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How did sand dunes and shingle bars form lagoons along the German and Polish Baltic coasts?

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Map showing the physical features, major cities, and national boundaries around the Baltic Sea, North Sea, and English Channel regions of northern and western Europe. The map highlights coastal countries including the United Kingdom, Norway, Denmark, Germany, the Netherlands, Belgium, France, Sweden, Finland, Estonia, Latvia, Lithuania, Poland, and Russia, with sea depths indicated and key islands, peninsulas, and underwater trenches labeled. — Baltic Sea The Baltic Sea, the North Sea, and the English Channel.

Along the German and Polish Baltic coasts, lagoons—known locally as Haffs—formed through the interaction of wave action, sediment supply, and post-glacial sea-level changes.12

The process involves the following mechanisms:

Wave-Built Barriers: Following the retreat of the ice sheet and subsequent rises in sea level, wave action along the flat, low-lying southern Baltic coast transported and deposited bottom material, such as sand and shingle (pebbles).123
Formation of Spits and Bars: This material accumulated to form long, narrow shingle bars (Nehrungen) and spits that extend along the coastline. These features act as protective barriers, cutting off shallow coastal indentations from the open sea.134
Dune Development: Once these barriers were established, wind action further shaped the landscape by piling sand atop the shingle spits to create moving dunes, which helped stabilize and raise the height of the barriers above the sea level.123

These processes effectively isolated bodies of water, such as the Szczeciński Lagoon and the Vistula Lagoon, from the Baltic Sea.125