The coast of the Baltic is rocky only in the island-studded region at the head of the Baltic basin proper—a submerged lake-district—and the littoral generally is a typical morainic land, the work of the last great Baltic glacier. The southern margin of the Baltic is of peculiar interest. From Schleswig eastwards to Lübeck Bay the coast is pierced by a number of narrow openings or Fohrden, the result of encroachment of the sea caused by subsidence. East of Lübeck, as far as the mouth of the Oder, these give place to Bodden, ramified openings studded with islands: the structure here resembles that of Scania in southern Sweden, a region once joined to both Denmark and Pomerania by an isthmus which was severed by tectonic movements. Beyond the Oder the coast-line is unbroken as far as the Gulf of Danzig. It is then cut into by the estuaries of the Vistula, the Pregel and the Memel. Here the westerly winds have full play, and the coast is rimmed by a continuous line of dunes, which cut off the two great lagoons of the Frisches Haff and Kurisches Haff by sandspits or Nehrungen.
The drainage area of the Baltic is relatively large. According to the measurements of Sir J. Murray it extends to 461,450 sq. sea m. ( = 611,700 sq. English m.) The largest river-basin included in it is that of the Neva in the east, and next in size come the Vistula and the Oder in the south. The narrow parallel troughs, at right angles to the coast, which form the drainage-system of Sweden and western Finland, are a remarkable feature.
Levellings from Swinemünde show that the mean level of the Level. surface of the Baltic at that point is 0.093 metres (= .305 ft.) below the surface of the North Sea at Amsterdam, and 0.066 metres ( = .216 ft.) below its level at Ostend. A line of levels from Swinemünde through Eger to the Adriatic showed the mean level of the surface of the Baltic to be 0.499 metres (1.6 ft.) above that of the Adriatic Sea. The mean level of the surface of the Baltic rises about 0.5 metres (1.6 ft.) from the coast of Holstein to Memel, probably as a result of the prevailing westerly winds; this mean difference is exceeded with strong westerly winds, and disappears or is reversed with easterly winds. The waves of the Baltic are usually short and irregular, often dangerous to navigation. Destructive waves, probably caused by distant earthquakes, called Seebaren (cf. English "bores") have been recorded.
The range of the tides is about one foot at Copenhagen; within the Baltic proper ordinary tides are scarcely perceptible. There is, however, a distinctly marked annual rise and fall due to meteorological influences having a mean range of about 11.4 cm. (0.37 ft.), at Travemünde, and 13.9 cm. (0.46 ft.) at Swinemünde, the maximum occurring at the end of the summer rainy period in August.
The circulation of water in the Baltic proper must be considered Circulation. apart from the circulation in the channels connecting it with the North Sea; and in this relation the plateau connecting the islands Falster and Möen with the coast of Mecklenburg and Rügen must be taken as the dividing line. In the great basins and hollows from Rügen to the Gulfs of Bothnia and Finland the upper layers of water, from 30 to 70 metres (16 to 38 fathoms) in thickness, have almost the same salinity throughout. In these waters a vertical circulation is kept up by convection currents. Beneath these layers are masses of salter water, through which a thermal wave of small amplitude is slowly propagated to the bottom by conduction. These strata are practically stagnant, deficient in oxygen and surcharged with carbonic acid. Their salter waters must have been originally derived from outside, and must therefore have passed over the plateau between Falster and Mecklenburg, but their horizontal extension is checked by the ridges separating the deep hollows in the Baltic from each other. The inflow to the deep basins is intermittent, probably with a long period of flux and reflux.
The circulation in the channels connecting the Baltic proper with the North Sea is of a complex character. It is necessary in the first place to distinguish clearly between outflowing and inflowing waters; in practice this is easily done, as the outflowing water always contains less than 30 parts pro mille of salt, and the inflowing water more than 32 pro mille. The Baltic receives much more water by rainfall, discharge of rivers, &c., than it loses by evaporation; hence a surplus must be got rid of by an outflowing current which may be named the "Baltic Stream." The following general laws may be laid down with regard to this:—
1. That the Baltic Stream must be a surface current, because it originates from a redundancy of fresh water.
2. That, on account of the earth's rotation, the main part of the Baltic Stream must keep close to the coast of the Scandinavian peninsula.
3. That it must be a periodic stream, because the discharge of the rivers into the Baltic varies with the season of the year. In spring and summer the water from the Baltic is sufficiently abundant to inundate the whole surface of the Kattegat and Skagerrak, but in winter the sources of the Baltic current are for the most part dried up by the freezing of the land water.
All the waters which enter the Skagerrak or Kattegat as undercurrents can be found at the surface of the North Sea (q.v.). They may be divided according to their origin and salinity as follows:—