MAGNETICS AND ELECTROMAGNETICS ON MONKS MOUND AT THE CAHOKIA WORLD HERITAGE SITE NEAR ST. LOUIS, MISSOURI

In the summer of 2007, magnetic and electromagnetic conductivity surveys were performed on the third terrace of Monks Mound, Cahokia 6 miles (~10 km) east of St Louis in Illinois. An 18 m by 41 m rectangular grid was established on the third terrace of the 850 year old mound surface. Readings were taken on the rectangular grid with one meter spacing. A Geometrics G-856 proton precession magnetometer and a Geonics EM38 conductivity meter were used for the survey. The survey 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, fire pits), historic (metal, fire pits), or contemporary (metal). The larger, better-defined anomalies were numbered and labeled with their proposed sources. 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 suggested to confirm the interpretations is outlined. Techniques borrowed from the fields of environmental magnetism and rock/soil magnetism are suggested as the next step to better define the unknown features since these methods result in minimal surface disturbance. This would include but not be limited to the measurements of magnetic susceptibility (x), natural remanent magnetization (NRM), anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM) on collected soil samples. Excavation is unlikely because of its inherent destructive nature and the need to protect this unique archaeological site.