Soil formation in a toposequence of soils was studied in highly calcareous parent materialunder semi-arid conditions of southern iran. Soils with salic horizons (typic salortids) have been formed on aflood plain with shallow ground water, soils with nitric horizons (typic natrixeralfs) on a lower terrace with moderately deep ground water, and soils with argillic horizons (calcic haploxeralfs) on a higher terrace with very deep ground water. Exchangeable na, the major cause of clay dispersion and migration, is only effectivewhen excess salts have been leached out, salinization and alkalization, desalinization and solonetzation, and dealkalization are three successive stages in the formation of these soils. X-ray and electron optical analysis indicated that the clay-size minerals are of similar type, but differ in relative occurrence. 1.desert areas management M.S.C student, natural resources college, university of Tehran,iran 2. soil and water management professor, natural resources college, university of Tehran,iran 3.watershed management M.S.C student, natural resources college, university of Tehran,iran