Adjacent soils frequently exhibit different profile characteristics because of differing parent materials.
They are formed on geologically old parent materials in humid tropical climates, with rainforest vegetation growing in the natural state.
Parent materials influence soil formation through their mineralogical composition, their texture, and their stratification (occurrence in layers).
Superimposed on those broad, climatically determined, soil patterns are local variations caused by topography, groundwater conditions, and parent materials.
These soils naturally form on medium- to fine-textured parent materials under any climatic, topographic, and vegetative-cover conditions.
Clay minerals other than those mentioned above usually occur in various soils as minor components inherited from the parent materials of those soils.
These soils strongly resemble their geologic parent materials, as there has been insufficient time to alter these materials into soils with strongly developed horizons.
More subtle discontinuous lithosequences, such as those on glacial tills, show systematic variation of mineralogical composition or of texture in unconsolidated parent materials.
In practice, parent materials can be deposited continually by wind, water, or volcanoes and can be altered from their initial, isolated state, thereby making identification difficult.
Parent materials are mainly Pleistocene sands aged ~ 30 kyr BP.
Silty alluvium and loess comprise 86% of the parent materials.
