Profiles of sediments in the Basin of Mexico show an inverse relation between the percentages of fossil pine and oak pollen. Vegetation studies suggest that any marked increase in oak indicates an increase in available moisture, while consistently low percentages of oak indicate moisture deficiency. On this basis prolonged moist and dry periods have alternated. The more recent of these climatic shifts can be correlated stratigraphically with known phases of human occupation (Sears, 1951b).
The Archaic Culture, 2000 or 1500 to 500 or 400 B.C., and the Nahua, 800 or 900 to 1521 A.D., both flourished within the Basin during relatively moist conditions. These two humid periods were separated by a prolonged dry interval which overtook the Late Archaic during a time of very low lake level.
The Late Archaic level was then covered by a volcanic ash fall, and the culture shifted northward to higher ground. There it became the Teotihuacán which lasted from 500 or 400 B.C. to 800 or 900 A.D. Since this highly developed culture existed during a dry period, it must have made use of ground water. If so, the sources should have been unfavorably affected by heavy soil erosion known to have occurred, and by any deforestation, as Vaillánt (1941) suggests in explanation of the ultimate Teotihuacán collapse.
Conclusions based on pollen analysis are consistent with what is known of soil profiles and former lake levels.