ASSESSMENT OF SOIL DISTURBANCE USING MAGNETIC SUSCEPTIBILITY AND FLY ASH CONTENTS ON A MISSISSIPPIAN MOUND IN ILLINOIS

Abstract

The Twenhafel Mound, a Middle Mississippian burial mound in Jackson County, Illinois, was most likely built between A.D. 500 and A.D. 900. The existence of the mound has been known since the French settled in the nearby town of Cahokia in 1699. Since the time of European settlement, the mound has not been legally protected from treasure hunters. The land was privately purchased and settled in the 1850s. Many similar and adjacent mounds were excavated in a search for treasure or artifacts, leveled for farming, or hauled away. The primary objectives of this research are to determine the extent and time of mixing by excavation and/or tillage, the source of the mound materials, and the rate of diagnostic soil horizon formation. Fly ash was used as a time indicator. The depth of fly ash in the soil on the summit of the mound is consistent with the soil being tilled and mixed to a 40-cm depth. The sideslope of the mound has a similar significant depth of occurrence but has a higher amount of fly ash in the surface layer, suggesting that some tillage translocation and erosion of the summit soil has resulted in deposition on the sideslope. The soil developed in the alluvial fan adjacent to the mound has a similar texture and supports the theory that the mound was constructed from soil materials obtained from this alluvial fan rather than the theory that earth was carried from the adjacent bluffs formed in loess. Radiocarbon analysis of charcoal and organic matter found in the 80 to 130-cm layer suggests that 99% of the carbon is modern (<100 yrs old), which is consistent with the findings that trace amounts of fly ash in the layer were most likely moved into the layer by late historical human excavations and/or bioturbation.