Tallinn
Geography

Tallinn is situated on the southern coast of the Gulf of Finland, in north-western Estonia.

The largest lake in Tallinn is Lake Ülemiste (9.44 km (3.6 sq mi)), which serves as the main source of the city's drinking water. Lake Harku is the second-largest lake within the borders of Tallinn and its area is 1.6 km (0.6 sq mi). The only significant river in Tallinn nowadays is the Pirita river, in the eponymous Pirita city district. Historically, a smaller river, called Härjapea, flowed from Lake Ülemiste through the town into the sea, but the river was diverted into underground sewerage system in the 1930s and has since completely disappeared from the cityscape. References to it still remain in the street names Jõe (from jõgi, river) and Kivisilla (from kivi sild, stone bridge).

The length of the seaside coast is 46 km (29 mi), comprising three larger (Kopli, Paljassaare, and Kakumäe) peninsulas. The city has a number of public beaches, including those at Pirita, Stroomi, Kakumäe, Harku, and Pikakari.

The highest point in Tallinn, at 64 m (about 200 ft) above sea level, is situated in Hiiu, Nõmme District, in the south-west of the city. A large limestone cliff runs through the city. It can be seen at Toompea, Lasnamäe, and Astangu. However, the hill at Toompea is not geologically part of the larger limestone cliff.

The rocks and sediments underneath Tallinn are of different composition and age. Youngest are the Quaternary deposits. The materials of these deposits are till, varved clay, sand, gravel, and pebbles that are of glacial, marine and lacustrine origin. Some of the Quaternary deposits are valuable as they constitute aquifers, or as in the case of gravels and sands, are used as construction materials. The Quaternary deposits are the fill of valleys that are now buried. The buried valleys of Tallinn are carved into older rock likely by ancient rivers to be later modified by glaciers. While the valley fill is made up of Quaternary sediments the valleys themselves originated from erosion that took place before the Quaternary. The substrate into which the buried valleys were carved is made up of hard sedimentary rock of Ediacaran, Cambrian and Ordovician age. Only the upper layer of Ordovician rocks protrudes from the cover of younger deposits, cropping out in the Baltic Klint at the coast and at a few places inland. The Ordovician rocks are made up from top to bottom of a thick layer of limestone and marlstone, then a first layer of argillite followed by first layer of sandstone and siltstone and then another layer of argillite also followed by sandstone and siltstone. In other places of the city, hard sedimentary rock is only to be found beneath Quaternary sediments at depths reaching as much as 120 m below sea level. Underlying the sedimentary rock are the rocks of the Fennoscandian Craton including gneisses and other metamorphic rocks with volcanic rock protoliths and rapakivi granites. These rocks are much older than the rest (Paleoproterozoic age) and do not crop out anywhere in Estonia.
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