X-ray, differential thermal, and chemical methods were used to study the clay fractions of soil samples from 21 localities in the Pampa region of eastern Argentina. A Pleistocene loesslike sediment consisting of eolian arkosic and pyroclastic silt is the principal parent material, and detrital illite is the predominant clay mineral throughout the region. In the La Plata river coastal area, the parent material is a Holocene estuarine deposit, and the clays are composed of equal amounts of montmorillonite, kaolinite, and illite; in the area east of the Parana river the parent material is Pleistocene clayey silt and the clay fraction of associated soils is rich in montmorillonite. No authigenic clay minerals were observed in any of the soils.

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