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.

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Sediment Grains

All About Sediment Grain Size

by Andrew Alden 

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.

Andrew Alden

Professional geologist, writer, photographer, and geological tour guide

Thirty-seven years of experience writing about geological subjects

Six years as a research guide with U.S. Geological Survey (USGS)

Member of Geological Society of America (GSA), and American Geophysical Union (AGU)

Experience

Andrew Alden is a former writer for ThoughtCo who contributed hundreds of articles for more than 17 years. Andrew works as a geologist, writer, editor, and photographer. He has written on geological subjects since 1981 and participates actively in his field. For example, Andrew spent six years as a research guide with the U.S. Geological Survey, leading excursions on both land land and at sea. And since 1992, he has hosted the earthquakes conference for the online discussion platform, The Well, which began as a dialogue between the writers and readers of the Whole Earth Review. 

In addition, Andrew is a longtime member of the member of the Geological Society of America — an international society that serves members in academia, government, and industry; and the American Geophysical Union — a community of earth and space scientists that advances the power of science to ensure a sustainable future.

Andrew lives in Oakland, California; and though he writes about the whole planet and beyond, Andrew finds his own city full of interest too and blogs about its geology. 

Education

Andrew Alden holds a bachelor's (B.A.) degree in Earth Science from the University of New Hampshire, College of Engineering and Physical Sciences, in Durham, N.H.

Awards and Publications

Andrew Alden on Earthquakes ​(The Well Group, Inc., 2011)

Assessment of River — Floodplain Aquifer Interactions (Environmental and Engineering Geoscience, 1997)

Andrew Alden on Hosting (The Well Group, Inc., 1995)

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