Mainland Shelf 



201 



CLAY 



TETRAHEDRO 

 UNFOLDED 



BASINS 

 SHELVES 



BASINS 

 SHELVES 



GRAVEL 



Figure 177. Triangular composition diagram for classifying sediments on the basis of their contents of gravel (>2 mm), 

 sand (2-0.62 mm), silt (0.62-0.004 mm), and clay (0.004 mm). Circles and dots show many analyses for shelf and 

 basin sediments of the region. Modified from Shepard (1954). 



vantage for a particular situation and purpose. 

 Maps of shelf sediments, most of them 

 with fewer details than Figure 179, have 

 been made by many workers for many con- 

 tinental shelf areas of the world (see Liiders, 

 1939; Shepard and Cohee, 1936). Nearly 

 all such maps exhibit irregularities in the 

 distribution patterns that are difficult to ex- 

 plain. As a result the concept has arisen 

 that sediments of the shelves are so hap- 

 hazardly placed that the cause cannot be in- 



terpreted, and thus the inference must be 

 drawn that the patterns of similar ancient 

 sediments cannot be predicted from incom- 

 plete sampling at outcrops or wells. An ef- 

 fort to reach an understanding of the chief 

 distributive factors was made by Shepard, 

 Emery, and Gould (1949), who found that 

 many of the anomalous patterns can be re- 

 lated to bottom currents, exposure to large 

 waves, nearby large river mouths, con- 

 tiguous sand beaches, submarine basins and 



