NORTH SEA, a sea bounded east by the continent of Europe and west by Great Britain. Its area was given by Murray (1888) as 162,600 sq.m., and by Kossinna (1921) as 575,00o sq.km., or 222,000 sq. statute miles. Murray estimated the volume of the North sea at 11,200 cu.m., and Kossinna at 54,000 cu.km., or 12,960 cu.m., giving mean depths of : 61 fathoms (112 metres) and 51.4 fathoms (94 metres) respectively. The North sea is thus 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 south. In the south and east there is a broad coastal strip, over which the depth nowhere exceeds 4o metres. 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 20 metres and it increases eastwards to about 4o metres. In one part of the Dog ger, where the depth is not more than 13 metres, surf is observed in stormy weather as well as dirty water owing to the bottom deposits being stirred up. 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." A bottom configuration strange to the bed of the North sea is known as "Norwegian chan nel." This is a gully broad and more than 2 5o metres deep, closely following the Scandinavian coast, and extending into the Skagerrack, in which the depth increases to as much as 66o metres.
According to Jukes-Browne, the North sea, in its present form, first took shape as a result of the tectonic move ments indicated by the break between the older and newer Plio cene 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 with in 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 200 metres, 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.
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. It is not certain whether the rapid changes of coast-line now taking place in many parts are wholly due to the action of the sea or whether a secular sinking of the shores pro duces this effect. 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 com paring 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—since 1902 by expeditions sent out by the Swedish, Norwegian, Danish, German and British governments—under the supervision of the International Council for the study of the Sea (Copenhagen). The waters found in the North sea alcz classified as follows : (I) oceanic water of 35 pro mille salinity or more; (2) water of sa linity 34 to 35 pro mille, called "North sea" water; (3) water of salinity 32 to 34 pro mille, found along the coasts of Holland, Ger many, 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. The Atlantic water enters the North sea by the passage between the Orkney and Shetland islands, in which way the whole volume of water in the North sea could be re cruited in two years. Oceanic or Atlantic water also enters through the Strait of Dover; this inflow is, however, much smaller, and it would take some 20 years to fill the North sea.