The Gulf of Finland

The Gulf of Finland is the easternmost gulf of the Baltic Sea, surrounded by Estonia, Finland and Russia. An imaginary line between the Põõsaspea Cape in Estonia and the Hanko Cape in Finland marks its western border. The area of the gulf is 30 000 square kilometres. It is shallow, averaging 40 m in depth, but its floor has a very uneven topography. Even in the middle of the gulf, the water is sometimes only a few metres deep (the Savinkov Shallow). The maximum depth — 121 m — is registered southeast of the Keri Island.

The coastline of the northern coast is fjord-like: narrow inlets of sea alternate with the rocky promontories of the crystalline basement; there are also many small rocky islands. The southern coast is cut into the Palaeozoic sedimentary rocks. The North-Estonian Klint borders the gulf depression from the south. The upper part of the escarpment, exposed above sea level, continues under the sea, with one or several terraces, to the depth of 100 to 150 m below sea level

The depression of the Gulf of Finland and the North Estonian Klint have been shaped by the erosion of the pre-Quaternary large rivers. The soft Vendian and Cambrian clays and sandstones that were once exposed in the southern part of Finland and everywhere in the bottom of the Gulf of Finland, were eroded. The erosion was hindered by stronger Ordovician limestones, forming a high klint terrace and protecting the underlying softer rocks from rapid erosion. This ensured the preservation of the North-Estonian plateau and the North-Estonian Klint. The klint margin and the terraces were eroded by continental ice. The Baltic Ice lake, formed after recession of the glacier and the subsequent development stages of the Baltic Sea have washed the base of the klint, subjecting it to continuous slow erosion.

The modern coastline of northern Estonia reflects the fields of NW–SE directional drumlins of the bedrock topography. West of Tallinn, the largest peninsulas are the Pakri Peninsula and the Suurupi Peninsula. The cluster of small bays between the Suurupi and Viimsi peninsulas is called Tallinn Bay. The largest island in this bay is the Naissaar Island.

East of Tallinn and west of Lahemaa are the Viimsi and Ihasalu peninsulas and the Ihasalu and Kolga bays. The Aegna Island is located north of the Viimsi Peninsula. Several small islands in the Kolga Bay include Aksi, Koipse and Rammu islands, and towards the north, in the open sea, the Prangli and Keri islands. A neighbouring part of the coastline in the east, known as Lahemaa, includes the Juminda, Pärispea, Käsmu and Vergi peninsulas and the Hara, Eru and Käsmu bays. The largest island north of the coast is the Mohni Island.

East of Lahemaa, the coastline is close to straight; the only major deviations being the Letipea Peninsula and the Kunda Bay near Kunda. The Vaindloo Island — the northernmost island of Estonia — is located north of the Letipea Peninsula.

The waters of the Gulf of Finland are among the freshest in the Baltic Sea. The salinity of the brackish waters decreases from 6 ‰ in the western part to 2 ‰ in the eastern part of the gulf. The largest fluctuations of the water level occur in the eastern part of the gulf. When strong western winds have caused floods in St Petersburg, the water level at the Estonian coast has usually risen 1.5 to 2 m above the average sea level.

As the Gulf of Finland is continental and with rather low salinity, its eastern part is covered by ice during some winter months. This can disturb shipping routes during the winters. Along the northwestern coast of Estonia, the ice period is usually one month shorter than in the Narva Bay. During warmer winters, there is no ice cover in northwestern Estonia.

The largest harbours of the Gulf of Finland are Helsinki in Finland, St Petersburg in Russia and Tallinn in Estonia. Favourable ice conditions and deep bays give good perspectives to the Paldiski harbour. A noteworthy freight harbour is Kunda. Several small harbours are available for international use.

Cod, sprat and Baltic herring are commercially fished in the Gulf of Finland.

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