own coasts the sea-currents are sufficient to prevent such deltas being formed, and the sand and mud brought down by our larger rivers form sand- and mud-banks which are constantly changing in outline as the tides ‘sweep away parts of them. Thus the mouth of the Thames, a river which flows over soft rocks, is thickly strewn with mud-banks, while the mouth of the Tay, which is a river flowing over hard rocks, has in it countless sand-banks near Dundee.
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A vast river like the Amazon may have such a strong current that it carries mud for many hundreds of miles out to sea, and so discolours the ocean waters, while the smaller rivers of the British Isles deposit most of their sandy material close to their mouths, and the finer material is not enough to produce the marked effects seen off the mouths of the larger rivers elsewhere. Still there is fine material brought down, and it is carried out to sea, borne along by the marine-currents and eventually deposited.
To sum up the effect of rivers on the land-surface we may say that in its steeper parts a river tends to carve down into that surface, and the material it carries along is gradually sorted out and deposited either on the land- surface or beneath the sea; while in the flatter part of its course the general tendency is to carve laterally, especially at the outside of any of its windings.
Sea-Action:
Turning now to the action of the sea on the land we can in general see the results more clearly than in the case of river-action. The sea attacks the edges of the land, and its effect is very markedly different along different parts of the coast. Round most of the coast of Scotland, Wales, and the west of England the cliffs are formed of hard rocks, these, when pounded by the waves, are broken up and the masses of rock which fall form a beach.
In times of storm these masses may be hurled by the waves against the cliffs of which they once formed a part, and so assist in still further breaking them away. The sea is continually grinding the smaller boulders against each other and against larger ones, and so wearing them all down, the smaller ones being eventually carried away by sea-currents and deposited beneath calmer waters.
Thus a hard rock like a granite may eventually be worn down and some of its constituents give rise to sand; and so round a granite coast, such as is found near the Land’s End, splendid stretches of sandy beach are formed.
In East Kent, Sussex, the Isle of Wight, and in several other places, the coast-line is made of chalk containing layers of flint; and when the sea attacks this the soft chalk is easily worn away, and the sea for some distance becomes the colour of milk owing to the particles of chalk suspended in it, but the hard flints are left close to the shore and form a beach.
As every wave comes up some of these are rolled round and round, giving rise to a great noise like a roar or a rattle depending on the size of the stones. All this rolling in all sorts of directions tends to make the flints take up a different form from those washed down by a river, for the latter are acted on by a current which tends to move them in a much more constant direction.
Owing to the main tendency of the marine-currents in the English Channel being from west to east, these flints tend to move up channel, and so flint beaches are now found in places at some distance from chalk.
Coast-Erosion:
The amount of land which is removed in a year from the harder coasts of Scotland and Wales may be unnoticed, but the amount removed from the softer chalk may be more appreciable.
When sea, however, comes up against soft sands and clays, as along the east coast of England from Essex to Lincolnshire and Yorkshire, its effects are very much more pronounced. An extra high tide or a fierce storm may send great waves against these soft cliffs, and a single tide may do a great deal of damage.
The land on which many villages and towns were built has now disappeared, and the sea occupies their sites. This eating away of the land is spoken of as erosion. In general, the softer the rock the more quickly the sea removes it, and a band of harder rock tends to protect the coast from further ravages, and so forms headlands.
These results are constantly seen round our coasts. The Land’s End is of granite and the Lizard of very old and very hard rocks called gneisses, while the bay between them is of softer rocks. Start Point in Devonshire is another headland made of hard rock which has protected the sandstones, which form the south of Devon, from being worn away by the sea.
The hard limestone of the Isle of Portland no doubt stretched at one time to St. Alban’s Head. Wherever the sea has broken through it the softer rocks to the north have been rapidly washed away, but the limestone still runs to the south of Kimmeridge and Swanage, and acts as a protection there. Wherever, as in Lulworth Cove, the sea makes a breach, a bay is rapidly scooped out in the soft clays behind the limestone.
The chalk of the Isle of Wight has had the soft sands to the north and south of it removed by sea-action and remains forming on the west the promontory overlooking the Needles and, on the east side of the island, Culver Cliff.
Beachy Head and the chalk of the South Foreland project as headlands beyond a stretch of coast which is made of sandstones and clays. Though sometimes, as at Fairlight, near Hastings, the sandstone, being fairly hard, has caused a slight projection of the coast.
Dungeness is one great exception to the general rule thus shown by the consideration of the south coast. This is a projection not formed by the denudation of the neighbouring coast, but is one being actually built out into the sea by constant additions at the present time.
