In contrast, bench terraces ( Fig. 3)
have treads that are almost level from the outset, and are retained by walls of dry-laid stone. Before tillage can start, farmers fill them by hand with earth brought in from elsewhere, or let them trap earth eroded upslope. Under either scenario, they are more labor-intensive than metepantles ( Wilken, 1987, 96–128). Once maintenance is withdrawn, all terraces tend to disintegrate, as the slope recovers its natural gradient. Breached segments of risers (berms or walls) become points of initiation of gullies, which cascade from one selleck chemicals llc tread to another. Gullies also develop along unterraced access routes that separate flights of terraces laterally. The natural disintegration of a terraced slope thus triggers several of the processes mentioned above. They are more violent and the amounts of sediment mobilized greater in the case of bench terraces, because these modify gradient to a larger degree. In the case of metepantles, they could stop once the berms are selleck compound erased and the ditches silted up (LaFevor, 2014). Both scholars and Tlaxcalan farmers have repeatedly observed and measured the geomorphic processes in question on timescales of a human lifespan or shorter,
so that several cycles of degradation could have occurred within the 500-year span of the historical era. On slopes, their physical imprint is limited to tepetate surfaces, erosional pedestals,
Oxymatrine and vestiges of terraces. These are inherently difficult to date and provide only a terminus post quem. In matched depositional settings we can hope to find stratigraphic sequences that yield a higher resolution and a terminus ante quem. These are found in footslope colluvium, gully mouth fans, alluvial and lacustrine deposits. Historical evidence and an understanding of geomorphic process allow us to identify several sets of circumstances within the past six centuries that may have led to land degradation. Table 2 summarizes twelve of them. Most have been identified before by historians, geographers, soil scientists, or agronomers. For the prehispanic era, the traditional view is that of Heine, 1976, Heine, 1978, Heine, 1983 and Heine, 2003 who related population pressure, agricultural intensification, and accelerated soil erosion. He posited substantial degradation in the Postclassic, which roughly corresponds to row A. He is more terse on the historical era, but following the same logic, he would place renewed degradation in the 20th C. (rows H and I). These are within living memory, and because of the involvement of government-sponsored engineers, abundantly documented. Werner, 1981 and Werner, 1988, was their best chronicler, critic, and occasional unenthusiastic participant.