Kvarken ( (as opposed to South Kvarken); ) is the narrow region of the Gulf of Bothnia separating the Bothnian Bay (the inner part of the gulf) from the Bothnian Sea. The distance from the Sweden to the Mainland Finland is around , while the distance between the outermost islands is only . The water depth in the Kvarken region is only around . The region also has an unusual rate of land rising at almost a year.
Several attempts to cross the strait swimming have been made, but cold water and currents have usually been insurmountable obstacles. The first successful crossing was carried out by Lennart Flygare, Pavio Grzelewski and Tore Klingberg, who on 24 July, 2018, swam from Valassaaret (Valsörarna) on the Finnish side to Holmöarna in Sweden. It took them 12 hours 2 minutes to cross the strait.
During the Ice age, the Kvarken region was located underneath the Fennoscandian ice sheet. When the ice sheet retreated from the area around 9,600 years ago, the land, which had been compacted under the weight of the ice, rapidly expanded in a process known as isostatic rebound. The Kvarken region is known for having some of the fastest rates of this uplift on Earth, having risen 285 meters since glacial retreat. It continues to rise, demonstrating the effects of deglaciation on flat and shallow archipelagos. The Kvarken Archipelago is also notable for its unique moraine formations (called de Geer moraines), which form regular clusters that are parallel to the ice front. It is estimated that within about 2,000 years, the seafloor in this area will rise above water, splitting the Gulf of Bothnia into a southern gulf and northern lake.
In 2006, parts of the Kvarken Archipelago were added as an extension to the World Heritage Site of the High Coast (located on the western shore of the Gulf of Bothnia) in Sweden, because of its outstanding demonstration of isostatic uplift, and the influence of glacial retreat on the evolution of landforms and topography. The most Finnish parts of the High Coast/Kvarken Archipelago World Heritage Site are situated in the Korsholm municipality.
In the group of islands in the “middle” of the Kvarken region, in Swedish called Valsörarna – Finnish Valassaaret, is a lighthouse designed by who worked for Gustave Eiffel's engineering bureau. The structural similarity between the lighthouse (built in 1885) and the Eiffel Tower (built in 1889) is quite obvious. The lighthouse is now automated as are most lighthouses in Finland.
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