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Magnetics and Electromagnetics on Monks Mound at the Cahokia World Heritage Site near St. Louis, Missouri

D. J. Smith and J. L. Sexton
Southern Illinois University, Carbondale, [email protected] and [email protected]

In the summer of 2007, magnetic and electromagnetic conductivity surveys were performed on Monks Mound within the prehistoric Cahokia settlement located 6 miles (~10 km) east of present day St Louis. A 17 m by 40 m rectangular grid was established on the third (front) terrace of the 850 year old mound surface. Readings were taken on the rectangular grid at one meter spacing. A Geometrics G-856 proton precession magnetometer and a Geonics EM38 conductivity meter were used for the survey, which was designed and executed to detect anomalous features within the very shallow subsurface (one to two meters). It was hoped to delineate and categorize the sources of the anomalies. The causes are known to be natural (in this case limited to lightning strikes), prehistoric (pottery, kilns, hearths), historic (metal, fire pits, structure remnants), or contemporary (metal). The larger, betterdefined anomalies were numbered and discussed with possible sources proposed. Good correlation is observed between some of the anomalies for the magnetic and electromagnetic methods, while other anomalies were observed using only one method. Further testing to confirm the interpretations is outlined. Techniques borrowed from the fields of rock/soil magnetism are recommended as the next step to better define the unknown features since these methods result in minimal surface disturbance. Chiefly, this would involve measurements such as magnetic susceptibility and natural remanent magnetization in collected soil samples. Excavation is unlikely because of its inherent destructive nature and the need to protect this unique archaeological site.

AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas