6–14 to 6–17). Pollen diagrams for cores I through IV cover all or part of the timespan discussed in this article. Being the only ones in either Puebla or Tlaxcala that clearly reach into the historical era, they are of potential relevance, unfulfilled because their chronometric control is limited to two radiocarbon dates. Maize is present throughout their depth, in frequencies so high that lakeshore agriculture can be taken for granted. The presence of Eucalyptus at depths of up to 430 cm makes me suspect that much of the diagrams and the deposition belongs to the 20th C., when this Australian tree was widely used in reforesting dynamited or bulldozed tepetates.
Documentary references to find more rapid sedimentation in the wetlands can be found for almost any period. But, in these strongly depositional environments, it is the relative rate of sedimentation that matters, and this we know little about. In sum, the scarcity of positively identified alluvium later than the Early Postclassic SP600125 mw seems incommensurate with the amount of historical erosion inferred by fieldwork on slopes. If
this pattern holds, two hypotheses may explain it. One is that the sediment is still largely stored on slopes, and that terracing, despite its repeated failure, has decreased the connectivity of slopes and valleys. The other is that historical streams became overfit to such a degree that they exported most sediment to southern Tlaxcala or beyond. There are many potential caveats. Along some reaches, historical alluvium may lie buried under the active floodplains. Alluvial records are fragmentary, and quantitative estimates of historical sediment transfers nearly impossible in open-ended systems. Lakes may offer a partial solution, but in Tlaxcala they have been drained or flooded by reservoirs, and their topmost sediments disturbed in
a myriad ways. Chronology is the Achilles’ heel of most arguments presented above. The problems are both methodological and theoretical. In Tlaxcala nobody has committed resources to the radiometric dating of Postclassic and later deposits. Periodizations based on styles of material culture are coarse for the Postclassic, and virtually non-existent for the historical era. The theoretical challenge is to arrive at explanations Gemcitabine that integrate cultural and environmental processes operating on different timescales. Readers familiar with the terminology of Fernand Braudel (1987) will recognize in rows A–Y of Table 2 his conjonctures, while rows X–Z may be eligible for the status of structures. The insight from Tlaxcala is that, in geologically young tropical landscapes, ‘geological’ change is measurable on timescales of a human lifespan. Therefore, instead of being relegated to the longue durée, it can be used to index certain economic or social conjunctures.