PHYSIOGRAPHY OF THE SEA-SHORE 31 



the swiftest tidal currents follow the deepest channels, and are 

 therefore not as directly effective in shore processes as when 

 they infringe upon an exposed portion of the coast . . . how- 

 ever . . . they remove vast quantities of debris originally 

 eroded from the land by wave action or carried to the sea by 

 rivers, and then transported by longshore currents . . . until 

 brought within the influence of the inflowing or outflowing 

 tidal current. The inflowing tide sweeps the finer material 

 far up the bay, where it is deposited in mud flats and tidal 

 marshes . . . while the coarser sand is moved landward a 

 much shorter distance, often forming bars along the channels. 

 In estuaries because of the river water usually poured into 

 a bay the ebb current predominates over the flood, and the 

 direction of debris migration is prevailingly seaward " 

 (Johnson, op. cit.). 



Circulation of British Waters. — According to Herd- 

 man (19 1 9) three masses of sea- water of diff'erent origin and 

 character may enter or affect the British seas in varying 

 quantity. 



1 . Arctic water, such as normally surrounds Iceland and 

 E. Greenland, and may extend towards Norway, the Faroes, 

 and Shetlands. 



2. Atlantic (or Gulf Stream Drift) which impinges on the 

 western shores of Ireland, and may flood the English Channel 

 and extend round the Shetlands or down into the North 

 Sea. 



3. Coastal water, such as flows out of the Baltic, and, 

 mixed with other waters, bathes the coasts of N.W. Europe 

 generally, and to a large extent surrounds the British Isles. 



The Irish Sea is regarded by Herdman as primarily an area 

 of coastal water which is, however, liable to be periodically 

 invaded to a greater or less extent by bodies of warmer and 

 Salter Atlantic water (reinforced possibly by a portion of a 

 deeper outflowing Mediterranean current) carrying in oceanic 

 plankton ; and more rarely, perhaps, by Norwegian or Arctic 

 water causing an invasion of Northern organisms. 



The variations which are found in different years in the 

 nature and amount of the plankton at the same localities no 



