Pedological processes occur during subaerial diagenesis, in the rhizosphere, the zone in which the root system of plants is active. Some plant roots may penetrate deep into the sediment. As a result of extensive evaporation and calcium ion concentration, CaCO 3 is concentrated at the sediment/root interface in column formations that surround the roots. With soil evolution calcareous columns disintegrate progressively into lens-shaped or concretionary structures. At the same time the sparry fabric of the columns changes to a fabric consisting of micron-sized grains. In the transitional stages numerous perforations appear that suggest a leaching process is taking place. Quartz grains incorporated in the concretions are peripherally replaced by calcite spar composed of needle-like crystals of the size of thousands of angstroms. Hence in the concretions the CaCO 3 concentration exceeds that in the columns. This reaction between dissolution of quartz and precipitation of calcite is pH-dependent and takes place under alkaline conditions. Increase in concentration of CaCO 3 is paralleled by an increase in magnesium ions and iron oxides; conversely the strontium concentration falls. Soil processes tend to concentrate iron and magnesium and to deplete strontium. These changes under pedological conditions account for the advanced stages of subaerial lithification.