Geological History
The Baltic Sea somewhat resembles a riverbed, with two tributaries, the Gulf of Finland and Gulf of Bothnia. Geological surveys show that before the Pleistocene instead of the Baltic Sea, there was a wide plain around a big river called the Eridanos. Several glaciation episodes during the Pleistocene scooped out the river bed into the sea basin. By the time of the last, or Eemian Stage (MIS 5e), the Eemian sea was in place. Instead of a true sea, the Baltic can even today also be understood as the common estuary of all rivers flowing into it.
From that time the waters underwent a geologic history summarized under the names listed below. Many of the stages are named after marine animals (e.g. the Littorina mollusk) that are clear markers of changing water temperatures and salinity.
The factors that determined the sea's characteristics were the submergence or emergence of the region due to the weight of ice and subsequent isostatic readjustment, and the connecting channels it found to the North Sea-Atlantic, either through the straits of Denmark or at what are now the large lakes of Sweden, and the White Sea-Arctic Sea.
- Eemian Sea, 130,000–115,000 (years ago)
- Baltic ice lake, 12,600–10,300
- Yoldia Sea, 10,300–9500
- Ancylus Lake, 9,500–8,000
- Mastogloia Sea 8,000–7,500
- Littorina Sea, 7,500–4,000
- Post-littorina Sea 4,000–present
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