North Sea

Geology

The extent of the North Sea and the level of its water surface have varied considerably over geologic time. At the end of the Pliocene Epoch (about 2.6 million years ago), the North Sea basin south of Dogger Bank was part of the European mainland, and the Rhine River—joined on its left bank by the Thames—emptied into the sea about 250 miles (400 km) north of present-day London. During the Pleistocene Epoch (about 2,600,000 to 11,700 years ago), ice sheets advanced and retreated several times and deposited a thick layer of clay on the seafloor. At the time of the greatest advance, the ice covered all of the North Sea from a line joining the Thames estuary with the Dutch coast. The final retreat of the glaciers took place about 8,000 years ago; some centuries later, as its level continued to rise, the expanding sea broke across the land bridge that, until then, had linked Britain with France, and the waters of the North Sea joined with those of the English Channel. The present coastlines of the North Sea were probably not established until some 3,000 years ago.

Physiography

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Few parts of the North Sea are more than 300 feet (90 metres) in depth. The floor dips to the north and is generally irregular. In the south, depths measure less than 120 feet (35 metres); many shallow, shifting banks, presumably of glacial origin, have been reworked by tidal currents. These present serious navigational hazards. Off northern England the vast moraine (glacial deposit of earth and stones) known as Dogger Bank is covered to depths of only about 50 to 100 feet (15 to 30 metres). This is the location of one of the finest fishing areas in the sea. In contrast, the waters deepen in the Norwegian Trench, an unusual depression that runs parallel to the coast of southern Norway from north of Bergen around to Oslo. It is between 15 and 20 miles (20 to 30 km) wide and is some 1,000 feet (300 metres) deep in the vicinity of Bergen, reaching a maximum depth of about 2,300 feet (700 metres) in the Skagerrak. There also are some deep trenches in the western part of the North Sea, including Devils Hole off Edinburgh, where depths exceed 1,500 feet (450 metres), and Silver Pit, nearly 320 feet (95 metres) deep, off the bay of The Wash in England. These trenches may have been formed at the time of the last glaciation, when parts of the North Sea were free of ice, and rivers coming off the mainland could have eroded deep channels in the basin floor.

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Water and its Varying Forms

There is a marked difference between the rugged upland coasts of the north and the regular lowland coasts of the south. The glaciated mountain coastline of Norway north of Stavanger is broken by fjords, headlands, and an offshore fringe of thousands of rocks and islands. Below Stavanger the coast is less precipitous, and there are fewer islands. Scotland’s east coast also is composed of uplands—although it is less broken—and the resistant rocks continue south into England. In the vicinity of Flamborough Head in northeastern England, the cliffs are lower, and their less-resistant clays are subject to extensive erosion. In the Fens area of East Anglia, the coast is low and marshy, as it is in the delta region of the Netherlands. Most of the southern and southeastern coast is straight and sandy. The low, offshore Frisian Islands stretch from Waddenzee in the Netherlands to southwestern Denmark.

Hydrology

The North Sea waters are affected by the warm North Atlantic Current, which moves northward along the western side of the British Isles and enters the Norwegian Sea. Atlantic waters with salinities exceeding 35 parts per thousand enter the North Sea through the English Channel and between the Shetland Islands and Norway. Colder, less-saline waters come from the Baltic Sea through the Skagerrak, creating a counterclockwise circulation in the basin. Salinities generally range between 34 and 35 parts per thousand, with higher readings occurring off the coast of Great Britain and lower readings off Norway. Large quantities of fresh water also enter through the Rhine, Thames, and other rivers.

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Average January temperatures of North Sea surface waters range from 35 °F (2 °C) to the east of Denmark to 46 °F (8 °C) between the Shetland Islands and Norway. In July, coastal water temperatures from the Strait of Dover to Denmark exceed 59 °F (15 °C), while in the Orkney-Shetland region they reach only 54 °F (12 °C).

Climate

Average air temperatures vary in January from 32 to 40 °F (0 to 4 °C) and in July from 55 to 64 °F (13 to 18 °C). Winters are stormy and gales are frequent. Tidal ranges average between 13 and 20 feet (4 and 6 metres) along the coasts of Britain and in the southern estuaries, while the range to the north and east is less than 10 feet (3 metres). Because of the low-lying nature of much of the southern coast, abnormal tides can be disastrous. In 1953 a storm surge of nearly 11 feet (3.4 metres) above the mean high-water level inundated large areas of the delta region of the Netherlands. In England, surges up the Thames estuary have also affected surrounding lowlands and, prior to the construction of the Thames Barrier, once posed a threat to London itself.