SECT. 3] 



ESTUARIES, DELTAS, SHELF, SLOPE 



629 



this book). In the coastal marshes and estuaries bordering the North Sea from 

 Zeeland to the Elbe mouth, the combination of subsidence and eustatic shifts 

 of sea-level has resulted in most complicated stratigraphy. The marine deposits 

 related to the last two interglacial periods, which are referred to as the Hol- 

 steinian (or Needian) and the Eemian deposits (Woldstedt, 1950), are depressed 

 under the marshes, while they stand as subaerial terraces in coastal areas where 

 no subsidence occurs. During the last 10,000 years — the Holocene — the follow- 

 ing strata were laid down in order (Fig. 7): a layer of peat (the lower peat), 

 deposited before the post-glacial sea-level was high enough to reach these 

 countries; the older tidal flat deposits and marine clays, deposited behind 

 barrier beaches; the upper peat, which may tentatively be related to a fall of 

 sea-level by a few metres after the post-glacial climatic optimum; and young 



.Young dunes 

 sand bars 



Young marine cloy 



X A A '^ ■^ X 



X ^ Pleistocene sand 



Lower peat 



Fig. 7. Diagrammatic section through tlie Holocene deposits of Holland. (Simplified from 

 Pannekoek, 1956.) 



marine clays and sands, resulting from the fact that the small lowering of sea- 

 level was finally surpassed by the subsidence, which led to invasions of the peat 

 bogs by the sea. Such alternations of marine and terrestrial deposits, with peat 

 layers in the sub-surface, have morphological consequences in the coastal 

 marshes (see below). 



If we consider now the processes acting today, we must describe separately 

 the deltas, the estuaries and the related marshes. 



a. Deltas 



Deltas (see, for example, Russell and Russell, in Trask, 1955) consist of 

 a complex of distributary channels, natural levees bordering the channels, and 

 shallow lakes or swamps lying between them at a slightly lower level. The 

 coarser elements are concentrated in the channels and on the crests of natural 

 levees; the finer ones are found in inter-levee lakes or swamps, where clay can 

 settle to the bottom in quiet waters. Organic sedimentation is also important in 

 this environment. As the lakes are slowly filled up, they gradually enlarge 

 by wave erosion along their banks, and their contours become more and more 

 regular and rounded. During floods, crevassing can occur in the levees, so that 

 the quiet sedimentation in the lakes is disturbed and coarser layers consisting 



