5 Origin of upland white sand

The white sand is the result of removal by the action of subsurface water (soil water and groundwater) of clay and various minerals other than quartz from the brown sand. The abundance of subsurface water on the seemingly water-deficient tabuleiros, originally composed of thick and permeable sandy sediments, is explained by the existence of "hardpan" (a lateritic duricrust) formed near the surface. This hardpan acts as an impermeable layer to maintain a shallow groundwater table beneath the tabuleiro surface.

Results of penetration resistance (hardness) tests for the soil layers on the Conde upland (fig. 11.8) may indicate this process. The tests were carried out at several points along a line across the boundary of white and brown sands (A-B in fig. 11.5). In the soil profile of the white sand, a welldeveloped hard layer (hardpan) is found relatively close to the surface (2-3 m in depth), on which shallow groundwater remains. On the other hand, generally no hardpan or groundwater body is evident at a shallow depth in the brown sand profiles. In the soil profile at the narrow transitional zone between the white and brown sands, a thin hard layer develops nearly at the same depth as the duricrust in white sand, indicating the embryonic stage in the formation of the hard layer at a shallow level.

The existence of a shallow hard layer and groundwater under white sand is observed also in many outcrops of upland podzol in the ParaíbaPernambuco area, where groundwater issues as springs at the boundary between the layers of loose white sand and the underlying consolidated, yellow-coloured sand (hardpan).

The formation of the hard layer is related to the precipitation of iron and/or aluminum oxides at the level of the groundwater table or where soil water evaporates. Once such an impermeable hard layer is formed at a shallow level, the groundwater developed on it accelerates the eluviation of elements and clay in the surface soil layer, leading to the formation of white sand.