Geology of Finland subsequent uplift and erosion would have eroded all of these sediments. In Finland, Caledonian nappes overlie shield rocks of Archean age. Despite occurring in about the same area the Scandinavian Caledonides and the modern Scandinavian Mountains are unrelated. The ice sheet that covered Finland intermittently during the Quaternary grew out from the Scandinavian Mountains. By some estimates the Quaternary glaciers eroded away on average 25 m of rock in Finland, with the degree of erosion being highly variable. Some of the material eroded in Finland has ended up in Germany, Poland, Russia and the Baltic states. Ground till left by
Finland–Sweden border in a topographical description made in 1811. The border was decided to run through the geographical centre of the island (defined by coordinates) in the treaty of Åland in 1921 and in the treaty of continental plates in 1972. The Märket lighthouse, built in 1885 by Finland and operated by them, was left on the Swedish side. In 1981, the border on the island was modified into a curve to get the lighthouse on the Finnish side. The border is agreed to be regularly inspected every 25 years. During the independence of Finland, inspections have been made in 1926–1927, 1956–1957,