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Vistula discharges. This spit is 35 miles long, having an entrance at Pillau, where there is a bar having 10 feet of water over it. Another spit 18 miles long extends out from the west horn of the Bay of Danzig to the point of Hela.
On the east coast of America, and in the Gulf of Mexico and the Caribbean Sea, are to be found some very remarkable examples of sand-spits.
The best known of these is Sandy Hook, at the entrance to the bay of New York. The approach to the bay of New York from the Atlantic lies between two projecting sand-spits 6 miles apart. The flood tide coming from the south sets towards the Narrows in a north-westerly direction, while, on the north side, the flood current striking Long Island curls round and sets in a westerly direction towards the Narrows. The two spits, Sandy Hook on the one side of the entrance, and Coney Island on the other, have drifted in the same direction as the two currents of the flood tide. Sandy Hook is 6 miles long and three-quarters of a mile wide, the depth of water close to on both sides being from 25 to 30 feet, and the east side is exposed to the full range of the Atlantic Ocean. Coney Island projects 1} miles in a westerly direction, and is about half a mile wide. The way in which Coney Island projects across the channel, with Gravesend Bay behind it, bears a strong resemblance to Spurn Point on the Humber.
Mr. Lewis M. Haupt, in describing these sand-spits in a paper before the American Society of Civil Engineers, stated that, so far as his experience went, he found “ that the unceasing action of the breakers upon a receding shore-line, as affected by the direction of the flood tide and littoral currents, is far more effective than the prevailing winds. ... Taking the coast of New Jersey from Sandy Hook to Cape May, here, if the wind-wave theory be correct, and especially if the movement be due to the more violent gales from the north-east as asserted, the resultant travel of the beaches should be uniformly to the southward ; or from whatever quarter the maximum wind force may proceed, the sands should doubtless move to leeward, but always in the same direction for wind from the same quarter. It would seem anomalous, therefore, to find, as a matter of fact, that from a
1 “Littoral Movements on the New Jersey Coast, with Remarks on Beach Protection and Jetty Reaction,” Transactions of the American Society of Ciril Engineers, vol. 23, September, 1890.
certain point on the coast-line of Massachusetts and New Jersey the inlets and sand-beaches are travelling northward, and at other points southward and westward. But when it is noted that these directions correspond with those of the flood tide along this shore, it indicates more than accidental coincidence.”
North of New York harbour the coasts of Massachusetts and New Hampshire are bordered by cliffs of boulder clay and rock
Figures denote depth in fathoms.
Fig. 4.–Sand Spit, Cape Cod. which yield a large supply of sand and boulders. Past Cape Cod the drift of the flood tide is north and then south-westerly into Massachusetts Bay, and the sand has been drifted in the same direction, forming a very remarkable hook-spit, behind which the harbour of Provincetown lies land-locked in complete shelter.
South of New York harbour the coast-line is entirely devoid of cliffs, and the beach consists of sands bordered by dunes, the width being above 3 miles over a length of 80 miles.
From the waters of Chesapeake Bay to those of Biscayne Bay, a distance of about 700 miles, this natural ram part of sand is so continuous, and the lagoons which it shelters so connected, that a small boat could traverse the distance all the way without being exposed to the open sea.
From the Delaware river the direction of the drift changes from north to south, and barrier beaches of sand extend in an almost unbroken line along the coasts of Virginia and Carolina to Cape Florida. These sand-beaches lie upon the seaward side of large lagoons, the water in some of which is sufficiently deep to provide good harbours.
In the Gulf of Mexico there are several examples of harbours protected by sand-spits. St. Joseph's Bay, which carries a depth of from 24 to 36 feet, and has an entrance through which large vessels can enter, is protected by a long spit; and Mobile Bay is also protected by a similar spit, by which the entrance to the bay has been reduced from 20 miles to about 3 miles, the navigable waterway being only 1 mile wide. The entrance to Galveston Bay is through a narrow opening between two sand-spits. From Galveston to the mouth of the Rio Grande, the coast of Texas is formed by an almost continuous barrier beach of sand, enclosing a lagoon of an area of 1500 square miles. This sandbank is unbroken for a distance of 110 miles.
