THE BALTIC SEA 



275 



of the ice lake-sea (12,000 to 13,000 years ago). The great variety and contrasts 

 displayed by the different parts of the Baltic Sea shores are connected with the 

 difference of its geological structure and with the history of its development 

 in the Quaternary Period. The boundary of the crystalline rocks of the Baltic 

 icefoot is adjacent to the top 

 end of the Gulf of Finland. 

 The coastline of the northern 

 and western parts of the Sea 

 is formed of granites and 

 gneisses (Finland and Swe- 

 den). The Quarken shapes of 

 the southern shores of Fin- 

 land, the Aland Archipelago 

 and Sweden were formed 

 when this area was sub- 

 merged. The shores of Swe- 

 den belong to the fjord type. 

 All these shores are rising 

 at a high rate, especially at 

 the head of the Gulf of 

 Bothnia (up to 1 -2 cm annu- 

 ally). Palaeozoic deposits of 

 the Russian shelf are laid 

 bare at the southern shores 

 of the Gulf of Finland ; far- 

 ther south they drop below 

 sea-level. The whole coast 

 from the Gulf of Riga to 

 Jutland consists mainly of loose Quaternary deposits exposed to considerable 

 destruction by the sea. Large masses of alluvium formed during this process 

 are transported from west to east, and then from south to north. As a result, 

 large sand wash forms are created — the characteristic peninsulas and shoal 

 heads of the southern coast. Wind action leading to the formation of powerful 

 dune belts, at times up to 60 m high, is a feature of the southern and eastern 

 shores of the Baltic Sea. 



Fig. 1 1 6. Change of salinity with depth at the 

 seafloor of the Baltic Sea (Kiillenberg). 



Temperature 



The bottom topography described above, together with the contour of the 

 coastline, exerts a very strong influence on the hydrological conditions of the 

 Baltic Sea. It brings about a relatively small water-exchange with the North 

 Sea, the formation of considerable stagnant zones with poor development of 

 life in the deep hollows and, finally, the distinctive set of conditions in the 

 Gulf of Bothnia. In the first place temperature conditions are affected. During 

 the season of the year when the water column has its lowest temperatures 

 (February), the surface waters of the northern parts of the Baltic (Gulfs of 

 Bothnia and Finland) are below 0°. In the warmer southern parts, the tempera- 

 ture is slightly above 2°. The two northern gulfs have an ice cover over most 



