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This paper summarises the results of on-going research on the valley floor by a multidisciplinary team of archaeologists and specialists in soils, pollen and phytoliths. The research project is designed to chart the history of human occupation of this complex and frequently flooded area during the Holocene and the interaction between this environment and the human population. In the mountains surrounding much of the upper and central Cauca valley there is a gap in our knowledge of human occupation of a thousand years and more with the most recent dates for preceramic sites in the third millennium B.C.; furthermore these later sites appear to be far less numerous than those known from earlier in the Holocene. This "archaeological silence" comes to an end about 700 B.C., by which time the area was inhabited by established farmers with sophisticated pottery. The period of silence is precisely when we would expect to find evidence for early Formative activity in the area, including an intensification of horticulture. Project results, so far, include evidence for local climatic fluctuations with wetter and drier periods, tentative evidence (burning) for human disturbance of the environment from very early in the Holocene (late IXth millennium B.C.) and firm paleobotanical evidence from one or probably two sites for cultivation (maize and arrowroot) associated with burning or other forms of disturbance of the vegetation within the period of "archaeological silence". Although no artefacts from this period have been found so far, a considerable number of sites were located from the late Formative period (Ilama and its daughter culture, Yotoco), testifying to a much more intensive occupation of the valley floor and its floodlands at this time than had been documented previously.

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This paper summarizes the results of ongoing archaeological and palaeoecological research on the floodplain of the river Cauca and adjacent piedmont, with the aim of reconstructing an outline history of the human occupation of the region and its interaction with the environment, beginning in the Late Pleistocene. The difficulties of locating occupation sites in this landscape cannot be overestimated. Although valuable for its preservation of palaeosols, the accumulation of sediment deposited by the river Cauca and its tributaries during periods of flooding can be extremely rapid and the earlier archaeological sites (1000 B.C. to 500 A.D.) are generally buried beneath several metres of alluvium. Under these circumstances, traditional site survey has limited potential and the most promising strategy is the location of palaeosols with evidence of human activity based on soil analysis and palaeobotanical studies. Sedimentology from a dozen deep cores is providing invaluable information on the environments that early human populations in the area would have enjoyed or coped with. The cores testify to a highly dynamic river Cauca and its tributaries resulting in a series of rapid local environmental changes. Besides extensive periodic flooding, past populations were probably affected by tectonic events since numerous faults cross the region. Volcanic ash is a component of many of the soil cores but much was redeposited material from earlier falls, probably during the Pleistocene. Direct archaeological evidence of human activity during the Early and Middle Holocene is limited to a mastodon skeleton with butchering marks on its ribs, and to a surface find of a stone tool (azada) characteristic of this period. Of the numerous fertile palaeosols detected in cores, some have agricultural characteristics and there is evidence of fires, possibly for clearing fields, from the late seventh millennium B.C. while by the third millennium B.C. a site provides evidence of burning in combination with the cultivation of maize (Zea mays) and arrowroot (Maranta arundinacea). While a relatively large number of archaeological sites testify to human activity over much of southwestern Colombia during the Early and Middle Holocene, research has drawn attention to a period of archaeological silence between 2500-1000 B.C. when Formative societies were developing in other regions of South America. An important focus of this project is the search for traces of human activity during this period. The palaeosols have important potential for acquiring information on this question since stratigraphical evidence suggests that many lie within this time range. By the Late Holocene (c. 500 B.C.) the Ilama population was established in that region of the alluvial valley centred on Lake Sonso and the town of Palmira, followed by Yotoco and, further south, Malagana; in contrast, in the northern sector of the valley evidence for this sequence remains tentative. Here the only settlement site located so far with very late Formative characteristics produced an ...