Monumentality in Western Amazonian formative societies: geometric ditched enclosures in the Brazilian state of Acre

In Amazonia, monumentality has traditionally been considered characteristic of the late pre-colonial densely populated complex societies. Recent archaeological fieldwork concerning the geometric earthworks in the Brazilian state of Acre has shown that the southwestern Amazonian interfluvial zone was a significant setting for long-term large landscape modifications. We describe the geometric ditched enclosure sites of Acre as early monumental public spaces reserved for ceremonial purposes, analogous to the central Andean ceremonial-civic centers of the Formative period. The geometric earthwork sites contain contiguous ditches and embankment structures of varying forms enclosing areas typically 3-10 hectares in size. Documented cultural features are sparse within the enclosed areas. Making use of satellite imagery, aerial photographs, and pedestrian surveys, 360 earthwork enclosures have been recorded in southwestern Amazonia. Our radiocarbon dates suggest that construction and use of geometric earthworks began at the latest around 1000 BC, and prevailed in the region until 1400 AD. The relatively small number of ceramics recovered from the geometric ditched enclosure sites appear to be local substyles of the same tradition, sharing certain attributes with contemporary ceramic traditions of the upper Amazonian region. This, and consistency in ceremonial earthwork architecture, indicate close cultural interaction between communities that built and used the earthwork sites, and imply probable relationships also with the central Andean area.