The Riedensee was formed after the last ice age as a shallow depression in a ground moraine area. As the glacier melted, meltwater sands up to five meters thick were deposited. The rise in sea level in the Littorina transgression led to permanent flooding and a beach lake was formed that was integrated into the compensating coast. The lake was separated from the Baltic Sea by a beach wall and only flooded during storms and the associated high water. The shallower areas of the lake became boggy with sedge and reed peats influenced by salt water and swamp forest. Due to coastal retreat over the last few millennia, these peats, up to five meters thick, are currently openly visible in the beach area.