SECT. 3] ESTUARIES, DELTAS, SHELF, SLOPE 635 



b. Estuaries and marshes 



In these the processes and rates of deposition on schorres or high marshes have 

 been studied and measured in Europe (Wales, Denmark, England, Brittany, 

 Poitou, Normandy) by means of sand layers scattered in patches between the 

 plants on the muddy or silty marshes. Borings show the thickness of mud 

 which has settled upon the sand (Richards, 1934; Nielsen, 1935; Steers, 1938- 

 1948, 1946; Guilcher and Berthois, 1957; Verger, 1956; Elhai, 1959). The rate 

 of retreat of the low cliffs at the outer edge of the schorres has also been mea- 

 sured by Guilcher and Berthois. 



The rate of accretion is extremely variable, ranging from 0.5 mm in five 

 years to 8 mm per month. It depends on the marsh, and, in any individual marsh, 

 on the height and on the distance to big creeks acting as feeders, and also on 

 the type of vegetation. The relative importance of these factors differs according 

 to the marshes. In Brittany and Poitou, it was found that the height is generally 

 the controlling factor if the low marsh (or high slikke) is not considered: the 

 maximum accretion takes place on the lowest parts of the schorre. In north 

 Norfolk, England, the distance to the creeks is important in determining the 

 accretion rate, particularly if dense Obione colonies grow along the creeks. 

 Obione is also of particular importance in Normandy, since the thickest de- 

 position occurs in it. Irregularities in deposition exist from year to year, but the 

 sand patches are very rarely removed. 



At the same time, the edges of the schorres are retreating at the places where 

 they are cliffed. The rate of retreat is also highly variable, since the recession 

 is caused by blocks of hardened mud falling down from time to time onto the 

 mud flats. Photographs repeatedly taken in the same areas show that these 

 blocks are progressively disintegrated and their mud returns once more to 

 suspension in the water, which in turn feeds the marsh surface and allows it to 

 continue to grow. This may be defined as a true cyclic evolution of the 

 mud. 



On low marshes or high slikkes, the same method has been used in Salicornia. 

 The rate of deposition has proved to be higher than on schorres in many cases ; 

 but deposition alternates with erosion, since sand patches have often dis- 

 appeared, sometimes after they had first been covered by a rapidly growing 

 layer of mud. The conclusion is that deposition is more irregular than higher 

 up on the marsh, probably because Salicornia is an annual plant the vegetal 

 cover of which no longer exists in winter. Moreover, the cover is always more 

 sparse than on schorres, and as the plants are stirred by the wind they gather 

 some mud and can erode the fiats. 



The fact that deposition generally exceeds erosion in marshes has been ex- 

 plained by Van Straaten and Kuenen (1957) as follows. The velocity of the tidal 

 current entering the marsh decreases as the current goes farther into the marsh. 

 In such a current, there is a time lag between the moment at which it is no 

 longer able to hold its sedimentary material in suspension and the moment at 

 which this material reaches the bottom. This "settling-lag effect" increases the 

 distance for transportation to the inner reaches of the marsh. On the other hand, 



