Silt

Definition

A loose granular substance resulting from natural erosion or from splitting of larger rock and sand particles and having two meanings: a textural class if used in terms of aggregates of silt-sized grains; or a size class if used in terms of a single grain.

The two most widely used size classification schemes to define grain size are the Unified Soil Classification System (USCS), ISO 14688, and the Krumbein phi scale (Tables 1 and 2). The Unified Soil Classification System combines silt with clays into fine-grained sediment (often referred to as “mud”) but distinguishes the two based on their plasticity and organic content. ISO 14688 grades silt between 0.002 and 0.063 mm. The Krumbein phi scale defines silt as particles 0.0039–0.0625 mm. There are other less common classification systems. As a textural class, the soil or sediment beds can be called silt if the silt content is greater than 80% (ASTM 2000).

Table 1 Silt sizes according the...