628 GtrHiCHER [chap. 24 



of continental deposits outcropping in the immediate vicinity (Guilcher and 

 King, 1061): and. in Newport Bay, California, nnid is supplied by slumping of 

 shales and siltstones outcropping in the surrounding cliffs (Stevenson and 

 Emery, 1958. p. 51). 



In tropical countries, where observations are not so numerous, it may be 

 assumed in a number of cases that estuarine mud is also derived from the 

 continent. A typical example may be cited at Majunga, Madagascar, where the 

 Betsiboka estuary has been filled up at a tremendous rate by red lateritic muds, 

 as a consequence of deforestation in the catchment area of the river (LaFond, 

 1957). The lagoons stretching along the coast of the Gulf of Benin in Ivory 

 Coast, Ghana, Togo and Dahomey, are likewise fed by rivers during the annual 

 floods, and a large part of the suspended mud spreads into the sea through the 

 outlets (Guilcher, in Russell, 1959). 



Thus, the mud deposited in estuaries and tidal marshes seems to be generally 

 ascribed to a terrestrial supply, according to the evidence given by accurate 

 investigations. The case of the Wadden Sea in Friesland appears to be a 

 notable exception. 



E. Processes of Deposition and Erosion 



Present-day sediments in estuaries and deltas are the ultimate result of 

 deposition ])rocesses which were in progress during the so-called Flandrian 

 transgression, corresponding to the melting of Pleistocene ice-caps. Practically 

 all the river mouths in the world rest upon a sedimentary column filling an old 

 valley which was carved during Pleistocene low sea-levels. The few exceptions 

 consist of small hanging valleys in which estuaries are lacking. Examples for 

 Great Britain may be found in Codrington (1898) and for Brittany in Guilcher 

 (1948, pp. 157, 320 and 584-585). In the United States, the borings in Chesa- 

 peake Bay (Hack, 1957) have shown that the bottom of the old valley system 

 lies at 200 ft below present sea-level near Annapolis. The gradients of the 

 profiles of some tributaries steepen near the main river; this is an inheritance 

 of the Pleistocene deepening. The oldest deposits of the fill consist of coarser 

 sediments (sand and gravel) than the present estuarine muds. This occurs 

 widely in, for example, the Mississippi valley and in many European estuaries 

 and is probably related to overloaded and braided-channel streams at the end 

 of the Pleistocene. Another typical area in this respect is the Texas and south- 

 western Louisiana coast, for which data are very accurate (LeBlanc and Hodg- 

 son, in Russell, 1959; Shei^ard and Moore, 1955; Gould and McFarlan, 1959; 

 and others). 



The eustatic rise of sea-level is not alone responsible for the recent aggrada- 

 tion in estuaries and deltas. In several large coastal plains, a long-continued 

 subsidence must also be taken into account. Two typical examples are the Lower 

 Mississippi Plain and the Low Countries along the North Sea. In both areas, the 

 Pleistocene is some 2000 ft thick under present sea-level in its thickest sections 

 (see for instance Akers and Hoick, 1957; Pannekoek, 1956; and Chapter 13 in 



