622 



GUILCHER 



[chap. 24 



Between the high marsh and the sHkke a transition area sometimes occurs — the 

 "low marsh* {haute slikke in French papers) — with a sparse cover oi Salicornia. 

 But the Salicornia zone often disaj^pears and is replaced by a low cliff, vertical 

 or overhanging, wliich abruptly bounds the schorre from the slikke. In many 

 marshes of the English Channel and the southern ])art of the North Sea, 

 Spartina toivnsendi has rapidly spread in recent times. Corresponding zones, 

 with somewhat different species, exist on both coasts of North America (Ste- 

 venson and Emery. 1958; Johnson. 1925). These halophytes are able to endure 

 large clianges in salinity, according to the time of the tide, the range of the tide 

 and the season (rain, river discharge, etc.), but each zone has its particular 

 ecology. On the other hand, peat can grow in the inner parts of the marshes, 

 which usually remain outside the reach of the salt or brackish water. 



Salt pan in schorre 

 >eek in marsh 



Pleistocene 

 periglacial deposits ^ 



^ V V , Bed-rock 

 ^ ^ X X X X ^ 



y-^^- Pleistocene ^^ Y. X ^ Bed-rock y y 

 iVIuddy sand periglociol deposits X X X Y V V V 



Fig. 2. Diagram showing parts and section of a small estviary in Brittany. 



The marshes are fed by large tidal creeks (Dutch: geulen) meandering into the 

 slikke, and smaller creeks (Dutch: prielen) dissecting the schorre. Straight 

 sections are rare, and very pronounced meanders are much more common than 

 in ordinary rivers, but their general evolution is the same, consisting of cut-offs, 

 captures, etc. Interconnections between creeks are frequent. The ebb current is 

 more important in creek evolution than the flood current, since the former 

 flows during a longer time than the latter. But on many slikkes in watts or 

 estuaries, the channels show an inter-digitated pattern: some of them are 

 flood channels which gradually shoal as they enter the watt, whereas others are 

 only followed by the ebb current: these are deepest in their inner parts and 

 gradually merge into shallow banks at their outer reaches. Such ebb- and 

 flood-channels in the Scheldt estuary have been described by Van Veen (in 

 Waddensymposium, 1950), and on several tidal flats around Great Britain by 

 Robinson (1956), and elsewhere. The surface of the high marsh between the 

 creeks includes ponds which are devoid of plants and are known as salt pans, in 

 which the water becomes highly saline at neap tides by evaporation, a fact 

 which may account for the absence of vegetation. Salt pans are often remnants 

 of silted-up creeks, although some of them may have other origins. 



In the intertropical belt, grass is generally replaced by mangrove in tidal 



