When water moves through the soil, it moves small colloidal-sized particles with it. These particles of clay, iron, humus, and calcium carbonate will be deposited in zones below the surface or in the subsoil. The zones are called illuvial zones and the process is referred to as illuviation. Illuvial zones are most often referred to as B horizons, and the subscript with the B designates the kind of material translocated (i.e., B_t = clay accumulation).