The stereotypical topographic feature of historical Ostrbothnia was the coastal plain north of Suomenselkä, in the modern regions of Ostrobothnia, Southern Ostrobothnia and Central Ostrobothnia. It is glacial (Weichselian glaciation) seabed, flat with a few hills, and characterized by rivers formed at the end of the glaciation period which run southeast to northwest. Post-glacial rebound is still causing the ground level to rise at a rate of about 9 mm per annum. Due to the flat terrain, this causes the Gulf of Bothnia to recede several kilometers each century, impacting navigation and the harbours of coastal cities in the region. The bedrock in the area, where exposed, features southeast-northwest oriented lines carved by rocks pushed by the advancing ice mass during the glaciation. When the ice melted, huge boulders (glacial erratics) and sand pits were also left behind.