SECT. 3] 



ESTUAKIES, DELTAS, SHELF, SLOPE 



637 



from creeks to watersheds makes easy the deposition of suspended particles 

 reaching the watersheds at the end of the flood, whereas the particles reaching 

 the outer parts of the channels at the end of the ebb have more difficulty 

 in settling in these deeper waters. This accounts for the deposition of fine sedi- 

 ments, especially on watersheds and inner marshes (Fig. 3). 



Berthois's studies (1953-1958) in the Loire estuary have also added much to 

 the knowledge of the processes. Deposition is more active in warm water than 

 in cold water (from which we can infer that its rate should be high in mangroves) 

 and in salt water than in fresh water; it occurs principally at slack tide, and 

 not during the ebb as Francis-Boeuf (1947) suggested. It reaches a peak in a 

 lense of immobile water which has been proved to persist for several hours near 

 the bottom in a part of the Loire estuary, and also in smaller Breton rivers, 

 especially during the ebb. The silting up of the lateral marshes is favoured by a 



(b) 



vvvv vvvv 

 vvvvvvvv 



Fig. 16. Inversion of relief by shrinking of peat in Flanders. (From Tavernier, 1947.) 

 (a) Deposition of old marine clay; (b) formation of peat; (c) peat eroded by creeks; 

 deposition of young marine clay and sand; (d) shrinking of peat and inversion of 

 relief. 



transverse pulsation of the zone of maximum turbidity, which occurs during the 

 flood. Glangeaud (1938) has also explained the median mud-banks in estuaries 

 as a consequence of vertical eddies on both sides of the channel, with descending 

 and converging currents which feed the banks. Marshes lying to the lee of sand 

 dunes are often very sandy, owing to the winnowing of dunes by wind, as in 

 Kerry, Ireland (Guilcher and King, 1961). 



When marshes include peat lenses, a drying-up of the marshes causes the 

 peat to shrink to a small fraction of its original volume. In Flanders, the Nether- 

 lands and elsewhere, the shrinkage of the peat leads to an inversion of relief 

 (Fig. 9) : when the sea invaded the peat bogs, creeks were carved into the peat and 

 were partially or completely filled with sand and clay, while clay settled on the 

 flats where the peat had not been eroded. After the marshes have been reclaimed, 

 the peat shrinks beneath the clay which now occurs in depressions, whereas the 

 former creeks stand as winding ridges at a higher level (Tavernier, 1947; Panne- 

 koek, 1956). Peat also shrinks when it is compressed by a sand bar retreating 

 landwards and encroaching on the marsh (Johnson, 1925). Whether the marsh is 



