Geological History
Deglaciation following the Weichsel glaciation took place in the Lake Ladoga basin between 12,500 and 11,500 radiocarbon years BP. Lake Ladoga was initially part of the Baltic Ice Lake (70–80 m. above present sea-level), a historical freshwater stage of Baltic Sea. It is possible, though not certain, that Ladoga was isolated from it during regression of the subsequent Yoldia Sea brackish stage (10,200-9,500 BP). The isolation threshold should be at Heinjoki to the east of Vyborg, where the Baltic Sea and Ladoga were connected by a strait or a river outlet at least until the formation of the River Neva, and possibly even much later, until 12th century AD or so.
At 9,500 BP, Lake Onega, previously draining into the White Sea, started emptying into Ladoga via the River Svir. Between 9,500 and 9,100 BP, during the transgression of Ancylus Lake, the next freshwater stage of the Baltic, Ladoga certainly became part of it, even if they hadn't been connected immediately before. During the Ancylus Lake subsequent regression, around 8,800 BP Ladoga became isolated.
Ladoga slowly transgressed in its southern part due to uplift of the Baltic Shield in the north. It has been hypothesized, but not proven, that waters of the Litorina Sea, the next brackish-water stage of the Baltic, occasionally invaded Ladoga between 7,000 and 5,000 BP. Around 5,000 BP the waters of the Saimaa Lake penetrated Salpausselkä and formed a new outlet, River Vuoksi, entering Lake Ladoga in the northwestern corner and raising its level by 1–2 m.
The River Neva originated when the Ladoga waters at last broke through the threshold at Porogi into the lower portions of Izhora River, then a tributary of the Gulf of Finland, between 4,000 and 2,000 BP. According to some new data, it happened at 3,100 radiocarbon years BP (3,410-3,250 calendar years BP).
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