The composition of the initial material in which some Aridisols are forming that contain argillic, natric and calcic horizons does not readily explain their internal enrichment in phyllosilicate clays or carbonates. Thus, it has been suggested that aeolian inputs may explain this enrichment. However, in some settings subsurface water enriched with clays and especially carbonates may also account for formation of Bt, Btn and Bk(m) horizons.