Page:EB1911 - Volume 19.djvu/816

 NORTH SEA, a sea bounded E. by the continent of Europe and W. by Great Britain. At its southern end it communicates by the narrow Strait of Dover with the English Channel, and so with the Atlantic, and towards the north it widens out gradually to 345 m. between St Abb’s Head and the coast of Denmark, and narrows again to 270 m. between Duncansby Head and the coast of Norway. To the north of Scotland it communicates with the Atlantic westwards by the Pentland Firth and the channel between the Orkney and Shetland Islands, and northwards with the Norwegian Sea.

Its total area is given by Murray as 162,600 sq. m., and by Krümmel as 571,910 sq. km., or 220,820 sq. statute m. Murray estimates the volume of the North Sea at 11,200 cub. m., and Krümmel at 53,730 cub. km. or 12,890 cub. m., giving mean depths of 61 and 48 fathoms respectively. The North Sea is thus on the whole shallow; its bed is part of the continental shelf on which the British Isles stand, and it slopes upwards with fair regularity from north to a broad coastal strip over which the depth nowhere exceeds 20 fathoms, and the whole south-eastern part of the area is less than 30 fathoms deep. In about its middle latitude the Dogger Bank crosses the North Sea from east to west, extending for about one-third of the whole distance; near the English coast the depth here is under 10 fathoms and it increases eastwards to about 20 fathoms. South of the Dogger there are local depressions, mostly of small area, in which the depth is as much as 45 fathoms, as in the “Silver Pit.” Krümmel points out that a line drawn from the northern edge of the Dogger to the middle of the Skagerrack constitutes a rough boundary of the shallow southern basin, the depth increasing very slowly beyond this line to the “Norwegian Channel”—a deep gully closely following the Scandinavian coast, and extending into the Skagerrack, in which the depth increases to as much as 400 fathoms.

According to Jukes-Browne, the North Sea, in its present form, first took shape as a result of the tectonic movements indicated by the break between the older and newer Pliocene deposits. The southern end of the North Sea was probably little affected by the general subsidence which occurred during the Glacial period; its boundary in this direction was apparently within the present land area of France and Belgium, while a narrow inlet may have run westwards between France and England in the present position of the Strait of Dover. Meanwhile immense quantities of ice detritus from Scotland and Scandinavia were deposited in the North Sea, to a thickness of perhaps 600 ft., and the whole region was subsequently raised above sea-level, constituting the “structural surface” upon which the present river system was developed as a series of tributaries to a great river which formed a continuation of the Rhine. Finally the land subsided again, the plain of the North Sea was again submerged, and the western inlet of Pleistocene times became the Strait of Dover.

For reasons which will be sufficiently obvious from the historical sketch just given, the coasts of the southern part of the North Sea are of no great height. In England they consist of low cliffs with sandy beaches, while on the continental side are immense flats and marshes, with parts below sea-level protected by sand-dunes and artificial dykes. Suess has shown that no evidence is forthcoming of tectonic movement since the Bronze Age, and the rapid changes of coast-line now taking place in many parts are therefore wholly due to the action of the sea, which is probably specially effective on account of the relatively recent opening of the Strait of Dover. The erosion of the North Sea coasts has been made a subject of minute study (in England especially by the British Association and a committee of the Royal Geographical Society), and Harmer has obtained interesting results by comparing the British and Continental coasts as characteristic “weather” and “lee” shores.

The physical conditions of the waters of the North Sea have been extensively studied by expeditions sent out by the Swedish, Norwegian, Danish, German and British governments; and since 1902 by the International Council for the Study of the Sea, which owes its origin mainly to the work of the earlier expeditions. Professor Pettersson of Stockholm, to whose initiative much of this work is due, classifies the waters found in the North Sea as follows: (1) oceanic water of 35 pro mille salinity or more; (2) water of salinity 34 to 35 pro mille, called “North Sea” water; (3) water of salinity 32 to 34 pro mille, found along the coasts of Holland, Germany, Denmark, and Norway, and called “bank-water”; (4) water of 32 pro mille salinity or less, belonging to the stream flowing out from the Baltic. Of these (1) and (4) are to be regarded as “in-flowing” waters, while the others are due to mixture, which may or may not take place in the North Sea itself. The oceanic water consists of a mixture of waters of Atlantic and Polar origin; it enters the North Sea from the north-west partly from the Norwegian sea, and partly from the Faewe channel by the passage between the Orkney and Shetland islands, and makes its way southwards along the coast of Scotland, especially during the early summer months.

The International Council, and more particularly the North Sea Fisheries Investigation Committee of the Fishery Board for Scotland, have studied the periodic and irregular variations in the distribution of these waters in minute detail; and the results, extending and confirming the observations of the earlier observers, have established the conclusion that the supply of fresher coastal waters from the land on both sides of the North Sea is greatest in late summer, after the occurrence of the maximum inflow of oceanic water. The autumn and early winter months accordingly represent a period of mixing rather than of inflow, and this mixing is clearly an extremely complicated process, depending on the relative amounts of the mixing waters (which are themselves liable to great variation), on their temperature and salinity, and also on the action of winds and tides. In the southern part of the North Sea area tidal action alone is sufficiently vigorous to ensure complete mixing of the waters from surface to bottom at all times.

The tides of the North Sea are of great complexity, and have not been fully investigated. The tidal wave of the Atlantic enters by the Strait of Dover and by the channels in the north. In the latter place a division into two parts takes place, one wave travelling southwards along the coast of Scotland in comparatively shallow water, while another moves with greater speed across the deeper water to the Norwegian Channel, and thence southwards to the Skagerrack and the Danish coast. The southwards-moving waves are greatly retarded in the shallow water over the Dogger Bank; the trough of the “Silver Pit” accordingly gives the Scottish wave a strong easterly component, and the three systems—the Scottish, Norwegian and Channel waves—meet to the east of the Dogger, producing complicated interference phenomena. Along the English coasts the tidal streams are for the most part normal, the flood stream running south to south-east and the ebb north to north-west, but on the Continental coast the movements become very complex on account of the varying influence of the waves from different sources.,

The North Sea is particularly rich in organisms of all kinds, and the abundance of food attracts fish in such quantities that the North Sea fisheries are the most productive in the world. Flat fishes, and those feeding at the bottom on smooth ground, are chiefly caught by means of the trawl. The favourite trawling-grounds are the Dogger Bank in winter, and the shallow waters off the Continental coasts in summer; these yield halibut, soles, turbot, brill, plaice, cod, haddock, whiting, &c. In rough ground where the trawl cannot be used, hook- and line-fishing are carried on most successfully, and “mid-water” fish are also taken in this way, although the trawl and line-fishing overlap considerably. Herring and mackerel are caught by means of drift-nets. The herring fishing off the British coasts exhibits a remarkable variation during summer and autumn, beginning in Shetland in June, and becoming progressively later southwards, until it ends off the Norfolk coast in November. Various attempts have