200 



Sediments 



grain-size distribution included between the 

 quartile percentages from which the sorting 

 coefficient and skewness are computed. 

 Broadening the base of computation to in- 

 clude the middle 68 per cent, as can be done 

 by using Inman's (1952, 1953) method, im- 

 proves the statistics but fails to increase ap- 

 preciably the understanding of the general 

 sediment pattern. 



In order to improve on the median di- 

 ameter-sorting coefficient-skewness method 

 of presentation, a set of three maps showing 

 the percentage of sand, silt, and clay may 

 be used (Fig. 176). This presentation still 

 requires some map-to-map comparison, but 

 the results are simpler of comprehension 

 than those by the first method; moreover, 

 these maps can include virtually the entire 

 grain-size distribution of the samples if 

 close contour intervals of percentage are 

 chosen. This method still requires several 

 maps and fails to indicate relative amounts 

 of calcium carbonate and other important 

 and unique characteristics of some of the 

 sediments. 



A further improvement in easy visualiza- 

 tion is that of classifying the sediments into 

 types and mapping the distribution of each 

 type. These types may be drawn on the 

 basis only of the percentage distribution of 

 grain sizes, using a triangular composition 

 diagram to define the size terms. Several 



variations of such diagrams have been used 

 by diff'erent workers. Shepard (1954) sug- 

 gested one based on sand-silt-clay relation- 

 ships. A supplementary one used at Uni- 

 versity of Southern California (Fig. 177) is 

 based also on gravel-sand-silt plus clay 

 ratios as being more applicable to some shelf 

 sediments of this region. An example of its 

 use is given by Figure 178, a map of the 

 sediments off Santa Barbara. 



For special purposes the sands may be 

 screened out of the whole sediment samples 

 and examined separately with a binocular 

 microscope to provide data on source of the 

 sediment. The results can be plotted as 

 contour maps of chief constituents (Emery. 

 Tracey, and Ladd, 1954, Figs. 22, 43, 50, and 

 56), as pie diagrams (Shepard and Moore, 

 1954), or as mappable classes of sediment 

 coarse fractions (Fig. 178). 



A last sort of map of sediment types is 

 one that combines grain size with a descrip- 

 tion of color and composition, for instance, 

 green silty shell sand. Examples of such 

 maps are given for the shelves off" Santa 

 Barbara (Fig. 178) and off' San Diego (Fig. 

 179). Such a map of types indicates at a 

 glance the general nature and distribution 

 of sediments. In spite of the large variety 

 of kinds of presentation, standardization of 

 maps of sediments should not be attempted 

 because each kind of map has its own ad- 



25 40 



Figure 176. Percentages of sand, silt, and clay in Santa Monica Bay based on 510 samples. Dashed line is shelf-break, 

 about 90 meters. Adapted from Terry, Keesling, and Uchupi (1956, Figs. 16, 17, and 18). 



