Geochemical model of sand silicification during glacial periods. (a) Quartz solubility according to temperature. Silica solubility decreases by lowering temperature. Hytec geochemical modelling (Van Der Lee et al., 2003). (b) Hydrological and thermal conditions at a plateau edge. (c) Silica precipitates from groundwater on meeting cold ground. (d) The degree of oversaturation of the water is proportional to the rate of water cooling and results in a sequence of silica minerals arranged along the flowpath. (e) Silica precipitates in the active layer above permanent cold ground or even permafrost.