Why is the High Coast high and the Kvarken archipelago low? This is a much older story than the latest Ice Age. About 600 million years ago, the High Coast and the Kvarken archipelago were part of a same flat surface called a peneplain. Tensions in the Earth’s crust caused faults or shifts, and some of the crust was pushed upwards, including the area we now call the High Coast. 

The bays and valleys of the High Coast were formed during millions of years, when cracks in the bedrock wore down by erosion. This happened because the high landscape was much more exposed to erosion than more flat areas. The tops of the High Coast mountains are the remains of the peneplane from 600 million years ago. The High Coast is the only area around the Baltic Sea where high hills reach to the sea.

In the Kvarken Archipelago, the old peneplane still remains. That is why the Kvarken Archipelago is so flat. The highest point of the archipelago is just over 20 metres above sea level, and water depth is 25 metres at its most. In comparison, the waters of the High Coast can be 290 meters deep and the highest points reach 350 meters.