Classical plotting and characterization of grain size distributions.
The input data should consist of, first, one column of sieve sizes, in mm, from smallest to largest. The smallest size can be 0, which is interpreted as phi = 10, or 0.001 mm (1 µm). This can be used for unanalyzed sediment in the bottom pan. The largest sieve size should be larger than the largest particle in the sample and will contain no sediment. This is used for defining an upper size limit.
The second column contains sample weights for each size fraction, in any unit, or percent. The last value is assumed to be 0 and does not need to be entered. Additional columns can contain sample weights for additional samples. The example below defines two samples, with the sieve sizes in the first column:
| 0 | 4.0 | 5.2 |
| 0.020 | 7.1 | 5.3 |
| 0.063 | 9.3 | 7.1 |
| 0.125 | 9.4 | 8.0 |
| 0.25 | 11.3 | 9.1 |
| 0.5 | 8.1 | 4.3 |
| 1 | 4.2 | 2.0 |
| 2 | 2.1 | 1.5 |
| 4 |
Past follows recommendations and equations of Blott and Pye (2001). Grain sizes are reported in the logarithmic phi scale (Krumbein scale), where φ = 0 corresponds to grain diameter 1 mm, more positive values are finer, and more negative coarser.
Gravel is defined as φ < -1 (D > 2 mm); sand is -1 < φ < 4 (0.0625 mm > D > 2 mm); mud is φ > 4 (D < 0.0625 mm).
The sediment name (textural group) is defined on the proportions of gravel, sand and mud, following Folk (1954) and with linear interpolation when sieve sizes do not correspond to the definition boundaries. The refined classification that also includes proportions of silt and clay within the mud class is not included in Past.
Mean grain size is computed both with the method-of-moments (MOM) and the Folk & Ward (1957) “graphical” method. MOM is potentially more accurate when the sediment analysis is complete and finely binned, but the F&W method is more robust and therefore recommended for less complete analyses. Results are given both in φ units and in mm.
Sorting is defined using standard deviations. Standardized names of degree of sorting follow Folk & Ward (1957), from “very well sorted” (σ < 0.35) to “extremely poorly sorted” (σ > 4.00)
Skewness is a somewhat less commonly used parameter. Because skewness is very sensitive to incomplete sieve analysis, only the more robust Folk & Ward method is calculated. Standardized names of skewness follow Folk & Ward (1957), from “very coarse skewed” (SkI < -0.3) to “very fine skewed” (SkI > 0.3)
For more details and equations, see the Past manual.
References
Blott, S.J., Pye, K. 2001. GRADISTAT: A grain size distribution and statistics package for the analysis of unconsolidated sediments. Earth Surface Processes and Landforms 26:1237–1248.
Folk, R.L. 1954. The distinction between grain size and mineral composition in sedimentary-rock nomenclature. Journal of Geology, 62:344–359.
Folk, R.L., Ward, W.C. 1957. Brazos River bar: a study in the significance of grain size parameters. Journal of Sedimentary Petrology 27:3–26.