Grain size is the average diameter of clasts (particles) of clastic sediments and rocks. Grain size is the primary parameter in sedimentary geology to divide clastic rocks and sediments in different classes for classification purposes. This subdivision is based on the Udden-Wentworth scale, which differentiates four major classes of sediments (and corresponding sedimentary rocks) that are further divided in sub-classes:
– gravel or ruditic grain size (> 2 mm), which includes pebbles (2 – 64 mm), cobbles (64 – 258 mm) and boulders (> 258 mm). The corresponding rocks are conglomerate and breccia (further differentiated based on roundness);
– sand or arenitic grain size (0.0625 – 2 mm) and the corresponding sandstone;
– silt or siltitic grain size (0.004 – 0.0625 mm) and the corresponding siltstone;
– clay or lutitic grain size (<0.004 mm), whose lithified counterpart is claystone.

The 0.0625 mm boundary between silt and sand divides particles that are visible to the human eye (i.e. sand) from particles that are too small to be recognized in the field (silt and clay). The recognition of the smaller grain sizes is possible through special techniques (sieve analysis, thin sections, SEM). In the field, geologists normally classify silt + clay together as mud (pelitic grain size), comprising all grain sizes < 0.0625 mm.

Why is grain size important?
Siliciclastic sedimentary rocks are the result of erosion, transport, and deposition. The grain size of a sediment reveals important information about (1) the agent of erosion/transport, and (2) the velocity of the current that transported the sediment. Some sedimentary processes (e.g. gravity flows like debris flow) produce deposits where different grain sizes are mixed together (poorly sorted deposits), whereas other processes, like wind or waves, are selective and produce well-sorted sediments, since they can transport only certain grain sizes. Hence, the study of grain sizes in sedimentary rocks allows us to understand the processes that formed them.

How to estimate grain size
There are several fairly accurate methods to measure grain sizes like sieve analysis and contouring/counting on thin sections. In the field, we can estimate grain size through a visual comparison with grain size charts like the one I have drawn, below. These charts are normally centered on the arenitic grain sizes, since sand grains are difficult to measure, whereas for gravels we can use rulers or measuring tapes.