All About Sediment Grain Size

Close of of several rocks.
John Burke / Photolibrary / Getty Images

The grain sizes of sediments and sedimentary rocks are a matter of great interest to geologists. Different size sediment grains form different types of rocks and can reveal information about the landform and environment of an area from millions of years prior.

Types of Sediment Grains

Sediments are classified by their method of erosion as either clastic or chemical. Chemical sediment is broken down through chemical weathering with transportation, a process known as corrosion, or without. That chemical sediment is then suspended in a solution until it precipitates. Think of what happens to a glass of saltwater that has been sitting out in the sun. 

Clastic sediments are broken down through mechanical means, like abrasion from wind, water or ice. They are what most people think of when mentioning sediment; things like sand, silt, and clay. Several physical properties are used to describe sediment, like shape (sphericity), roundness and grain size.

Of these properties, grain size is arguably the most important. It can help a geologist interpret the geomorphic setting (both present and historical) of a site, as well as whether the sediment was transported there from regional or local settings. Grain size determines just how far a piece of sediment can travel before coming to a halt. 

Clastic sediments form a wide range of rocks, from mudstone to conglomerate, and soil depending on their grain size. Within many of these rocks, the sediments are clearly distinguishable--especially with a little help from a magnifier

Sediment Grain Sizes

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 following is a simplified version of the much more detailed USGS version. 

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

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. 

Clastic Sedimentary Rocks

Sedimentary rocks form whenever these sediments are deposited and lithified and can be classified based on the size of their grains.

  • Gravel forms coarse rocks with grains over 2 mm in size. If the fragments are rounded, they form conglomerate, and if they are angular, they form breccia.
  • Sand, as you may guess, forms sandstone. Sandstone is medium-grained, meaning its fragments are between 1/16 mm and 2 mm. 
  • Silt forms fine-grained siltstone, with fragments between 1/16 mm and 1/256 mm. 
  • Anything less than 1/256 mm results in either claystone or mudstone. Two types of mudstone are shale and argillite, which is shale that has undergone very low-grade metamorphism. 

Geologists determine grain sizes in the field using printed cards called comparators, which usually have a millimeter scale, phi scale, and angularity chart. They are especially useful for larger sediment grains. In the laboratory, comparators are supplemented by standard sieves.

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Alden, Andrew. "All About Sediment Grain Size." ThoughtCo, Apr. 5, 2023, Alden, Andrew. (2023, April 5). All About Sediment Grain Size. Retrieved from Alden, Andrew. "All About Sediment Grain Size." ThoughtCo. (accessed June 2, 2023).