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Later the melting of the ice raised the sea level to its present mark, and the ecologically important land bridge across the Strait of Dover finally was submerged about 8,000 years ago.
Hurd Deep, is one of a group of anomalous deep, enclosed troughs in the bed of the western channel.
Precipitation averages 28 to 39 inches (700 to 1,000 millimetres) per year.
Depths range from 6 to 160 feet, with such elongated banks as the Varne and the Ridge greatly constricting shipping lanes.
Because the English Channel, unlike the Irish or North seas, lay beyond the action of Pleistocene glaciers, superficial deposits are either very thin (three feet or less) or entirely absent.
They represent a complex reworking of deposits of various ages, and their distribution reflects tidal streams.
Where the streams are strong, the seabed is bare except for pebbles; decreasing velocities give rise to sand and gravel ribbons and waves (the latter up to 40 feet thick) and to thick beds of fine-grained deposits in sheltered areas, notably the Gulf of Saint-Malo.
Surface salinities decline eastward from slightly less than the Atlantic level of 35.5 parts per thousand; coastal salinity readings are further reduced by the influx of river water, especially from the larger French landmass.