ABSTRACT

The tidal flats on the southeastern border of the North Sea are developed behind a chain of barrier islands. In this area the grain size of the sand decreases from the inlets towards the inner shores and the tidal watersheds behind the islands. Likewise the clay content increases, so that muddy sands and even sandy muds are formed. This condition is explained by the action of tidal currents combining with other, mainly biological phenomena. Pellets and flakes are formed and these are carried inwards in consequence of the "settling lag effect" and the "scour lag effect." In estuaries and tidal lagoons similar conditions may be responsible for lateral changes in clay content. In ancient basins such lateral increase in fine grained material may indicate approach to the mainland shore and increasing distance from passes between sandy barrier islands. This study is an elaboration of suggestions offered by Postma (1954). A more detailed account with extended bibliography and frill acknowledgements has been given in an earlier paper (van Straaten and Kuenen, 1957).