Geologically the Karelian Isthmus lies on the southern edge of the Baltic Shield's crystalline bedrock. During the final part of the last Weichselian glaciation, deglaciation in the central parts of the Isthmus started as early as 14000 BP, when it formed the bottom of a large lake dammed by the surrounding ice sheet. During further deglaciation, at the time of the Baltic Ice Lake, an early high water stage of the Baltic Sea, when the ice sheet retreated to Salpausselkä, the upland area of the Isthmus remained a large island and many upland lakes emerged.