During the following millennia, the tidal waters ebbed and flowed. The seas continued to rise slowly, and this meant that the water encroached further and further across the flat plains of the Wadden Sea. As waters flow inland, they deposit a sediment containing both sand and mud. The sand is the heaviest of these two. Therefore, it already begins to settle once the current slows down offshore. The mud, however, only settles once the waters come to a stop further inland. This is why sand is found at the shoreline and the more nourishing silt is found in the river valleys further inland.
A coastline built on mud
During the past 8-9,000 years, the sea level by the shores of southwest Jutland have risen by 17 metres. This means that during that same period, the sea and the pulse of tidal waters have deposited new coastline up to 17 metres thick. Drilling down through the layers of salt marsh has revealed that, under many metres of deposited silt-clay, there is a layer of peat and sand from a time before the tidal waters took over.
Rømø itself is an example of how tidal waters can create a coastline. Rømø is named after a so-called 'rimme', which originally described a low embankment off the coast. However, the layer of sand thickened as water flowed across the 'rimme'. The sand then swirled together to create the dunes that formed the island. There is so much lee behind the dunes that the water remained still long enough to deposit mud, and the salt marshes began to take shape. This is how tidal waters created the unique landscape of the Wadden Sea.
Storm surges and dykes
The deposited clay soil is rich in nutrients. This is why the salt marshes are so rich compared to the moorland plains further inland. Throughout the ages, the salt marshes have been the bread and butter of the relatively large population living by the coastline of the Wadden Sea.