Mapping plow zone soil magnetism to delineate disturbed archaeological site boundaries

This paper presents the results of a magnetic susceptibility survey at Ocmulgee National Monument, a large archaeological site located in central Georgia. Ocmulgee consists of numerous bluffs, each subjected to plowing and erosion. Archaeological remains preserved differentially better on some bluffs, while remains on other bluffs have been obliterated. Magnetic susceptibility readings were recorded in the plow zone on Mound X Ridge, an area of the site that is significantly disturbed. Although the archaeological site is no longer intact, superparamagnetic micro-particles from daub, ceramics, or charcoal have been mixed from 100 years of plowing and remain in the plow zone. The goal of the project was to delineate the northern boundary of this activity area. Thirty posthole tests were excavated and the presence or absence of artifacts from each posthole test was recorded. Magnetic susceptibility values were also recorded in the plow zone following excavation of each test. The positive correlation of high susceptibility values with presence of artifacts indicates this method is useful for mapping disturbed site boundaries of plowed archaeological sites. This method is quicker than more traditional invasive testing and does not further disturb archaeological remains if posthole tests are not excavated below the plow zone.