638 GUILCHER [CHAP. 24 



peaty or clayey. multi])le sand barriers, or cheniers, alternating with long, 

 narrow swamps, are a common feature and point to successive stages in pro- 

 gradation: such areas have been studied in the Guianas (Van der Eyk, 1957, 

 with an excellent map; Vann, in Russell, 1959; and others), in Texas and Loui- 

 siana (Price, 1955; LeBlanc and Hodgson, in Russell, 1959; Byrne, LeRoy and 

 Riley, 1959; Gould and McFarlan, 1959; and others), in Dahomey (Guilcher, in 

 Russell, 1959), etc. 



Laminations, or the presence of thin successive layers, are expected to occur 

 in estuarine and deltaic deposits. Actually, the situation is not always the same, 

 since it dejiends on processes of deposition and on ecological conditions (Van 

 Straaten, 1959; Hantzschel, in Trask, 1939). Salt marshes (schorres) show 

 laminations which are often graded "as a result of decreasing competency of the 

 currents during each stage of inundation of the marshes". On banks of tidal 

 creeks in flats, other laminations are due to variations in competency of tidal 

 currents, and show irregularities related to local erosion. Laminations are also 

 found in some kinds of deltaic deposit, but in other tidal environments they 

 disappear owing to burrowing animals, especially in tropical mangroves where 

 innumerable crabs, mud-skippers (gobies), etc. hide in the mud. At the places 

 where such disturbances occur, it does not seem possible to study the deposition 

 by the method of artificially scattered sand patches. 



2. Continental Shelf 



The simplest conceivable distribution of sediments over the continental shelf 

 includes well sorted sands near the shore, followed by silts and clays more 

 and more poorly sorted on the median and outer parts of the shelf as the depth 

 increases and the waves become less efficient in stirring the sediments on the 

 bottom. Such a distribution would fit with a well graded slope of the shelf, 

 consisting of a smooth curve, concave upward, which may be defined as the 

 profile of equilibrium of the shelf. Before accurate sedimentological investiga- 

 tions were carried out on the shelf, this feature was expected to be of general 

 occurrence, and, according to H. C. Stetson (in Trask, 1939, 1949), it is actually 

 found in sections off the east coast of the United States. 



However, Stetson himself admits that "the fully graded profile . . . would be 

 the exception . . . and represents the theoretical form that should eventually be 

 arrived at, but seldom is". The profile shows irregularities in several New Eng- 

 land traverses, and examples are given in the same area by Stetson in which the 

 sediments near the outer edge of the shelf "are coarser than some of those found 

 in shallower water near shore". As early as 1939, Twenhofel insisted on the same 

 idea, which was also supported by findings of pebbles in the Celtic Sea and in 

 the Bay of Biscay at depths ranging from 30 to 55 fm, where the waves are no 

 longer able to stir them (Furnestin, 1937). 



Shortly after World War II, Shepard gave in his treatise (194S) the results 

 of an extensive examination of detailed charts covering the principal shelves 

 around the world. He came to the same conclusion already reached for the 



