Wallows are circular soil depressions created by repeated bison (Bison bison L.) dust-bathing. Despite more than a century of bison absence from the Great Plains and lack of evidence on wallow persistence, many studies have classified grassland soil depressions as ‘relict’ wallows. We studied bison wallowing on a tallgrass prairie site in Oklahoma where bison were reestablished in late 1993. Bison use of existing soil depressions fitting descriptions for relict wallows located before reintroduction and bison formation of new active wallows were documented from 1993–1995. Bison avoided existing depressions, instead forming active wallows in different locations. Bulk density and soluble salt, sodium and clay content of soils in existing depressions were significantly higher than soils in active wallows or nearby reference sites. These depressions occurred primarily on shale-derived soils rich in silt and clay, whereas active wallows were formed mostly on sandy loam soils overlying sandstone. The spatial distribution and soil conditions of these depressions suggested pedogenic, rather than animal disturbance, origins. Soil sampling beneath the depressions revealed a dense clay lens located above a natric (high exchangeable sodium content) horizon, both of which were absent from soils in nearby reference sites. Natric soil horizons, known as claypans, are formed by pedologic processes common to soils derived from marine shales. Thus, these soil depressions, and possibly other ‘relict’ bison wallows, are not persistent soil disturbances resulting from bison wallowing, but small patches of landscape and soil heterogeneity resulting from variation in underlying geological materials.