Pollen analysis of two deep lacustrine cores under Mexico City indicates a series of moist-dry oscillations with longer trends of changing temperature—a climatic record which appears to extend as far back as early Wisconsin. The individual moist pulsations, although marked by warmth, were evidently times of glacial nourishment; the dry phases, some of which were cool, were evidently times of abatement.
The upper 50 m of the profiles indicate two episodes of maximum glaciation; the details that follow the more recent agree with present knowledge of the Cary-Mankato-Latest sequence of the Wisconsin. Under milder conditions between the postulated Taze-well and Cary glacial maxima, soil might have formed on exposed till. The base of the profiles indicates warm-moist conditions with falling temperature to about 50 m. Because of tectonic and volcanic disturbance, it is not clear whether this cooling represents a separate advance or merely an early phase of the first maximum above the 50-m level.
For the tropical Mexico City area, it is shown that the moist periods of glacial nourishment were also relatively warm.