Fluctuating shorelines

Isobases for recent movements in the Baltic Sea region. From Ekman 2001.

Isobases for recent movements in the Baltic Sea region. From Ekman 2001.

Baltic Sea is the largest body of brackish water in the world. It occupies a basin formed by glacial erosion during the last few ice ages. As the Baltic Sea does not come from the collision of the earth plates but is a glacially scoured river valley, it is a shallow sea with an average of only 55 m of depth. And the reason for the very existence of this sea – the Ice Age is actually constantly present in the archaeology of the region, no matter of the time period of study. This is because of the Ice Age being the reason for the phenomenon called shore displacement that is constantly changing the shorelines of the Baltic Sea region. During the Ice Age, the earth’s crust was pressed down by the ice, and after the melting of the ice, the crust is striving to attain its former level. Thus, the shore displacement comprises changes of the earth crust but also changes of the world sea (isostatic resp. eostatic changes of the shoreline), which result in transgressions and regressions of the water level – in fluctuating shorelines. The ice-sheet was thickest and thereby heaviest on the territories of the central and northern Baltic Sea region and the land still rises in these areas; while on the southern territories of the Baltic Sea, eustatic changes, i.e. the rise of the water level has been and still is characteristic leading to the submerged shorelines.

Åland around 5000 BC. From Stenbäck 2003

Åland around 5000 BC. From Stenbäck 2003

Most of the coastal landscapes of the Baltic Sea region were ice-free from around 13.500 BC. But as large parts of Åland are very low – the highest point, Orrdalsklint, being only about 130 m above sea level – we can talk about first people on Åland only from the end of the Mesolithic period, from around 5500-5000 BC, when there was a land to dwell on and the sea stood about 55 m higher than today. From the cognitive point of view, it is quite so fascinating that one just has to go to the outer archipelago of the central or northern Baltic Sea region to meet a newborn landscape very similar to that of thousands of years ago met by first inhabitants of, among others, the Åland Islands…

Newborn landscape in the eastern archipelago of Sweden

Newborn landscape in the outer archipelago of eastern Sweden

The data for recent movements in the earth’s crust are known fairly exactly. It is on this basis, the reconstructions of the shorelines at different times are generated. For example, Åland is today experiencing isostatic rebound of approx. 5 mm per year meaning that 100 years ago the water level was about 50 cm higher and 1000 years ago it was about 5 m higher. However, the process of shore displacement through time has not been linear and steady, it varies between different places and the crust has moved irregularly. Furthermore, it has slowed down considerably if compared with the beginning of this process. Therefore, a certain level, say 5 m above sea level, isn’t characteristic for the whole of Åland at some point of time, there are local differences. And case-to-case approach must be applied in the archaeological studies on a more specific level (say, when you happen to be interested in finding out how close a certain longhouse-like feature in Saltvik really was to a site-contemporary shoreline).

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