Page 29 - Petrology of Sedimentary Rocks

Grain Size Scales and Conversion Tables

The grade scale most commonly used for sediments is the Wentworth (I 922) scale
which is a logarithmic scale in that each grade limit is twice as large as the next
smaller grade limit. The scale starting at Imm and changing by a fixed ratio of 2 was
introduced by J. A. Udden (I 898), who also named the sand grades we use today.
However, Udden drew the gravel/sand boundary at Imm and used different terms in the
gravel and mud divisions. For more detailed work, sieves have been constructed at

intervals 2 2 and 4 2. The 4 (phi) scale, devised by Krumbein, is a much more
convenient way of presenting data than if the values are expressed in millimeters, and
is used almost entirely in recent work.

U. S. Standard Millimeters Microns Phi ($1 Wentworth Size Class
Sieve Mesh #/ (I Kilometer)
-20
4096 -12
1024 -10 Boulder (-8 to - 124)
Use 256 -8 II
wire 64 -6 Cobble (-6 to -84) UJ
squares I6 -4 Pebble (-2 to -641)
5 4 -2 2
6 3.36 -I .75
7 2.83 -I .5 Granule z
8 2.38 -I .25
- IO 2.00 -I .o
I2 I .68 -0.75
I4 I.41 -0.5 Very coarse sand
I6 1.19 -0.25
- I8 1.00 0.0
20 0.84 0.25
0.71 0.5 Coarse sand
z; 0.59 0.75
- 35 -l/2 - 0.50 - 500 1.0
40 0.42 420 1.25
45 0.35 350 1.5 Medium sand P
50 0.30 300 1.75 2
-60 - l/4 - 0.25 - 250 a
70 0.210 210 Z5 v)
0.177 177 2:5 Fine sand
10”: 0.149 149 2.75
- l20- l/8 -0.125- 125
140 0.105 105 E5
170 0.088 88 3.5 Very fine sand
200 0.074 74 3.75
-230- I/ I6 -0.0625- 62.5 - 4.0
270 0.053 53 4.25
325 0.044 44 4.5 Coarse si It
0.037 4.75
- l/32- 0.031- 3”: 5.0 P
Analyzed l/64 0.0156 15.6 6.0 Medium silt
I/ 128 0.0078 7.0 Fine silt
bY -l/256 0.0039 E . 8.0 Very fine silt
0.0020 2.0 9.0
Pipette 0.00098 0.98 10.0 Clay
0.00049 0.49 I I .o (Some use 2@ or
or 0.00024 0.24 12.0 9$ as the clay
0.00012 0.12 13.0 bound ry)
Hydrometer 0.00006 0.06 14.0 I

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