Archaeomagnetic anomalies of midcontinental North American archaeological sites

Abstract

Where the magnetizations of artifacts and their soil environment contrast sufficiently, magnetic mapping can provide a rapid overview of their distribution which will help optimize the time consuming and expensive effort of excavation. Magnetic artifacts frequently encountered at midcontinental archaeological sites include iron objects, baked-clay features, wells, pits, and structural trenches. Iron objects and baked-clay features normally yield strong and readily detectable anomalies in the near-surface geomagnetic field. Hence, magnetometry is routinely warranted when the location and distribution of these artifacts are of archaeological interest. Magnetic detectability of such soil artifacts as wells, pits, and trenches, however, is ultimately a direct function of the iron oxide content of the site’s soil environment. Thus, the glaciated terrain of the midcontinent generally is most suited to magnetic exploration for soil artifacts, because these soils normally exhibit relatively high iron oxide fractions. However, little more is known currently about the magnetic properties of soil features for midcontinental sites, so that the feasibility of the magnetic method for archaeological exploration of these features is never really certain until an actual survey is carried out. Several examples from a magnetic survey of the Fort Ouiatenon archaeological site (12T9) in northern Indiana are given to illustrate these generalizations.