Under the classic Bryan-Antevs model the climatic history of the Southwest has been divided into (1) pluvial, (2) early postpluvial or Anathermal, (3) mid-postpluvial or Altithermal, (4) late postpluvial or Medithermal. Pollen analysis of radiocarbon-dated pluvial deposits shows that the desert grassland of southern Arizona and the short grassland of the Texas High Plains were invaded by a pine forest or pine savanna during the last pluvial period. Sulphur Spring-age alluvial deposits at Double Adobe, dated as pluvial in age by Sayles and Antevs, do not contain the pine-pollen record associated with pluvial conditions elsewhere; six radiocarbon dates from deep alluvial deposits at Double Adobe range from 7756 to 9350 years B.P. On the basis of pollen content and radiocarbon-age estimates, the beds are of early postpluvial rather than pluvial age, and thus the Sulphur Spring-stage artifacts found there are also early postpluvial in age.
Under the Bryan-Antevs model the Altithermal, 7500-4000 B.P., is considered an especially hot, dry period. Sediments associated with three radiocarbon dates within this interval contain slightly more pollen of mesic and hygric indicators than do sediments postdating the Altithermal. Interpreting the results literally, the Altithermal was wetter, not hotter and drier, than at present. The limitations of pollen analysis are not fully understood. But the traditional view of a hot, dry Altithermal in southern Arizona cannot be verified from fossil pollen evidence available to date.