A pedologic cover on the Occidental Plateau in Botucatu, Sã o Paulo State, was studied using a structural analysis approach. In upper landscape positions yellowish red to dark reddish brown (5YR hue) and sand-loamy to loam-sandy oxisols occur that exhibit a microaggregated structure. In lower landscape positions, soils (Nitisols) have higher clay levels, with redder colors (2.5 YR hues) and microaggregated structure at depth (oxic horizon) and a polyhedral one in subsurface (argillic horizon). The character of the soil toposequence largely reflects the underlying lithosequence: in higher landscape positions soil materials are sandy and are related to the underlying Cretaceous sandstones of the Bauru Group; in downvalley directions the sequence cuts Jurassic basalt of the Serra Geral Formation. The transition between sandstone and basalt is frequently marked by the presence of conglomerates rich in quartzite pebbles. The most important organizations in the pedological cover is presented in lateral distribution (scheme): the oxic horizon group with microaggregated structure; the sombric horizon; the stone-line and the charcoal fragments. Special attention was paid to transitions between the different pedological organisations.

Soils with oxic horizons that display microaggregate structures are distributed generally from upvalley to downvalley across the whole of the pedological cover. Microaggregated structure consists of oval packed microaggregates, with diameters that range from 50 - 500 microns. The usual explanation is that microaggregation is owed to physico-chemical processes, that is in stable union with certain forms of iron in connection with kaolinite. Evidence observed on the Botucatu Plateau, however, suggests a biological origin, mainly ant and termite activity.

The stone-line appears as a subsurface pebbly layer in the pedological cover. It has a wide distribution, not only in the research area but generally throughout the tropics. Though the composition of its component clasts is varied, weathering-resistant quartzite pebbles are rather common. Quartzite clasts prove stone-line allochthony where the subjacent substrate is basalt. On the matter of stone-line genesis, many authors favor the explanation that stone-lines are of sedimentary (erosional/depositional) origin, which would explain their wide spatial distribution. Considering pedological cover in Botucatu, however, that explanation cannot apply because the stone-line is present across the lithodependent lateral differentiation. The basalt related filiation grade of the stone-line overlaying materials increases from upstream to downstream. Considering the autochthony rising of the stone-line overlaying materials from upstream to downstream, the vertical translocation of soil material by ants and termites is the only mechanism that can explain stone-line genesis in this case.

The sombric horizon is a superficial paleo-horizon buried by soil materials vertically translocated by the soil fauna, mainly ants and termites. A quantitative estimate of the amount material translocated by soil fauna is presented. Abundant charcoal fragments are also present in the soil, and radiocarbon measurements on these allow us to estimate the rate of the bioturbation processes. On this basis, the amount of soil that has been biogenically translocated is estimated to be between 10.000 and 13.000 ton/ha. Charcoal that was subject to bioturbation and translocation has been dated at 4400 radiocarbon years BP. Rates of soil translocation in this soil range from 0.2 to 0.3 mm/year, or 2 to 3 cm/100 years, or 20-30 cm/1000 years. This suggests that stone-lines can form in the tropics by bioturbation rather quickly, and without invoking major climate or other environmental change.