In the Paleozoic, 300–400 million years ago, all the delta region was covered by a sea. Modern relief, eminences, were formed by glacial scouring. Its retreat formed the Littorina Sea, the level of which was higher than its successor the Baltic Sea. Then, the Tosna was flowing in the modern lower half of the Neva as today, into the Litorinal Sea. In the north of the Karelian Isthmus, the sea was united by a wide strait with Lake Ladoga. The Mga then flowed to the east, into Lake Ladoga, near the modern source of the Neva. Thus the Mga then was separate from the Tosna/lower-Neva basin.