III. SEDIMENT CLASSIFICATION 



Index or classification properties of sediments used in this report are derived 

 from measurements of the grain size, water content, liquid limit, and plastic limit; 

 the latter two collectively are called Atterberg limits. 



Geologists and civil engineers in the United States commonly use slightly differ- 

 ent grain-size scales, respectively the Wentworth (1922) scale and the Massachusetts 

 Institute of Technology or M.I.T. scale developed by Gilboy (Glossop and Skempton, 

 1945). These scales are compared and the scale used in this paper is given In Table 2. 

 At the Hydrographic Office, the distinction between silts and sands follows Wentworth 

 and is made at 62.5/u or about 4 ^ units ' . Clay-size particles are of particular signif- 

 icance in fine-grained sediments and the maximum size of clay minerals (comprising 

 most of the clay-size fraction) appears to be nearer 2 than 4^ according to Grim (1942, 

 p. 229; 1953, p. I) 2 . 



TABLE 2. COMPARISON OF GRAIN-SIZE SCALES 





Wentworth Scale 



M.I.T. Scale 



Scale Used In This Paper 





microns 



phil 



microns 



phi 



microns 



phi 



Sand 



2000 to 62.5 



-1 to 4 



2000 to 60 



-1 to 4.06 



2000 to 62.5 



-1 to 4 



Silt 



62.5 to 3.9 



4 to 8 



60 to 2 



4.06 to 9 



62.5 to 2 



4 to 9 



Clay 



<3.9 



>8 



<2 



>9 



<2 



>9 



After Krumbein (1936) 



Consequently, the M.I .T . silt-clay separation at 2/x or 9 rf is preferred (Table 2) . 

 Nomenclature (Fig. 3) follows the triangular diagram system devised by Shepard (1954). 

 In Table 1, the modifying prefix to the name of the sediment is based on its predominant 



In the phi notation (Krumbein, 1934, p. 76), phi is equal to the negative logarithm 

 to the base two of the diameter in millimeters „ 



Baver (1940, p. 14-17) relates the development of 2/j. as the upper limit for clay. 

 On -the other hand, Mielenz and King (1955, p. 213-216) summarize evidence for 

 larger clay-size particles. 