On the east coast of Africa the tidal current sets along the shore from south to north. Durban harbour is situated in a large inlet or bay having an area of 74 square miles. This bay is completely sheltered by a sand-spit 2 miles long which extends in a northerly direction across the harbour.
These spits are proofs of the permanence of banks of sand in deep water; and if taken in conjunction with similar submarine banks to be found in many parts of the coast in the open sea, show that channels dredged in sand, if properly designed with regard to the set of the tides, will remain permanent without the aid of any training walls. This has been also demonstrated by works recently carried out for the improvement of channels through sand-bars and in sandy estuaries.?
A further lesson to be learnt is that a single pier, having a I “The Geological History of Harbours,” U.S. Geological Survey, by N. S. Shaler. Washington, 1894.
? " The use of Suction Dredgers for the Improvement of Tidal Channels,” by W. H. Wheeler. Fourth Luternational Navigation Congress, Brussels, 1898.
curved form carried in the direction of the prevailing current, is sufficient for the protection of a harbour. All sand-spits trend in this direction, and there is no instance of two spits running out at right angles to the shore in the way in which many artificial piers for the protection of harbours have been projected.
Sand-dunes.—Low, sandy, exposed shores are frequently bordered by mounds of sand blown off the beach by the winds, known as “dunes ” or “ denes,” in Cornwall as “towans," on the south-west coast as “burrows,” and in Lancashire, Cheshire, Lincolnshire, and Norfolk as “meals” or “meols,” and on the north-east coast as “bents.” Thus the sand-hills at Gibraltar Point,
Fig.5. on the Lincolnshire coast, are described in old maps as “meals." Taylor, in his “Geology of Norfolk,” describes the ridges of sand by which the harbours of Cley, Blakeney, Wells, and Brancaster are defended from the sea in north gales as “meals." The sand-banks at Southport are described as “meols," and there are villages in Lancashire and Cheshire having the same name. At Lowestoft they are known as “ denes.”
These dunes afford protection to the low land lying behind them from high tides, and in some cases advance on the land, covering it with sand, and burying houses and churches, and in some cases whole villages.
As to the theory relating to the formation of sand-dunes, see paper by Dr. Vaughan Cornish in the Geographical Journal for March, 1897.
But for the protection of dunes on low coasts whole provinces would be covered by the sea, and but for the protection of its dunes there would be little left of Holland and the Netherlands. Kohl says that the people in Holland and Denmark “deal as carefully with their dunes as if dealing with eggs, and talk of their fringe of sand-hills as if it were a border set with pearls. They regard them as their best defence against the sea. These dunes are connected with their system of dykes, and sentries are posted all along their length to repair and guard them against wanton injury.”
The sand on dunes is held together by the roots of the grass known as “marram," "star-grass,” or “sea-mat” (Psamma arenaria), the roots of which penetrate a long way into the sand for moisture, attaining sometimes a length of 36 feet, and thus hold the sand together, while the grass checks the action of the wind on the surface.
Taylor, in a paper in the Philosophical Magazine, says that a single plant of mat-grass, which he designates Arundo arenaria, will have lateral shoots radiating from a single stem 10 to 12 yards long, forming a circle 20 yards in diameter, and that a plant of this grass in one year will multiply itself five hundredfold.?
These sandhills retain a considerable amount of moisture, fresh water being often found by sinking wells to a depth of a few yards. Andresen states that the amount of moisture at 1 yard below the surface amounts to 2 per cent. of the bulk, and after rain 4 per cent., and lower down to as much as 33 per cent. by measure.3
Other plants that can live in sand also soon become established, such as the sea buckthorn (Hippophæ rhamnoides); the sea wheat-grass (Triticum junceum); sea rocket (Cakile maritima); saltwort (Salsola kali); sea sowthistle (Sonchus maritimus); sand carex (Carex arenaria); sea lyme-grass (Elymus arenaria).
The mat-grass is sometimes cut for thatching and similar purposes, but this practice is considered detrimental to the maintenance of the dunes. Its leaves are nutritious food for
1 Min. Proc. Inst. C.E., vol. xxi. Paper on Sea-dykes at Schleswig.
? “Natural Embankments formed against the German Ocean on Norfolk and Suffolk Coast,” Phil. Mag., Series 2, vol. 2, 1827.
3 Marsb, “ Man and Nature,” quoting Audresen’s “ Om Klitformationem."