626 GUiLCHER [chap. 24 



the mineral y)articles are enclosed in fibrous networks. Foam also includes small 

 mineral (grains, givintr them more })U()yancy and allowing tiieir transportation 

 to the higher parts of the marsh (W'aksman, 1933; Trask, 1939-1955; Bourcart 

 and Francis-Ba3uf. 1942; Francis-Boeuf, 1947; Rajcevic, 1957). Nevertheless, 

 the organic fraction is always considerably smaller in estuarine muds than 

 the mineral fraction, which usually exceeds 90% by weight (Berthois, 1955). The 

 exceptions are peats (which are not truly muds) and tangues deposited on the 

 watts of north-east Brittany (Bourcart and Charlier, 1959), in which the lime 

 content is unusually high (more than 60% sometimes) and consists of com- 

 minuted fragments of marine shells and spicules of s])onges. These tangues are 

 comparatively coarse, since the clay is almost comi)letely lacking, and fine 

 sand predominates with some silt. Fine calcareous sediments deposited in 

 shallow water in warm seas, and euxinic or poorly ventilated basins, where the 

 organic matter content is much higher, are not considered here. 



Iron is found in estuarine muds, in percentages ranging from 2.1 to 7.3, in 

 examples given by Bourcart and Francis-Boeuf (1942, p. 21). Along with the 

 organic content, it acts as a binding factor, and bacteria, which live in very 

 large amounts at the surface of the tidal flats, seem essential in its fixation 

 (Harder, 1919). 



The water content of the soft mud may exceed the weight of the dried sedi- 

 ment, and those who have walked on tidal mud flats have experienced how deep 

 one can sometimes sink into them. However, mud is plastic, and the footmarks 

 remain impressed into its surface for a long time, even after many tides have 

 l^assed over it. 



D. Source of Sediments 



The source of the mineral fraction in deltas is obviously the catchment area 

 of the river, or a j^art of it. Van Andel (1955) has thus shown, by means of 

 heavy-mineral analyses, that "at least four-fifths of the Rhone delta sediments 

 have been derived from the Alps; a comparatively small contribution is made by 

 the Massif Central". This is not surprising, because the tributaries coming from 

 the Alps are by far the most important ones. According to the same author, the 

 sand in the Lower Rhine is su]i])lied by reworked Pleistocene deposits, which 

 came partly from Switzerland and partly from the Rhenish Schiefergebirge 

 (Van Andel, 1950, p. 114). 



Concerning estuaries and tidal marshes, the problem of the source of the 

 sediments is more complex. A mass of material in suspension is carried to and 

 fro with the rising and falling tide, but whence this bouchon vaseux (Glangeaud, 

 1938) comes is disputed, and various views are held. Many Dutch authors, 

 especially Crommelin ( 1 940, 1 947), have come to the conclusion that all fractions 

 of the sediments of the Wadden Sea in West Friesland are derived from the 

 North Sea, since the heavy minerals are the same as those found in the fine 

 particles on the bottom of the North Sea, and diff'erent from those carried in 

 suspension in the rivers Ems, Weser and Elbe. Van Straaten and Kuenen (1957) 



