Fig. 3 Stratigraphy of cores and excavations carried out at SM4.

SM4 is placed above fluvial sands that overlay clay deposits (A). Both the sand and clay show high levels of hydromorphism: red and yellow staining due to the formation of iron oxides. These hydromorphic conditions are typical of tropical soils subjected to repeated cycles of humid and dry conditions, as it happens in regions with strong seasonality. However, currently, these sediments are found below a wetland (B), suggesting that these iron oxides are a relict feature of the past, when the average water table was meters below its current depth. A view from the north (C) shows how today the forest island is surrounded by tall marsh vegetation and connected to the mainland by a modern causeway. Photo credits: (B) Source image: DigitalGlobe, Google Earth, and (C) José M. Capriles, PSU.

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