3.1: Grain Size

Udden-Wentworth Grain Size Scale

The Udden-Wentworth grain size scale is the most common one used by geologists and forms the basis for subdividing clastic sedimentary rocks based on clast size. We tend to make the most basic subdivisions based on size because the maximum clast size is a function of the amount of energy in the system. We've used this classification scheme as the basis for Table 4.1.1, which combines grain size, Phi units, and more detailed naming of clastic sediments and clastic sedimentary rocks.

Table \(\PageIndex{1}\): Grain size classification from Wentworth (1922) and phi scale from Krumbein (1934). Settling velocities from http://www.filtration-and-separation...g/settling.htm and entrainment (erosion) velocities from http://en.Wikipedia.org/wiki/File:We...Size-Chart.pdf; both assume spherical particles of quartz.

Phi Units

In hydrogeology, we commonly describe sediment size in terms of phi units (Φ), where the conversion to real world units is:

Diameter (mm) = 1/2ⁿ
where n = phi (Φ) value

They key thing to remember about this is that the bigger the phi value the smaller the diameter of the particle.

 

Sorting

Sorting is a measure of the uniformity of grain size in a specimen. A well sorted sample will have relatively uniform grain size whereas a poorly-sorted sample has a wide range of grain sizes. Geologists generally apply positive-sounding terms to uniform grain size because those samples have the most porosity and thus have the best potential for fluid flow and storage.You can estimate sorting visually (Figure 3.1.1) or measure it quantitatively using sieve analysis and plotting up the data on a cumulative distribution plot and histogram (Figure 3.1.2).  Once plotted, you can use the equations and techniques in Figure 3.1.2 to determine mean (average), median (middle value when data is sorted smallest to largest), and mode (value that occurs most frequently) values for grain size, as well as calculate a numeric value for sorting and skewness (a measure of the symmetry of grain size distribution).

Table \(\PageIndex{1}\): Sorting and skewness terminology and values from Folk (1966)

 

Note

Engineers and geologists live in opposite worlds when it comes to grain size distribution. They use the term "grading" to describe the distribution of grain size. For them, compaction is what is important and a well-graded specimen has a wide range of grain sizes (can be densely compacted) and a poorly graded specimen has uniform grain size and does not compact as well.

 

Figure \(\PageIndex{1}\): Illustration of spherical sand grains (white) and pore spaces (blue) showing changes in sorting and porosity (Michael C. Rygel via Wikimedia Commons; CC BY-SA 4.0).

 

 

Figure \(\PageIndex{2}\): Example grain size data shown in tabular, histogram, and cumulative distribution curve form.  Median, mean, mode, sorting, and skewness can be calculated from this data using the equations on the bottom right of the diagram.  Techniques and equations from Folk (1966) 

 

Additional Readings and Resources

• Folk, R. L., 1966, A review of grain‐size parameters. Sedimentology, v. 6, no. 2, p. 73-93.
• Krumbein, W. C.m 1938, Size frequency distributions of sediments and the normal phi curve. Journal of Sedimentary Research, v. 8, no. 3, p. 84-90.
• Wentworth, C. K., 1922, A scale of grade and class terms for clastic sediments. The Journal of geology, v. 30, no. 5, p. 377-392