It has been hypothesized .
It has been not proven , that waters of the Litorina Sea , the next brackish - water stage of the Baltic , occasionally invaded Ladoga between 7,000 and 5,000 BP .
It has been hypothesized , but not proven , that waters of the Litorina Sea , the next brackish - water stage of the Baltic , occasionally invaded Ladoga between 7,000 and 5,000 BP .