Folk's Classification of Sediments

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Folk Diagram for Sediment

The SZC sand-silt-clay graph
Click the image for the larger version. (c) 2013 Andrew Alden, licensed to (fair use policy)

Robert Folk first published this diagram, along with the system of sediment classification it represents, in 1954. Since that time it has become an enduring standard among sedimentologists and sedimentary petrologists, along with the Shepard sediment classification.

Siliciclastic Sediments

Like Folk's classification diagram for gravelly sediment, this scheme is for use on siliciclastic sediments -- not high in either organic matter or carbonate minerals. The difference is that this diagram is for sediments with less than 10 percent particles of gravel size, larger than 2 millimeters. (Folk devised a separate classification scheme for carbonate rocks that is also still in wide use.)

Before using this diagram, researchers carefully analyze a sediment sample to determine its content in the three classes of particle size: sand (from 2 millimeters to 1/16 mm), silt (from 1/16 to 1/256 mm), and clay (smaller than 1/256 mm). Here is a simple home test using a quart jar for making this determination. The result of the analysis is a set of percentages, which describe a particle size distribution.

Take the percentages of silt and sand first, and determine the ratio of the two numbers. That tells where to put the first mark on the bottom line of the diagram. Folk's classification is unusual in specifying the term "mud" for a sediment in which sand and silt are more or less equally mixed. After that, draw a line from the point on the bottom toward the Clay corner, stopping at the percentage that was measured for the clay content. The location of that point gives the correct name to use for that sediment sample.

Sedimentary Rocks

The Folk classification is also used on sedimentary rocks. For that purpose, thin sections are made from a rock specimen and the sizes of a large number of randomly selected grains are carefully measured under a microscope. In that case, just add "-stone" to all of these names.