silts

Silts are a fine-grained sediment consisting of particles typically between 0.063 and 0.002 millimeters in diameter. On the Wentworth scale they lie between sands and clays. Silt grains are usually well sorted and may be coated with clay minerals, which can enhance cohesion when wet and influence plasticity. They often have a high surface area relative to their size and can be deposited in relatively quiet water conditions.

Depositional settings for silts include rivers and floodplains, lake bottoms, deltas, and offshore marine environments where water movement is minimal. Wind-blown silts form loess, a widespread and often highly fertile deposit. Over geological time, accumulations of silt may lithify to form siltstones, a fine-grained sedimentary rock.

In soils, the silt fraction contributes to texture along with sand and clay. Silty soils typically hold more water than sandy soils and have moderate fertility, but drainage can be slower than for sands. They are more susceptible to erosion by water than sands and can be less stable when disturbed. The presence of clay minerals on silt grains can raise cohesion and plasticity, influencing how the soil behaves under moisture and load.

Silt in the natural environment is important for sediment transport and landscape formation. Its small size allows long-distance transport by water and wind, shaping features such as floodplains, deltas, and wind-swept plains. In geology and soil science, silts are distinguished from clay by grain size and from siltstone by the presence of lithified rock.