Two old drift units called Poti-Malal and Seguro have been differentiated in the Río Grande basin based on relative-age criteria, stratigraphical relationships, morphology and fission-track dating. A tephra dated at 0.226±0.025 Ma was deposited on the Poti-Malal drift and underlies the Seguro outwash, which is inferred to equate with marine oxygen isotope stage 6. The stratigraphical position and age suggest that the tephra post-dates the Poti-Malal glaciation and that it is older than the Seguro drift. The Poti-Malal glaciation must be at least as old as Early-Middle Pleistocene, and the Seguro glaciation is assigned to the penultimate glaciation. The tephra unit may have been deposited during marine oxygen isotope stage 7. 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