Evidence of leaching below the average depth of water storage is commonly observed in Aridisols and is explained by: (i) more humid paleoclimates, and/or (ii) the influence of occasional, exceptionally large precipitation events. Examination of soil forming processes in arid zones invariably requires consideration of possible paleoclimatic influences (i.e. some features in the soil may have formed under conditions quite different from those operating at present), the periodic occurrence of large precipitation events that can punctuate the otherwise dry environment of these regions and local variation in factors that prescribe soil genesis. It seems to be contradictory that horizons accumulated with clay, sodium, salts, gypsum, or silica occurs in Aridisols which is associated with illuviation of those materials. A prerequisite for leaching or eluviation/illuviation is rainfall. Aridisols occur on landscapes that are more than one million years old, a time scale that has allowed for development of accumulations of clay, carbonates, and silica.