In this paper we present new data on the precolumbian geometric ditched enclosures identified in Acre State, western Amazonia, Brazil. Remote sensing and ground survey have revealed 281 earthworks, located mainly on the edges of high plateaus overlooking the river valleys drained by the southeastern tributaries of the Upper Purus River. Excavations have shown that the few existing cultural materials are concentrated on the slopes and in the bottoms of the ditches, as well as on small mounds that were likely remains of houses, whereas the central, flat enclosed areas lack major archaeological features. New radiocarbon dates place the initial stage of earthwork construction as early as ca. 2000 B.P. We suggest that the building of these geometric earthworks may have been a regionally shared phenomenon, especially among the Arawak and the Tacana peoples, who used them for special gatherings, religious activities, and, in some cases, as village sites.

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.

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Interpretations of the Amazonia prehistory have changed significantly in the last few decades, as the complexity and diversity of the Amazonian cultures are beginning to be documented and understood. Earthworking, a long-term conscious anthropogenic landscape alteration, was a widespread phenomenon throughout the South American tropical lowlands. A variety of earthworks has been documented in the Southwest Amazon, including ditches and embankments of different shapes and sizes, roads, extensive raised fields, canals, causeways, and artificial wetlands linked to adjacent mounds and forest island settlement sites. A field survey and test excavations were undertaken in the region of Riberalta, in the Bolivian Amazon. The purpose of these investigations was to study the distribution and characteristics of the pre-Columbian occupation in the region. We found different types of sites, some without visible earthworks, indicating fairly dense occupation on river bluffs and terra firme, but lacking long permanence in the same location. The earthwork tradition prevailed in the Riberalta region from at least 100 B.C. until the period of European contact. The function of the less-complex earthworks may have been to enclose the occupation areas, and in some cases, to serve as canals. Compared to the variable layout of the sites, the ceramic assemblages of the region are relatively homogeneous. A central objective for future research will be to determine if the earthwork sites correlate with a single or multiple cultural traditions.Key-words: Southwest Amazonian archaeology, earthworks, ceramic traditions.

Geometric earthworks located in the interfluvial zone of the Upper Purús and Madeira river tributaries in the southwestern Amazon are formed by ditches of varying shapes and sizes along with contiguous exterior embankments and roads terraced with low backfill banks. The earthworks in the Brazilian state of Acre were used from 1200 B.C. to the fourteenth century A.D., indicating a continuous, collective cultural institution, a generalized regional phenomenon characterized by local variants and alterations and probably not exclusively restricted to a specified ethnic group. The carefully planned position of the earthworks in the landscape and the recurring geometric forms represented in this earthwork architecture suggest functions that were part of a tradition of shared ideology related to rituals and/or the sociopolitical activities of ancient Amazonian peoples. Recent archaeological fieldwork on two composite earthwork sites, Fazenda Atlântica and Quinauá, situated in the core location of the geometric earthworks, provides further evidence for the predominantly ceremonial use of these constructions.