Magnetics and Electromagnetics on Monks Mound at the Cahokia Mounds State Historic Site near St. Louis, Missouri

Author

Dominic Smith, Southern Illinois University CarbondaleFollow

Date of Award

1-1-2008

Degree Name

Master of Science

Department

Geology

First Advisor

Sexton, John

Abstract

In the summer of 2007, magnetic and electromagnetic conductivity surveys were performed on the third (front) terrace of Monks Mound, Cahokia, 6 miles (~10 km) east of St Louis in Illinois. A 17 m by 40 m rectangular grid was established on the third 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. The survey was designed and executed to detect anomalous features within the very shallow subsurface (one to two meters deep). 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 modern (metal). The larger, better-defined anomalies were numbered and discussed. Magnetic anomaly interpretations include a root cellar (4), outhouse (10), house (6), and metal features (7, 8, 14, 15). The certainty of the interpretations varied and there was often multiple interpretations possible. The EM anomaly interpretations proposed include metal, pit, grave, and animal burrowing. 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 in the thesis.