Grain size (particle size) in sediments and soils

Classification and common ranges

Several classification schemes exist; one widely used in geology is the Wentworth scale, which groups particles into named classes. Typical size categories (rounded values) include:

• Clay: less than about 0.0039 mm (very fine particles)
• Silt: about 0.0039–0.0625 mm
• Sand: about 0.0625–2 mm
• Gravel: greater than 2 mm, subdivided into granule, pebble, cobble and boulder

For example, the term "pebble" commonly refers to particles roughly 4–64 mm across, while "granule" and "cobble" denote adjacent size intervals within the gravel range. Many texts convert sizes to the phi (φ) scale, where φ = −log2(diameter in mm), to simplify statistical handling of distributions.

Measurement methods and interpretation

Grain size is determined by different methods depending on particle size and required precision. Coarser samples are sorted with sieves and measured directly by diameter; fine silts and clays are characterized by sedimentation techniques (hydrometer) or by laser diffraction and image analysis. Grain-size distributions are frequently plotted as histograms or cumulative curves to summarize sorting (spread of sizes), skewness and modal size. These parameters help infer depositional environments such as rivers, beaches, dunes or glacial deposits.

Grain size also guides practical decisions: foundation design, filtration, reservoir evaluation and agricultural tillage all depend on an understanding of particle-size composition. In rock descriptions, the same vocabulary describes clastic material within sandstones or conglomerates; precise terminology distinguishes whether a rock is termed sandstone, conglomerate, or mudstone based primarily on the dominant grain-size fraction (diameter, sediment, clastic rock).

Historical development of grain-size classification reflects both laboratory advances and practical needs. Early 20th-century work standardized names and ranges; later, instrumental techniques allowed finer resolution and numerical analysis. Modern studies combine field description with instrument data to reconstruct transport history and depositional processes. For further technical guidance and classification details see authoritative references or online resources (classification guides).

Notable distinctions: grain size is distinct from grain shape and composition — two sediments with identical size distributions may behave very differently if grains are angular, flat, or composed of different minerals. Understanding size alongside texture, mineralogy and fabric gives a more complete picture of sediment origin, history, and engineering behavior.