Sedimentation on lower-lying areas occurs by a combination of : 1) accumulation of fine clastic material derived from the flood water, and phytolithic silica, both of which are mixed into the sandy substrate by illuviation and bioturbation and, 2) precipitation of fine-grained amorphous silica from the groundwater, induced by transpiration by aquatic grasses and sedges. Accumulation of silica in the soil profile produces a proto-silcrete. This sequence grades laterally into carbonate-dominated island soils.