The effects of river-action in our own islands are not, as a rule, very harmful to man. A field near a river may be slightly eaten away, or a flood may bring down with it mud and deposit it over low-lying country, or, as in the case of the Norfolk floods of the autumn of 1912, a river may loosen railway embankments and rise so as to flow into towns and villages; but the waters soon go down and the life of the district returns to its accustomed paths.
But even in such cases there may be obvious permanent effects ; thus a stream which turned a mill-wheel in Norfolk cut itself a new course and left the mill and its wheel— when the big flood, referred to above, abated — high and dry.
The effect of marine denudation may, however, be much more pronounced. We have already stated that towns and villages on the east coast of England have vanished. There is evidence that along the coast of Yorkshire many villages have vanished since the fourteenth century, while the cathedral town of Dunwich in Suffolk, with five great churches, has now entirely disappeared with the exception of a part of one of the churches.
This is at present situated on the very edge of the cliff, the sea having cut the land back so that the churchyard wall has fallen and the skeletons of the bodies which were buried there have some of them fallen on to the beach below.
Recent measurements near Southwold, a few miles to the north of Dunwich, have shown that the cliff edge is vanishing at the rate of 350 feet in nine years.
Naturally this rapid disappearance of the land has caused great difficulties where it has occurred, and the whole question of coast-erosion has recently been the subject of an inquiry by a Royal Commission.
It is in some ways comforting to find that this Commission reports that the resulting gain round our coasts by deposition and the reclamation of lands at one time flooded by the tides more than balances the loss by erosion. But it is little comfort to the landowner of Suffolk, who sees his fields disappearing year by year, to be told that a landowner in some other county has his lands increasing in acreage.
Kent and Sussex show some marked alterations in their land area during historic times. The north-eastern corner of Kent is still called the Isle of Thanet, and in Roman times it was indeed an island. Now, however, only marshy flats separate it from the mainland, and where Roman ships sailed the modern cow feeds.
On the Sussex coast are situated some of the old Cinque Ports from whose harbours in the seventeenth century ships sailed straight out to sea, but now they are many miles from the coast.
Coast Alterations in Historical Times:
The alteration in the shape of the coast-line between Hythe and Winchelsea in recent times has been very striking. In Roman times the river Rother used to flow along the northern edge of what is now Romney Marsh, past Appledore, through Lympne and into the sea at Hythe, and there was a port at the foot of Lympne Hill.
There is a charter of the year A.D. 833 which probably refers to the river as flowing from Lympne to Hythe. But later this course was deserted and the stream ran from Appledore to New Romney. The exact date of this alteration is unknown, but it was probably before the Norman Conquest.
The old haven near Hythe was quickly silted up, as it no longer had the stream to scour it out, and there came an accumulation of shingle which completely blocked the inlet some 350 years ago.
But the mouth of the river did not long remain at New Romney. In the thirteenth century storms occurred which probably caused banks of sand and shingle to accumulate, and forced the river to seek another channel. It changed its course at Appledore and swung round to the south, falling into the sea at Rye.
The port of New Romney then began to decay, and the inlet there is now nearly filled up by sand and mud, being only covered by sea at high water near the time of spring tides.
All through these years the sea-currents have been bringing to this region masses of flint pebbles derived from the chalk cliffs of the south of England, and these flints have been steadily accumulating, building up the country through which the Rother has wandered by various channels, and has brought down sand and mud in its stream.
The actual point of Dungeness has been built up by these flint pebbles, and it is constantly creeping out into the sea. The first accurate map of the district was made in Queen Elizabeth’s reign, and the distance from the end of the point to Lydd Church was then just three miles. In 1844 the distance was three miles and seven-eighths, which gives an average increase of about six yards per year.
Another part of our island which has altered much in historical times is that near the Wash. The rivers Ouse, Nen, Welland, and Witham all pour down into this inlet, bringing large quantities of material suspended in their waters. Early in historical times it seems probable that Cambridge was close to the sea. Now a large stretch of fen-land stretches round the, so-called, Isle of Ely for many miles to the north, and the deposition of material still goes on.
As a recent writer has put it:
“Cambridge we must conceive in the thirteenth century, and in the ages before it, as a seaport, situated at the farthest southern extremity of the inland sea, which in its modern, reduced dimensions we call the Wash.”
The district of Holderness, lying between the Humber and Flamborough Head, is another one which has suffered much from erosion in historical times. The coast-line is formed of soft clay, sand, and gravel, and every time the tide comes up against such material some is washed away.
