When the area of the present Baltic Sea was covered by a huge ice sheet during the Weichselian glaciation, about 20,000 to 70,000 years before present, the edge of the ice moved on land as tongues of glaciers; these carved out channels.
Some of these Förden and fjorde are believed not to have been carved out by the ice directly, but to have been washed out by flows of water below the ice (tunnel valleys).