curved form carried in the direction of the prevailing current, is Sand-dunes.-Low, sandy, exposed shores are frequently on the Lincolnshire coast, are described in old maps as "meals." These dunes afford protection to the low land lying behind 1 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,1 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 hundred fold.2 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 per cent., and lower down to as much as 33 per cent. by 4 measure.8 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); 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. 2 “Natural Embankments formed against the German Ocean on Norfolk and Suffolk Coast," Phil. Mag., Series 2, vol. 2, 1827. * Marsh, "Man and Nature," quoting Andresen's "Om Klitformationem." E cattle, and it is also used abroad for making cordage and netting. The protection of these sand-dunes from injury, as already stated in Chapter I., was considered of such importance as to be the subject of legislative enactment. The growth of the reed or grass can be promoted in bare places, or newly formed dunes, by removing tufts from the older dunes or the inland banks, and planting them in the sand. On the coast of Holland the dunes are extended and repaired by setting in the sand, in rows about 1 foot apart, tufts of marramgrass. The holes are dug with the hand, the tuft placed in, and the sand pressed round it. One or two rows of reeds are set in the sand, projecting about 4 feet from the surface. The sand drifting along the beach is caught by the reeds, almost burying the tufts of grass, which soon make their way through. As the sand grows up, fresh plantings of grass and reeds are made. By this means bare places in the dunes are extended seaward, and the toe of the slope made good when it has been cut out by storms. In heavy gales gullies are cut through the dunes, which rapidly increase in width and depth, and through which the water will be apt to run at high tides. These demand careful attention and repair, by making barriers of reeds and bushes, and replanting the marram-grass until the gap is restored. All knolls and vertical faces formed on the seaward face of dunes also should be from time to time levelled and trimmed to a gentle slope. The bulk of the sand of most dunes is composed of grains of quartz, but in places where shell sand predominates on the beach, the dunes are also composed of this. In Jutland the bulk of the sand consists of yellow quartzite grains mixed with black titanic iron; in gales the dunes are furrowed with alternate ridges and depressions, the former composed of sand and the latter lined with the iron grains. In Prussia dunes are in places sprinkled with oxide of iron, which has given the sand a red colour. The size of the grains on English dunes varies from 1% to 200 of an inch, the average size being of an inch in diameter. The largest grains are to be found at the base of the dune, and the finest grains at the top. The slope and surface is greater and more regular on the lee than on the windward face of the latter, standing after a gale at an angle of from 5 to 10 degrees, the slope of the former being about 30 degrees with the horizon. Sand-dunes are to be found on several parts of the English coast, but they do not rise so high, and are not so extensive, as on the opposite shores of Belgium and Holland. The Lincolnshire coast is protected by a range of sandhills 30 miles in length, and from 15 to 20 feet high. On the Norfolk and Suffolk coasts there are also long ranges of dunes, varying in height from 10 to 50 feet, which form a protection to the low land behind them. In Norfolk, churches and villages have been buried by the advancing sandhills. Eccles, the church, at one time buried, became again uncovered owing to the sand drifting further inland. The dunes on the South Coast are, with a few exceptions, unimportant. In the Bristol Channel, near Braunton Burrows, the sandhills are lofty and nearly a mile wide. Port Talbot, which lies open to the channel, is protected by hillocks of blown sand, which rise 50 feet above the sea. Near Ogmoor, in Glamorganshire, the dunes reach 2 miles inland, and attain a height of ' 150 feet, the houses adjacent being rendered uninhabitable, and the channel of the river filled up for a length of 2 miles. In Cornwall, along the north-west coast, there are extensive dunes or "towans." In St. Ives and Perran Bays the dunes cover several square miles, composed of shell sand, chiefly mussels. On the east of Padstow the sandhills, which consist largely of shell sand, are 100 feet in height, and a church has been buried by the sand. (These sands are used largely for manure. In several places farm buildings and houses have been buried by the blown sand. At Marazion the sandhills are 130 feet high, and at Penzance 193 feet. The former sands are chiefly granitic. The ancient church of Perranzabuloe, which was buried by the drifting sand, became recently uncovered by the further advance of the dunes. On the West Coast, the principal hills of blown sand extend between the Mersey and the Ribble, and between Lytham and Blackpool. These in places are upwards of 2 miles in width, and vary in height from 30 to 80 feet. Along the shores of the Moray Firth and Aberdeenshire several parishes have been wholly or in part buried by sanddunes. Barrow, a town on the east coast of Ireland, near the mouth of the river Bann, was buried beneath the sand, for many years a single chimney from one of the houses showing above the surface. On the island of Coll, in the Hebrides, the sandhills are prevented from encroaching on the land owing to the sand being held together by the sea bent-grass (Elymus arenaria). During gales in winter the wind plays havoc with these hills, scooping out in one place a hollow 50 feet deep, and, in another, levelling the windward side to an almost vertical face; but in calm weather the damage is made good, and the sea-grasses mat and bind the shifting soil with their long spreading roots. On the opposite side of the English Channel and North Sea the dunes extend in an almost unbroken line from Calais to the Texel, the dunes beginning soon after the chalk cliffs cease. On the French coast they vary from a quarter to nearly a mile in width, and are from 50 to 80 feet in height; and along Belgium from 1500 to 2000 feet in width, and from 50 to 60 feet in height, the highest being over 130 feet above sea level. Holland depends almost entirely for its protection from the sea upon the sand-dunes, which are from 1 to 3 miles wide, and from 40 to 50 feet in height. Elie de Beaumont states that on the coast of Brittany the sanddunes, which extend over a length of 12 miles, driven by the winds from the north-west, and advancing landward, rendered a canton uninhabitable by covering it with 20 feet of sand. The tower of the church and chimneys of the buried houses are now occasionally visible. The most striking examples of dunes are to be found on the shores of the Bay of Biscay, where the sandhills extend for 150 miles from the cliffs at Biarritz to the Point de Grave at the mouth of the Gironde, occupying an area of about 500 square miles. Along this space at one time there existed a vast bay, having an area of 1,700,000 square miles, which became covered with sand raised above sea-level, and as bare of vegetation as the desert of Sahara. The southern portion of this vast sandy area is known as the Landes. The dunes, which consist of a series of hills and valleys, vary in width from 300 yards to nearly 5 miles, or an average width of 3 miles, and attain a height in some parts of over 300 feet. The greatest elevation is obtained at the centre of the line of dunes, the height near the Gironde decreasing to 20 or 30 feet. The size of the grains of sand varies with the height, the smallest grains being found at the top of the highest hills. |