MICROSTRATIGRAPHIC INSIGHTS INTO THE SITE FORMATION PROCESSES OF THE SUB-MOUND 51 BORROW PIT AT CAHOKIA MOUNDS, ILLINOIS, USA

Central to the model of how urbanism rapidly emerged at the ancient North American city of Cahokia is the interpretation that layers of fill found in the sub-Mound 51 pit represent accumulations from large-scale ritual feasting events. The borrow pit, which Cahokians filled with cultural material and subsequently buried when constructing Mound 51, is located on the eastern side of the Grand Plaza. It measures at least 56 meters long, 19 meters wide, and up to 3 m deep. Pauketat et al. (2002) conducted detailed analyses of the well-preserved cultural materials from salvage excavations by Charles Bareis (1966-1972), and found that the fill dates to the Lohmann phase (AD 1050-1100). Analyses of 17 soil thin sections prepared from unanalyzed soil monoliths collected by Bareis coupled with new insights on Cahokian practices related to watery places are allowing us to better understand how this feature was used and its importance to the emergence of Cahokia. New observations show that accumulations interpreted as the first zone of fill (Zone G) were a continuation of natural water-laid sedimentation identified at the base of the pit (Zone H), albeit with the deposition of artifacts. Zone F, interpreted originally as a massive fill event of burned thatch, is actually comprised of multiple depositional events, with burned plant remains interbedded by short-term surfaces rich in microartifacts. Relatively clean fill of Zone E caps Zone F, which is then buried by fill rich in ash, artifacts, and other domestic refuse in Zones D1 and D2. A sequence of water-laid deposits of Zones A-C covers a living surface atop Zone D before Mound 51 buried the pit. These findings show that the depositional history of the sub-Mound 51 pit and cultural practices driving these events at the onset of urbanism at Cahokia were much more complex than originally interpreted.