(named after the mollusk Yoldia), the name for the phases in the development of the marine basin that occupied the Baltic Sea depression during its gradual liberation from the ice of the final Pleistocene glaciation. In the periods between these phases the basin was a freshwater lake fed by meltwaters of the retreating glacier (the Baltic Glacial Lake).
Three marine phases are distinguished. The first and second Yoldian seas owed their existence to the penetration of seawater from the east across the Baltic-White Sea divide; the third Yoldian Sea, the one usually meant when the term is used, was linked to the ocean in the west through what is now central Sweden. The end of the last phase of the Yoldian Sea coincides with the end of the late glacial period.