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|
|>1||Very coarse sand||0|
|>1/16||Very fine sand||4|
|>1/256||Very fine silt||8|