Genesis of a Vertisol and an Associated Mollisol in Northern Utah

The genetic relationship between a Vertisol (Chromoxerert) and an associated Mollisol with an argillic horizon (Palexeroll) was investigated. These soils occur in the Wasatch Mountains of northern Utah and have similar parent materials, altitudes, and slope gradients. Erosion is prevalent on the Vertisols because the wyethia ( Wyethia amplexicaulis ) plant community does not adequately cover the soil surface during the wet season of fall through early spring. Dense stands of Gambel oak ( Quercus gambelii ) and a continuous O horizon on the Mollisols protect them from erosion. The Mollisol subsoil and the Vertisol solum both had cracks, slick‐ensides, high clay contents, high coefficient of linear extensibility (COLE) values, and smectite as the dominant clay mineral. These characteristics of high shrink‐swell activity were most strongly expressed in the Vertisol. The Mollisol subsoil clay was overlain abruptly by a loam surface soil. The textural difference was attributed to lessivage, in‐situ clay formation in the subsoil, and eolian contributions mixed with pre‐existing surface soil by soil animals. Eolian additions were inferred from the presence of easily weathered silt and clay‐size minerals in the Mollisol A horizons and within the zone of mixing in the Vertisol. These minerals were not present or were less abundant in the Mollisol subsoil and the Vertisol C horizon. The formation of these Vertisols was probably initiated when erosion removed the A horizons of the Mollisols, or their precursors, exposing the cracking, clayey argillic horizons at the surface. Modification of vertic subgroup criteria to include all soils with significant shrink‐swell activity in subsoils close to the surface, regardless of surface cracking, is recommended.