The 600 square mile volcanic area of the Sierra Pinacate, in extreme northwestern Sonora, Mexico, comprises a unique geologic and ecologic enclave in which archaeological remains have been undisturbed by erosion. Situated in an essentially waterless desert, its semipermanent waterholes have been foci of human activity since man's first entry into the region. Three geologic phenomena indicate the chronology of artifacts in the region. Desert pavements formed by aeolian deflation of loess are altithermal phenomena. Two degrees of thickness of desert varnish on pavement and associated artifacts provide evidence that man was present during a pluvial period preceding an altithermal episode centering around 17,000 B.C., as well as throughout the well-known Pluvial period. The formation of caliche below ground level helps place excavated artifacts in sequence. M. J. Rogers' term “Malpais” is revived and assigned to the earlier pluvial occupation, basal to “San Dieguito Phase I (SD I)” of the Pluvial. Diagnostic criteria which involve both archaeologic and geologic elements have been developed for both periods, and their extension to related areas is briefly discussed.