During the Ice Age, thick ice masses pressed down the earth's crust. When the ice melted, the land began to rise. Right after the ice melted this rising happened very quickly. The land rose the most in the Merenkurkku archipelago region and the least in southeast Finland. The different speeds of rising caused the land to incline toward the southeast. This leaning then caused changes in the water systems: the waters flooded the area where the land rose less steeply, the old outlet channels dried up and new ones were created in the flood areas. With the opening of new outlet channels and the rising of the land, the old banks were left on dry land. The oldest banks lean in the direction of the steepest land uplift and are situated highest above sea level.
During the Stone Age, the population settled in the treeless area between the shoreline and the edge of the forest. Because settlements were located right along the shore, we can determine their age by examining ancient shore surfaces. Dating settlements by means of ancient shores is called shore displacement chronology. The land continues to rise slowly. Today, in the Merenkurkku archipelago the land is rising about 9 mm a year, and in southeast Finland, about 2.5 - 3 mm a year.