EPICOxNTINENTAL SEDIMENTATION 493 



The Dutch flats are situated between a row of islands and the 

 shore of the mainland. There are watersheds that connect these 

 islands with the coast. Here the ebb waters diverge and the 

 watersheds are the first parts to emerge during ebb tide. Hence, 

 each inlet supplies the tidal waters for the area between two 

 watersheds. Each inlet has a major channel 20 to 50 m deep with 

 strong currents and a coarse bottom co\'er of shells, pebbles, clay 

 pebbles, and sand. This channel branches inward in all directions 

 until small creeks are found which may run practically dry at 

 low tide and reach close to the mainland coast and to watersheds. 

 The bulk of the area is occupied by flats which fall dry at low tide. 

 These are composed dominantly of sand, but certain areas are 

 more muddy. 



During the last centuries the land has been protected by dykes. 

 Up against these the flats tend to collect fine sediment, and now 

 and then new polders adjoining the land have been dyked in. 

 Extensive mussel banks and a dense population of various shells 

 living below the surface of the sand flats have an important func- 

 tion because they filter finely distributed suspended clay out of 

 the water entering from the open sea and collect it in faecal and 

 pseudofaecal pellets. These minute grains sink much more rapidly, 

 and the result of the presence of the shells is that the clay can 

 settle during slack water and tends to accumulate in the tidal flat 

 area. Natural and artificial vegetation close to the shore enhances 

 the accumulation ot sediment. Another form of coagulation is the 

 production of floccules by chemical and biological influences. 

 Compaction of clay beds, growth of algal films on the bottom, and 

 reduction of wave action are three more factors promoting the 

 accumulation of clay in the tidal flats. 



If there were no other mechanism at work, the result would be 

 to cause a higher clay content in the deposits than is found out- 

 side the row of islands. But once a certain stage had been reached, 

 a balance should be struck between accumulation and dissipation. 

 Ingoing and outgoing waters would contain an equal amount of 

 suspended clay. 



Two facts show that the situation is more complicated. One is 

 that for long periods of the year a marked storage of mud may be 

 seen to take place. The other is that the concentration of suspended 



