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Sediment Grain Size Categories

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Sediment Grain Size Categories
Cobble-sized clasts

The grain sizes of sediments and sedimentary rocks are a matter of great interest to geologists. They determine grain sizes in the field using printed cards called comparators, especially for the larger sizes. In the laboratory, comparators are supplemented by standard seives.

The Wentworth scale was published in 1922 by Chester K. Wentworth, modifying an earlier scale by Johan A. Udden. Wentworth's grades and sizes were later supplemented by William Krumbein's phi or logarithmic scale, which transforms the millimeter number by taking the negative of its logarithm in base 2 to yield simple whole numbers.

The size fraction larger than sand (granules, pebbles, cobbles and boulders) is collectively called gravel, and the size fraction smaller than sand (silt and clay) is collectively called mud.

Sediment Grain Sizes

Millimeters Wentworth Grade Phi (Φ) Scale
>256 Boulder –8
>64 Cobble –6
>4 Pebble –2
>2 Granule –1
>1 Very coarse sand 0
>1/2 Coarse sand 1
>1/4 Medium sand 2
>1/8 Fine sand 3
>1/16 Very fine sand 4
>1/32 Coarse silt 5
>1/64 Medium silt 6
>1/128 Fine silt 7
>1/256 Very fine silt 8
<1/256 Clay >8
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