Geological History
The lake is of glacial-tectonic origin and is a small remnant of a larger body of water which existed in this area during an Ice Age. In geologic terms, the lake is rather young, formed – like almost all lakes in northern Europe – through the carving activity of the inland ice sheets in the latter part of the last Ice Age, about 12,000 years ago: In Paleozoic Era (300–400 million years ago) the entire territory of the modern basin of the lake was covered with a shelf sea lying near the ancient, near-equatoric Baltic continent. Sediments at that time – sandstone, sand, clay and limestone – form a 200-meter thick layer covering the Baltic Shield which consists of granite, gneiss and greenstone. Retreat of the Ice Age glacier formed Littorina Sea. Its level was first 7–9 meters higher than at present, but it gradually lowered, thereby decreasing the sea area and forming several lakes in the Baltic region.
Read more about this topic: Lake Onega
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