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Where precipitation is ample (udic moisture regimes) and evidence of illuviation clear, argillic-horizon genesis is often easily understood. In drier climates (aridic to ustic), argillic horizons are often interpreted as relict features of the Pleistocene. However, in high montane, western landscapes where climate falls in between these two extremes, the role of aeolian clay inputs vs. in-situ weathering contributing to clay formation is of debate. Two mollisols, one developed in quartzite alluvium (Charkiln series - 3 pedons) and one in limestone alluvium (Troughspring series - one pedon) were examined to determine whether atmospheric dust or local geologies contribute to their development, and whether Bt horizons in both pedons were argillic horizons. We hypothesize that both soils have argillic horizons and that clays are derived from site parent materials. Thin-section specimens of Bt horizons, and sand and silt fraction grains were prepared and examined using a petrographic microscope. Regional dust XRD mineralogic data were compared to XRD rock fragment and soil mineralogic data. Clay films, while seen in all field pedons, were very few and faint in the Troughspring series. Illuviation of clays was evident in thin sections of the Charkiln pedons but not the Troughspring peclon. In the Charkiln series, illuviation is observed as clay-depleted A horizons, clay-accumulating B horizons, clay bridging, clay films on grains and in channels. In the Troughspring pedon, clay-depleted A horizons and clay-accumulating B horizons, very few clay films on grains and some pressure features in packing voids are seen, but grain bridging and coatings on channels are not observed. While local geologies influence soil mineralogy in both series, aeolian dust deposition is as significant and its affect is further enhanced by organic-acid weathering from an overlying pinyon-juniper woodland and the current ustic climate regime. While evidence of an argillic horizon is seen in the Charkiln series, excess Ca in soils is believed to be inhibiting illuviation in the Troughspring series and thus preventing argillic-horizon development. (C) 2009 Elsevier B.V. All rights reserved.