Soil texture refers to the relative percentage of sand, silt, and clay within a soil layer. Only particles < 2 mm in equivalent spherical diameter  are included in soil texture descriptions because most physicochemical activity occurs in this fine-size fraction, although assignment of soil textural classes involves consideration of particles > 2 mm —termed “skeletal grains” because of their low water-holding capacity. Soil texture and textural classes are an especially important aspect of modern soil research because they affect water-holding capacity and base saturation, which relate to agronomic productivity. However, modern soil texture and textural class assignments are not appropriate for paleosols, which are composed of sediments and sedimentary rocks that are either partially or completely lithified. The textural contrast between soils and paleosols is reflected in the different grain-size definitions used for soil texture  and sedimentary texture