The genetic relationship between a Vertisol and an associated Mollisol with an argillic horizon was studied. These soils are taxadjuncts of the Hawkins and Ostler series, respectively. They occur in the Wasatch Mountains of northern Utah and have similar parent materials, altitudes, and slope percentages. Ostler soils have a dense cover of Gambel oak (Quercus gambelii) and occur preferentially on north aspects. Mule's ear (Wyethia amplexicaulis) is the dominant vegetation on Hawkins soils, which are most frequent on south aspects. Erosion is prevalent on Hawkins soils because the mule's ear does not adequately cover the soil surface during the fall, winter, and early spring. The dense oak woods and the litter cover on the Ostler soil protect it from erosion. Both soils dried sufficiently for cracks to develop in their clayey portions. The Ostler soil was drier, probably because it has more and longer transpiring vegetation than the Hawkins soil. The Ostler subsoil and the Hawkins solum both had cracks, slickensides, high clay contents, high COLE values, and smectite as the dominant clay mineral. These characteristics of high shrink-swell activity were most strongly expressed in the Hawkins soil. The Ostler subsoil clay was overlain abruptly by loam surface soil. The textural difference was attributed to lessivage, eolian contributions mixed with preexisting surface soil by soil animals, and in situ clay formation in the subsoil. Eolian additions were inferred from the presence of easily weathered silt and clay-size minerals in the Ostler A horizons and within the zone of mixing in the Hawkins soil. These minerals were not present or were less abundant in the Ostler subsoil and in the Hawkins C horizon. The formation of Hawkins soils (Vertisols) probably resulted from erosional exposure of the clay subsoil of Ostler soils or their precursors.