ORIGIN OF RECENT DUTCH TIDAL FLAT FORMATIONS 



by 



L. M. J. U. van Straaten 

 University of Groningen, Netherlands 



In the Dutch Wadden Sea ((the tidal flat area south of the Frisian (barrier) 

 Islands)), as well as in the estuaries in the southwestern part of the Netherlands, 

 three main environments can be distinguished (Fig. 1): 



1) Above mean high tide level: salt marshes, thickly covered by vegetation 

 and dissected by numerous meandering creeks. They form more or less 

 narrow strips along the inner shores or dykes of the barrier islands and 

 along the mainland shore. One small marsh island is present in the 

 central part of the Wadden Sea. It is a remnant of extensive salt marshes 

 which have been removed by erosion since the Middle Ages. The salt 

 marsh sediments are composed of clayey sands, sandy clays and clays. 



2) Between the lines of mean high and mean low tide: tidal flats . With the 

 exception of the highest parts bordering the salt marshes they are nor- 

 mally devoid of vegetation other than algae. Tidal flats occupy the main 

 part of the Wadden Sea area. They are dissected by tidal channels and 

 by small, tributary gullies. The central and higher parts of the flats are 

 usually very sandy, except along parts of the southern (leeward) shores 

 and in deep embayments where they are more protected from wave action. 

 The material of the lower parts of the flats, along the channels, ranges 

 in composition from pure sand to (sandy) mud. A muddy composition is 

 especially found where the channel banks are rich in gullies. 



3) Below the low tide level: channel floor environment. Sandy deposits pre- 

 dominate on the channel floors, but in the interior parts of the Wadden 

 Sea there may be much clayey material. 



The sedimentary structures in these environments are discussed in 

 detail in other papers by the author. 



Sediments with the same characteristics as the estuary and Wadden Sea 

 deposits are found in the low-lying, dyked areas along most of the Dutch coast. 

 The greater part of them has been formed in earlier Holocene (Atlantic and 

 Subboreal) stages. Most sections through the deposits of both these older and 

 the recent tidal flat environments show a sequence of a thick series of channel 

 floor material, covered by thinner series of tidal flat and salt marsh sediments. 

 These sequences are formed in successive channel systems, the more recent 

 ones cutting through the older ones (Fig. 2). 



The sediment of the recent Dutch Wadden Sea has been supplied by the 

 flood currents out of the North Sea environment. This is proven among other 



