Wascana Creek Ash, located close to Regina in Saskatchewan, is correlated with Pearlette ash, type O (classification of Izett et al. 1972) on the basis of its mineral assemblage, shard habit, composition of glass, titanomagnetite and chevkinite. Directions of magnetization and normal polarization of clay immediately below Wascana Creek ash further support this correlation, which is essentially confirmed by the fission track age of volcanic glass and zircon that show Wascana Creek Ash to be 0.6–0.7 Ma.Pearlette ash, type O, is derived from vents in Yellowstone National Park in Wyoming and Idaho, situated about 800 km southwest of Wascana Creek in Saskatchewan. Its air-fall tephra is widely distributed in the form of isolated lentils over the midcontinent region of the United States where stratigraphic relationships suggest a late Kansan, Yarmouth, or early Illinioan age.Wascana Creek Ash and its host clay occupy an intertill position; they rest directly on sandy till, interpreted as part of the Floral Formation, and are covered by till of the Battleford Formation. Thus, the Floral Formation in Saskatchewan may well be as old as the Kansan—that is, decidedly older than most current estimates, which favour an early Wisconsin age.The horizontal disposition and palaeomagnetic characteristics of the host clay, uniformity of carbonate content of the underlying till, and absence of local glaciotectonic features strongly suggest that the tephra and clay are in situ and not glacial erratics that have been transported as blocks from an older deposit. Furthermore, extensive reworking of the tephra from an older horizon is considered unlikely in view of minor detrital contamination, fresh glass, and conspicuous crop of glass-sheathed heavies. However, the stratigraphic position of Wascana Creek Ash should not be considered as firmly established for these observations of necessity pertain to just one locality; further occurrences must be found and studied before its stratigraphic position can be confidently defined.