is a plain
, western Finland
, just 10 kilometres south of the town of Vaasa
. The plain is located in an impact crater
some 560 million years old (late Ediacaran
). The crater has a diameter of 6.6 km and a maximum depth of 300 metres. It is filled with Cambrian sandstones
leaving only the outer rim visible. There is also a central uplift, which is buried. After rising from the sea due to post-glacial rebound
, Söderfjärden was a wetland but was later drained. It is currently cultivated and is very clearly visible from air as a large, circular field. This makes Söderfjärden unique among the other impact structures in Finland which are at least partially under water.