ABSTRACT
Similar results that arise from different processes and causes (convergence or equifinality) and different results that arise from similar processes and causes (divergence) are two common problems of explanation in any historical science. Two cases rooted in geoarchaeological fieldwork in highland Mexico exemplify such problems. A widespread entrenchment of the stream network of the Nochixtlan Valley occurred at ca. A.D. 1000. Potential proximate causes include changes in runoff, sediment delivery from slopes, adjustments of stream gradient, or channel form. Each hints in turn at a plethora of converging ultimate causes in realms such as climate, demography, agriculture, or warfare, but choosing between them is impossible without widening research to include issues usually considered to be beyond the scope of environmental archaeology. Tlaxcala experienced cycles of demographic explosion and collapse between 500 B.C. and A.D. 100, and again between A.D. 1200 and 1600. Each cycle was accompanied by agricultural intensification and disintensification, but the form they took were very different. In the first cycle, population growth resulted in severe land degradation, in the second in widespread land improvement. The divergence seems to be due to differences in cultural context and accumulated historical experience. In order to satisfactorily resolve problems of explanatory convergence and divergence, environmental archaeologists must constantly shift attention between process and history, and actively contribute to endeavors such as chronology building, on-site excavation, and the long-term curation of collections.
