The host sediment is a desert loess, forming a gypsiferous sonochak soil with a thin salt crust. The nodules occur only in areas where the capillary fringe intersects the ground surface, usually where the surface is within 1.5 m of the water table. Nodules a few centimeters beneath the surface are white, very fine grained gypsum. Nodules at lower levels are of more coarsely crystalline gypsum. Phreatic groundwater brines, although saturated for calcium sulfate, are only moderately hypersaline, about 60 per mil. New evidence suggests that they are mainly of marine origin. Surface evaporation causes capillary brines above the water table to move upwards through the loess and to increase in salinity. A marked increase in the m Mg (super 2+) / m Ca (super 2+) ratio of the interstitial brines at the zone of fine-grained nodules demonstrates active precipitation there of calcium sulfate. It is because of the existence of a very high salinity zone (>300 per mil) with halite, at the top of the profile, that gypsum precipitation takes place beneath the surface so as to form nodules rather than a surface crust. Na (super +) and Cl (super -) ions, in addition to Ca (super 2+) and SO 2 (super 2-) ions, seem to be necessary for the development of gypsum nodules by the "per ascensum" mechanism.--Modified journal abstract.