SECT. 3] 



ESTUABIES, DELTAS, SHELF, SLOPE 



625 



sand fraction may be very large, and it decreases on tidal fiats and salt marshes, 

 where much higher clay contents occur. Examples of this distribution are found 

 in the Wadden Sea, West Friesland (Fig. 6), off Cuxhaven, East Friesland 

 (Gellert, 1952), and in North Friesland (Hansen, 1951). True sand may be seen 

 on parts of the outer watts bordering the North Sea and the English Channel, 

 and also on tidal flats behind sand-spits in Kerry, Ireland (Guilcher and King, 

 1961). This is connected with processes of deposition which are considered 

 below. As a consequence of the wide range of the grain size in muds, the sorting 



Fig. 6. Lutite {<2 y.) contents of sediments in Lauwerszee, Wadden Sea, West Friesland: 

 general inci'ease towards the coasts and the watersheds. (From Van Straaten and 

 Kuenen, 1957.) 



Legend— 1: 0-li%; 2: 1^-3%; 3: 3-5%; 4: 5-8%; 5: 8-12%; 6: 12-17%; 7: 17- 

 25%; 8: >25%; 9: marshes. 



is always much poorer than on beaches and in submarine sands on the shelf 

 where strong currents act upon them. In deltas, the sorting is also poor, and the 

 grain size is even more variable than in estuaries: the sediments can include 

 large pebbles in Mediterranean deltas, such as the Var delta in south-east 

 France, while in the Mississippi delta the coarser fraction consists of fine sand. 

 On the other hand, the particles are not the same in all parts of the deltas, since 

 they depend on the processes of deposition (see below). 



Estuarine and deltaic sediments also contain an organic fraction, consisting 

 of algal particles, small pieces of roots, remnants of planktonic animals, and so 

 on. The organic fraction sometimes constitutes a large volume, and it leads, 

 either in fresh or in salt water, to the formation of suspended flakes in which 



