Fossil soils in alluvial red beds from the Juniata Formation, near Potters Mills, in central Pennsylvania, provide evidence of soil-forming processes during Late Ordovician time. Paleogeographic and facies considerations indicate that the fossil soils formed on flood plains west of the Taconic uplift.

Most studies of paleosols of this age or older have considered soils developed on metamorphic or igneous basement rock. Alluvial fossil soils provide evidence of conditions during shorter intervals of weathering without problems of overprinting by successive and different weathering regimes. They can be recognized by the presence of trace fossils and the development of soil horizons and structures. Problems associated with such fossil soils include establishing the nature of the parent material and distinguishing clay formation in the soil from originally deposited fining-upward cycles.

These difficulties can be overcome by comparing paleosols in different stages of development, as indicated by degree of ferruginization, density of trace fossils, amount of clay, and abundance and size of caliche nodules. In modern soils, caliche forms in alkaline conditions under which TiO₂ is stable. Gains and losses of oxides (measured in grams per cubic centimeters) relative to TiO₂ in a strongly developed paleosol were compared with those of a weakly developed paleosol, taken to approximate the compositional range of the parent material. Concentration ratios indicate significant soil development of the strongly developed paleosol beyond the compositional range of the weakly developed paleosol. There was depletion of SiO₂ and enrichment of Fe₂O₃, Al₂O₃, and K₂O relative to TiO₂. Anomalous enrichment of K₂O has been documented in other fossil soils. Both x-ray diffraction studies and a strong correlation between K₂O and Al₂O₃ are evidence that most of the potassium is contained in sericitized illite.