77 



the sorting coefficient is the square root of the ratio of the 

 particle diameter representing 25 per cent of the sediment 

 weight to that representing 75 per cent of the sediment weight, 

 or So = Q25/Q75" ^* ^s, therefore, a dimensionless number 

 which is an index of the degree of sorting of the sediment; 



or, in other words, the extent to which the particle diameters 

 are spread on either side of the median diameter of the sedi- 

 ment. The sorting coefficient indicates the degree of uni- 

 formity of grain size and numbers close to unity indicate 

 uniformly distributed grains in a sediment. In the system 

 set up by Trask, sediments having sorting coefficients from 

 1,00 to 2.50 are well-sorted; those with sorting coefficients 

 from 2.50 to 4„00 are moderately-sorted? and those greater 

 than 4.00 are poorly-sorted. A sediment having a sorting 

 coefficient of 1.5 is not twice as well sorted as one having 

 a value of 3,0, because the coefficient is geometric. The 

 sorting of sediments is directly related to the median diameter, 

 and in some cases, listed by Inman (1949), sediments having a 

 median diameter of 0.18 mm are the best sorted; the sorting 

 being poorer for sediments both larger and smaller than this 

 value. However, in Santa Monica Bay, the sediments with 

 median diameters from .05 mm to 0.10 mm are the best sorted, 

 even though the relationship holds that coarser and finer 

 sediments have poorer sorting. This relationship is shown in 

 Figure 23 which has a point plotted for each sediment sample 

 with the median diameter as the abscissa and the sorting 

 coefficient as the ordinate. On this diagram there is a 

 prominent grouping of points for well-sorted sediment in the 



