SECT. 3] 



ESTUARIES, DELTAS, SHELF, SLOPE 



621 



tidal deltas and occur at inlets between barrier islands, as off North Carolina and 

 off Friesland (Fig. 1). They project inwards and outwards, and originate from 

 the slackening of the flood and ebb currents on both sides of the pass. Thus, 

 the concept of a delta is complex and comprehensive; protrusion forms are 

 diverse (Gulliver, 1898-1899); caution is necessary in definition. 



[ZH] Marsh 



Above 

 averoge low 

 water mark 



[^0 to -5m 



rrn-s to-iomi 



P77-10 to-20mJ 

 -20 to -30 m 



Fig. 1. Tidal delta and part of Wadden Sea in West Friesland. Inlet over-deepened by 

 tidal scour. (From Waddensymposivim, 1950, p. 448.) 



B. Description of Tidal Marshes 



A typical tidal marsh in temperate countries (Fig. 2) includes two main 

 parts. The lower part, which is exposed at low tide and is entirely under water 

 at high ordinary tide, is generally free of vegetation. It is referred to as the 

 slikke in international terminology. Sometimes, it bears a Zostera cover, or 

 mussel banks, or green algae, e.g. in the Netherlands and in Ireland (Van 

 Straaten, 1954; Guilcher and King, 1961). The higher part is the schorre (salt 

 marsh in England and in America), which is entirely covered by sea-water only 

 at high spring tides and always bears halophytes. On the schorre proper, or 

 "high marsh", different species of plants are found according to altitude: the 

 most widespread one in Europe is PuccineUia maritima; Aster tripolium and 

 Obione portulacoides are also numerous; with further increase in height, Juncus 

 maritimus is often of common occurrence (see Steers, 1946, for more detail). 



