The stratigraphic position of the Eocene-Oligocene boundary in southwest Alabama, based on macrofossils, differs from that based on planktonic foraminifers. To resolve the dilemma, the Yazoo and Red Bluff Clays and the associated foraminifers were studied at St. Stephens Quarry, Washington County, Alabama. On the basis of planktonic foraminiferal vertical distribution at this locality, the Eocene-Oligocene boundary is at the top of the Shubuta Member of the Yazoo Clay. Cribrohantkenina inflata (Howe), Hantkenina alabamensis Cushman, Hantkenina longispina Cushman, and the Globorotalia cerroazulensis group became extinct, and Globigerina ampliapertura Bolli and Globigerina gortanii (Borsetti) first appear near this horizon. This planktonic foraminiferal vertical distributio allows assignment of the upper Yazoo Clay to the Globorotalia cerroazulensis (s.l.) Interval Zone and the Red Bluff Clay to the Cassigerinella chipolensis-Pseudohastigerina micra Interval Zone.

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