In 1895 a Committee of the British Association for the Advancement of Science reported on the rate of erosion of our coasts, and the Director of the Ordnance Survey reported on the alteration of the coast-line in the Holderness district which had taken place between the two surveys of 1852 and 1889. At thirty-one spots he examined the actual breadth of erosion, and the greatest amount was 383 feet, the average amount being 215 feet, which meant a removal of close on 6 feet of land per year.
We are able to compare the acreage of various townships in this district at different periods, and find that between 1086 and 1800 the arable land of Easington had diminished from 2400 acres to 1300 acres, and that of Tunstall from 1280 acres to 800 acres.
A little to the east of the present Spurn Head was formerly the town of Ravenser. Founded by the Danes, it developed into a wealthy port, and in 1305 returned two members to Parliament and had two markets a week. In 1335 its warships are referred to, but in 1346 it only sent one ship to the Siege of Calais; in 1355 bodies were washed out of their graves in its churchyard, and in 1361 its merchants were driven away by the floods. In 1390 nearly all traces of the town had vanished.
In like manner in 1822 the village of Kilnsea, a little farther north, consisted of a church and thirty houses; on the Ordnance Map of 1852 there are some six or seven left, and the foundations of the church were visible at half- tide; now the last traces of Kilnsea have entirely disappeared.
Some distance inland from the village the Blue Bell Inn was built in 1847. In its wall is a tablet which states that it was then 534 yards from the sea.
Since that date other, measurements have been taken, and we find the following distances:
This gives an average encroachment of 5½ yards per year. The last burial in Kilnsea Church was that of a shipwrecked negro in 1823, for after this year the Church services were stopped. The tower fell in two portions, the first in 1826, the second in 1831. A room in the remainder of the village was devoted to divine service, and the church bell was hung over a beam and made to sound by throwing stones at it.
Since the Ordnance Survey of 1822 the village of Owthorne, north of Kilnsea, has quite disappeared. Its church steeple was 22 yards from the cliff in 1805, 8 yards from it in 1814, and fell in 1816; and in 1833 its site lay 18 yards to seaward of the cliff edge.
Phillips, in his Geology of Yorkshire, states that when he went there in 1826 only two gravestones remained in the churchyard, and on one of these was the inscription, “I must lie here till Christ appear.” On his next visit in 1828 all the gravestones had gone, while “the buried bones of former generations, which were seen projecting from the crumbling cliff, had a singular appearance, and, combined with the falling of the cliff and the roar of the destroying waves, filled the contemplative mind with solemn and awful reflections.”
The rate of erosion all along the coast from the Humber to Flamborough Head is very much the same, and some thirty villages or towns are known to have vanished. But all the material washed away by the sea is not carried out far from the land; the drift is from north to south, and so some of the coarser portions of the material derived from the decay of the land is moved southwards and assists in building up banks below the sea.
There are three large tracts within the estuary of the Humber which have comparatively recently been reclaimed- Sunk Island, Broomfleet Island, and Read’s Island.
Two sand-banks are marked in a map of 1660 as occurring where Sunk Island is now; in 1774, 1561 acres had been enclosed, and further enclosures were made in 1800, 1826, 1850,1880, 1897, and 1912; and now some 7500 acres of excellent land have been added to the district, and the area is no longer separated from the mainland.
About one hundred years ago large sand-banks were formed at Broomfleet, and in 1820 had grown well above high-water mark. In 1846, 130 acres were enclosed, but soon after that erosion took place, and the whole island vanished. In 1853, however, it began to form again, and in 1866 six acres were enclosed; in 1870 the enclosure increased to 60 acres, and by 1900 the channel between the island and the mainland was entirely silted up and some 600 acres reclaimed.
Early in the nineteenth century Read’s Island began to form; in 1840, 75 acres were enclosed; in 1861, 289 acres; and in 1886, 450 acres. Since then the island is being washed away at the rate of 4J acres a year.
Evidence such as that quoted above shows that the surface and edges of our islands are ceaselessly changing ; that in some places this change is so slow that it may require a vast time before it becomes obvious, while in others a few years are sufficient for a pronounced effect to be produced.
