Thus, in evaluating the occurrence and significance of enrichment it is important to evaluate mineral source(s), hydrology and relative age of the soil-landscape. Relative age is important because many of the processes of enrichment are necessarily time dependent. The processes associated with the accumulation of materials in Aridisols are: (i)lessivage or eluviation/illuviation of clays - argillic horizons, (ii) silication , i.e., the accumulation of silica - duripans, (iii) calcification , i.e., the accumulation of CaCO 3 - calcic or petrocalcic horizons. The hardening of soil material may lead to a decrease in volume of voids by infilling with salts and silica. This process is responsible for the formation of petrocalcic, petrogypsic horizons or duripans.