Two cores taken from near the center of Mexico City are each divisible into seven zones; the division is based on differences in texture, biota, waterholding capacity, and stratigraphy. Zones of the same number in each of the cores are similar in nearly all respects.
The lake and its surrounding basin are in the same physiographic and climatic province, and the rocks of the basin are consanguineous and are either pyroclastics or flows. Rocks in the Mexico City basin are predominantly glassy or hypocrystalline; many are vesicular and subject to rapid weathering and generally are easily eroded. Sediments derived from these rocks are rather sensitive indicators of variations in climate. Throughout the time sedimentation was taking place in the lake there were in this region many volcanic eruptions, and some tectonic activity, and these too have affected the vegetation and lake sediments, which include clays, fresh to weathered particles of ash and clastics from the flows, and authigenic, chemical, and organic deposits as well as pollen and plant fragments. A tentative history is given of the climates, volcanism, and tectonic activity in this area during the time represented by the sediments.