Accelerated Soil Erosion of a Mississippian Mound at Cahokia Site in Illinois

Abstract

About 1000 years ago prehistoric Indians built over 120 earthen mounds, including Mound 57, at the Cahokia site near Collinsville, IL. Loamy and clayey soil materials were apparently placed on the mound before 1100 CE by people of the Middle Mississippian culture. The objectives of this study were: (i) to determine the extent of soil formation during the past 850 yr, (ii) to determine the time period, extent, and direction of tillage, and (iii) to determine the erosion phase of soil on a mound and the extent of soil loss from erosion. The high amount and regular decrease in organic C with depth in 16 soil profiles suggested that since 1925 no tillage and little soil erosion had occurred on the sideslopes of M57. In the sideslope and footslope positions on the north, south, and east transects, the fine‐loamy soil materials with fly ash were thickest suggesting that any tillage translocation and soil erosion must have occurred after 1864. This resulted in the current elongated south to north axis which was parallel to the tract boundary and the direction of plowing. The amounts and distribution of the fly ash in the upper 20 cm indicated that cultivation between 1864 and 1925 may have mixed some fly ash into the 20‐cm deep tillage zone. Fly‐ash content of soil layers to the 40‐cm depth on the footslope was primarily a result of fly ash being deposited after the 1850s on the original soil surface and the subsequent deposition of sediment rich in fly ash.