Illuvial Horizon a horizon that has been enriched by addition of materials from the overlying horizons. The materials accumulated can be clay, iron, aluminium, humus, sesquioxides, carbonates, soluble salts or a mixture of two or more of these materials. The accumulations can occur in the form of coatings or nodules or can be distributed more or less uniformly in the horizon. In sub-humid sub-tropical regions like Bangladesh, illuviation plays a major role in developing the characteristics of soil horizon. Oriented clay skins or argillans occur in the subsoil and substratum of most terrace soils and in some hill soils, but they are rarely seen in floodplain soils. In only one soil series (Kashimpur, a deep red brown terrace soil occurring on the madhupur tract and barind tract) these clay skins are prominently developed on ped faces. [Sirajul Hoque]