The application of soil magnetic techniques for determining the original form of prehistoric earthworks

Abstract

Mounds located within the Cahokia Mounds State Historic Site in southwestern Illinois are being used as test cases for the development of this methodology. To date, work has concentrated on conical and platform mounds. Soil magnetic results from three mounds (Mound 36, Mound 62, and Mound 56) are presented below. Detailed topographic interpretations can be found in Dalan and Watters, 1994. We propose a methodology consisting of a combination of topographic and soil magnetic techniques as a relatively nondestructive and economic means of determining the original form of prehistoric earthworks. We have tested such an approach on earthen mounds at the Cahokia Mounds State Historic Site, a Mississippian period (ca. A.D. 900-1400) center located in southwestern Illinois. Erosion and sedimentation, exacerbated by cultivation and other modern cultural activities, have worked to blur and sometimes completely eradicate the original shapes of these mounds. Laboratory magnetic analyses are applied to provide an understanding of the erosional history of a mound, that, together with topographic data and an understanding of hillslope evolution, can be used to determine what it looked like during the Mississippian period occupation. To date, we have used this methodology to distinguish platform and conical mounds as well as to identify less common forms such as multi-terraced platforms.