SECT. 3] • ESTUARIES, DELTAS, SHELF, SLOPE 627 



support this conclusion with some reservations: the main source is the North 

 Sea; the River Ijssel may have suppHed some mud, but this contribution was 

 "only of subordinate importance". The evidence which has been produced is 

 convincing, and this result seems to be well established. 



Rajcevic (1957) also thinks that 75% of the mud in the Seine estuary is of 

 marine origin; however, evidence for this is scanty and doubtful, and, in the 

 writer's opinion, the Seine estuary needs more attention before a definite 

 conclusion can be reached. It might be supposed that a large supply of marine 

 particles is made easily available by the fact that, in the salt wedge which has 

 been proved to exist in many estuaries in the world, the resulting current goes 

 upstream at and near the bottom, whereas the resulting outflow to the sea 

 occurs near the surface. Such a distribution of the currents is encountered, for 

 example, in Chesapeake Bay (Pritchard, 1952). Nevertheless, Burt (1955) 

 assumes that, in Chesapeake Bay, the settling particles are mostly coming 

 from the rivers ending in the estuary. Spectrophotometer readings show that 

 the maximum of suspended materials in the estuary occurs in spring, a fact 

 which can be explained neither by the production of organic matter, which is as 

 large in summer as it is in spring, nor by the tidal currents, which have no 

 annual cycle, nor by the wind stress; on the contrary, the peak in river discharge 

 occurs in spring, and this is in accord with observations. It may be suggested 

 that the bottom current carries along particles of terrestrial origin, which 

 first travelled downstream in the surface current. 



A continental origin of the estuarine sediments has also been well established 

 by careful and numerous observations over a period of several years carried out 

 by Berthois in the Loire estuary (several papers summarized in Guilcher, 

 1956-1957; see especially Berthois, 1956). Multiple measurements at the entrance 

 of the estuary to the north of Saint Gildas Point have shown that turbidity is 

 always higher at all depths during ebb than during flood tides, so that a net 

 inflow of marine suspended material is impossible. The continental suspended 

 material can hardly be carried outside at neap tides and in periods of low river 

 discharge, but it is f)artly driven to the sea during the floods, or at spring tides 

 in summer, when the viscosity of the water is low. On the other hand, the mineral 

 fraction in the tangues of the Breton tidal flats has been supplied by Pleistocene 

 loess or head existing in nearby cliff's, and even from the foreshore beneath the 

 tangue; only the calcareous fraction (see above) is derived from marine shells. 

 Burrowing organisms (worms, lamellibranchs, amphipods, etc. ) largely contribute 

 to converting the loess into tangue (Milon, 1935). In the small estuaries along 

 the coasts of Brittany, thermal differential. X-ray and grain-size analyses and 

 mineralogy support the conclusion that the mud is derived from periglacial 

 Pleistocene deposits (rubble drift) covering the slopes of the estuaries (Fig. 2), 

 which are washed by waves at high spring tides. The deposits are clifFed: the 

 coarser fraction settles at the foot of the cliff's and the finer fraction is brought 

 into suspension in water and feeds the mud flats and tidal marshes (L, and C. 

 Berthois, 1954-1955; Berthois, 1955; Guilcher and Berthois, 1957). In Derry- 

 more marsh, Tralee Bay, Kerry, mud is also derived mostly from the washing 



