Archaeological Mounds in Marajó Island in Northern Brazil: A Geological Perspective Integrating Remote Sensing and Sedimentology Dilce de Fátima Rossetti, 1, * Ana Maria Góes, 2 and Peter Mann de Toledo 1 1 Instituto Nacional de Pesquisas Espaciais-INPE, Rua dos Astronautas 1758-CP 515, 12245-970 São José dos Campos-SP, Brazil 2 Universidade de São Paulo-USP, Instituto de Geociências—Programa de Pós-Graduação em Geoquímica e Geotectônica, Rua do Lago, 562 Butantã 05508-080 São Paulo, SP, Brazil Earthen mounds with archaeological artifacts have been well known in Marajó Island since the 19th century. Their documented dimensions are impressive, e.g., up to 20 m high, and with areas as large as 90 ha. The mounds, locally known as tesos, impose a significant relief on the very low-lying landscape of this region, which averages 4 to 6 m above present sea level. These features have been traditionally interpreted as artificial constructions of the Marajoara culture, designed for defense, cemetery purposes, or escape from flooding. Here, we provide sedimen- tological and geomorphological data that suggest an alternative origin for these structures that is more consistent with their monumental sizes. Rather than artificial, the Marajoara tesos seem to consist of natural morphological features related to late Pleistocene and Holocene fluvial, and possibly tidal-influenced, paleochannels and paleobars that became abandoned as depositional conditions changed through time. Although utilized and modified by the Marajoara since at least 2000 years ago, these earthen mounds contain a significant non-anthropogenically modified sedimentary substratum. Therefore, the large Marajoara tesos are not entirely artifi- cial. Ancient Marajoara cultures took advantage of these natural, preexisting elevated sur- faces to base their communities and develop their activities, locally increasing the sizes of these fluvial landforms. This alternative interpretation suggests less cumulative labor invest- ment in the construction of the mounds and might have significant implications for recon- structing the organization of the Marajoara culture. © 2009 Wiley Periodicals, Inc. INTRODUCTION Archaeological vestiges are widespread in the Brazilian Amazonia, with Marajó Island, in the northeast of the State of Pará, containing the most impressive sites stud- ied so far, many dating at least 2000 14 C yr B.P. (Roosevelt, 1991; Schaan, 1997, 2000, 2004; Schaan & Veiga-Silva, 2004). The sites consist of mounds several hundreds of meters long and averaging 3 to 7 m high, but which can locally reach up to 20 m in height. Despite the very gentle undulation, these mounds, locally known as tesos, highlight Geoarchaeology: An International Journal, Vol. 24, No. 1, 22–41 (2009) © 2009 Wiley Periodicals, Inc. Published online in Wiley Interscience (www.interscience.wiley.com). DOI:10.1002/gea.20250 *Corresponding author; E-mail: rossetti@dsr.inpe.br.