things by mineralogical analysis of sands, silts and clays. 



The direct source of the sand is the coastal barrier (notably its submerged 

 parts). This barrier has been built up during the Holocene, by wave action 

 transverse to the coast as well as by lateral beach drifting. The sand has there- 

 fore probably two original sources: a) Pleistocene sediments in the southern 

 North Sea basin, deposited by the Scandinavian glaciers, by meltwater streams 

 and by normal rivers, b) the Rhine (and Meuse) which brought sand to the coast 

 during (earlier stages of) the Holocene. The sand is transported into the Wadden 

 Sea area mainly by bottom traction. 



The clayey material of the Wadden Sea deposits is chiefly supplied out of 

 the stock of suspended material present in the North Sea waters, especially the 

 near-shore waters. This stock is presumably maintained a) by erosion of older 

 sediments cropping out on the sea floor, notably those on the submerged slope of 

 the coastal barrier, and b) by supply from rivers debouching into the North Sea; 

 the Rhine, the Meuse, the Scheldt, English rivers , in the first place the Thamesj 

 may also contribute, since a residual current exists in the North Sea which flows 

 counter-clockwise through its southern part. The nnechanism of concentration 

 of fine-grained material in the Wadden Sea area has been described elsewhere. 



The most important condition for the accumulation of the tidal flat deposits 

 is probably the presence of the coastal barrier, sheltering the area behind it 

 from the erosive effect of large sea waves. Cross sections at right angles to 

 the coast show that the top of the (terrestrial) Pleistocene sediments (or of the 

 thin early Atlantic peat or lagoonal deposits) which form the base of the tidal 

 flat sediments, slopes gently coastward. It seems to grade, beyond the coastal 

 barrier, into the present North Sea floor (Fig. 3). It therefore follows that 

 sedimentation in the Holocene was chiefly restricted to the area behind the 

 coastal barrier . As a matter of fact, Pleistocene and early Holocene deposits 

 crop out at several places on the North Sea floor. 



Geological information and carbon-14 datings show that tidal flat conditions 

 existed already in the Atlanticum. The evolution of the whole complex of coastal 

 sediments is thought to have been as indicated in Fig. 4. So long as the sea was 

 rapidly rising (during the Atlanticum) the coastal barrier could never become 

 very broad. Broadening of the barrier complex by addition of new beach ridges, 

 etc. could take place, however, during the subsequent stage, the Subboreal, 

 when the relative rise ' of the sea decreased considerably. Since the Roman 

 period the relative rise has probably increased again with resulting erosion of 

 the coastal barrier. The comparatively high sand dunes of the Dutch coast date 

 from this last period. 



The fact that the greater part of the present Wadden Sea is formed by tidal 

 flats, lying below the mean high tide level (Fig. 5) implies a certain lag of the 

 sediment supply by the flood currents. This supply is insufficient to raise the 



1) = absolute rise + subsidence of the land 



10 



