Page:VCH Norfolk 1.djvu/109

 ZOOLOGY MARINE ZOOLOGY THE marine fauna of Norfolk, as of East Anglia in general, presents several distinctive features of interest, in comparison with the fauna of other parts of the British coasts. The shore and sea bottom have an unusually uniform character, while the sea in this region is subject to an exceptional annual range of temperature. These two factors exert a marked influence in determining the character of the local fauna, both by limiting the number of common North Sea species fitted to live under these conditions, and by favouring the development of certain species to an unusual degree. The basin of the North Sea is geologically a great bight of the Norwegian Sea, and the great mass of its waters is still derived by tidal and drift currents from the northward, mixed with the cold waters which escape from the Baltic in spring after the melting of the winter ice and snow, and to some extent with the warm waters of the English Channel. The general North Sea fauna is, however, modified on the Norfolk coast by the shallowness of the bottom in the adjacent region of the North Sea. The North Sea is in fact cut obliquely into two portions by the Dogger Bank and its extensions towards the Yorkshire coast in the west and towards the Danish promontory in the north-east. This line, which may be called the Dogger ridge, separates a northern deep region (from 30 to 100 fathoms deep) from a southern shallow region, every- where less than 30 fathoms in depth. The trough of the deeper region is filled in summer with cold water, the temperature of which, in the height of summer, may be 1 2° or 1 3° F. less than that of the surface water in the same region, and even 1 5° or 1 6° F. below that of the water south of the Dogger. The presence of this underlying mass of cold water serves to moderate the temperature of the surface and coastal waters north of the Dogger, while the absence of such a layer south of the Dogger subjects the sea in this region to greater seasonal alternations of warmth and cold. The shallow shelving shores of East Anglia intensify this effect by exposing the coastal waters to the full influence of winter frosts and summer heat. Thus it results that the water along the coast of East Anglia is colder in midwinter and hotter in midsummer than along any equal stretch of coast in the British Isles. This feature must be a most im- portant factor in the physical conditions which determine the peculiar- 77