Weathering rinds on volcanic clasts in Quaternary deposits in the western United States contain only very fine grained and poorly crystalline clay minerals. Rinds were sampled from soils containing well-developed argillic B horizons in deposits about 105 yr old or more. The clay-size fraction of the rinds is dominated by allophane and iron hydroxy-oxides, whereas the B horizons contain abundant, well-crystallized clay minerals. The contrast between the clay mineralogy of the weathering rinds, in which weathering is isolated from other soil processes, and that of the associated soil matrices suggests a need to reassess assumptions concerning the rates at which clay minerals form and the sources of clay minerals in argillic B horizons. This study suggests that crystalline clay minerals form more slowly in weathering rinds than is generally assumed for soil environments and that the weathering of primary minerals may not be the dominant source of crystalline clay minerals in middle to late Pleistocene soils.