The method by which the foreshore in the Wash is now gradually being converted into land worth cultivating is somewhat as follows. The tide spreads over mud-flats, and during slack water deposits a certain amount of the material it holds in suspension—first the larger particles of silt, then the finer; much of the latter being removed by the out-going tide. Gradually the height of the deposit is raised, the strength of the ebb tide is weakened, and the finer particles settle. Then samphire and marine grass begin to grow, and assist in retaining the very fine particles. The soil thus becomes a mixture of very fine mud and organic matter derived from the vegetation which grows in it, and after twenty or thirty years is fit for enclosure. It is important that the level should be allowed-to rise almost to the height of the spring tides in order that the salt may be washed out of it.
The importance of the samphire and other salt-marsh plants lies in the fact that they can endure periodical immersion in sea-water and also live on a salt-saturated soil.
Deep-Sea Deposits:
We see then that there is plenty of evidence of changes going on from year to year on the surface of our islands and round our coasts, and the next question to be answered is, What is now going on beneath the deep oceans far from our shores?
The only way we can get a clue to an answer to this question is by looking at what is dredged up from time to time from the ocean-floors. Every scientific expedition that sails from our shores to investigate and explore the sea always carries out dredging operations, and in this way we have learnt much about the conditions of existence at the bottom of the oceans, the type of life found there, and the type of deposit now being formed.
As we might expect, it has been proved that round the shores of all continents to various distances the sand and clay washed out to sea are being deposited, but even beyond the limits of those deposits material is being constantly thrown down. The bottom of the Atlantic and Pacific is covered for many thousands of square miles with a grey ooze, and when this is dried and examined under the microscope it is found to be made up of countless tiny shells.
The animals which inhabited these shells live in the sea where the water is clear, and sink slowly, very slowly, through the ocean depths to the ocean-floor.
They take a very long time to do this, and as many of the shells are made of carbonate of lime, which is soluble in carbonic acid, the carbonic acid in the sea-water slowly attacks and, if the depth they sink through be great enough, completely dissolves them. Hence below a certain depth the sea-floor is no longer covered by a deposit chiefly formed of calcium carbonate, but of shells made of a much more insoluble substance called Silica.
At the greatest depths of all, even these siliceous shells have gone, and we find a red clay not made of shells at all. This is thought to be formed of the fine dust blown out into the air during volcanic eruptions and carried away from the land by winds. This, gradually settling down, may fall into the sea and then slowly sinks to its floor.
The grey Atlantic ooze is building up a deposit called Limestone, which is the characteristic rock formed beneath the clear waters of a sea into which no or very little sediment due to denudation is brought.
Just as a limestone may form beneath the clear waters of the sea, so one may form beneath the clear waters of a lake, and in this case it is called a Fresh-water Limestone.
Such deep-sea deposits when raised to the surface are found to be very fine grained. But it is quite conceivable that they may locally include a patch of coarse material. If an iceberg floats into a warmer sea than the one in which it was formed, it will gradually melt, and if it contains blocks of stone these will be showered down on the floor of the sea where the melting occurs.
Or if a tree-trunk containing rocks entangled amongst its roots floats down a river and out to sea, when the wood rots the stones will sink and perhaps become entombed in the fine deep-water ooze below. But such occurrences must be rare, and usually a deep-sea deposit is of a fine-grained nature.
Land Deposits:
Not only does deposition of material go on beneath water but also, though more rarely, on the land. In many places, where there are stretches of sand, high inshore winds blow the sand inland, and tall sand-dunes are produced. These can be well seen on the shores of Cardigan Bay, particularly near Harlech; they also occur in Cornwall, near St. Ives, and on the east coast they are found in Norfolk and elsewhere. In Scotland they can be seen on the coasts of Berwickshire, Fife, and Forfar, and on the west in Ayrshire.
These sand-dunes are not stationary, but the sand of one side is apt to get blown over the crest of the dune, and so the hill apparently moves inland. In this way a church tower at Eccles in Norfolk was completely buried under a sand-dune, and after many years the sand passed beyond it; and at last the church tower was once again exposed to view.
In order to prevent this movement of the sand, in some places, men plant grass or rushes on the sand, which on growing bind the surface together and prevent the wind from cutting into it. In certain very dry desert regions of the world sandy areas of great extent are known, and in them wind-borne sand forms deposits.
Another quite distinct set of deposits formed on land is given the name of Volcanic Deposits. Through holes in the surface of the earth explosions scatter big and small blocks of rock or finer material, called Volcanic Ashes, over the surrounding country. The larger fragments fall close round the hole and build up a heap of material called a Volcano, but the finer material may be carried for miles by the air-currents, and in the noted case of the eruption in the island of Krakatoa, near Java, in 1886, the very fine ashes were blown into the upper regions of the atmosphere and carried right round the world.