The Rockwall in Rockwall, Texas: A Study of Unusual Natural Magnetic Effects in Geoarcheological Surveys Produced by Mineral Oxidation
The city council of Rockwall, Texas, decided in 1985 to build a park containing an exposure of the “famed” rockwall after which the city and county of Rockwall derive their names. This wall, thought to be man-made because of it's unusual brick-like appearance, was first discovered in 1851. As a result of weathering, the wall has no surface exposure and crews working for the city had not been able to find it. Geological studies, published between 1927 and 1950, clearly demonstrate that the rockwall is one of a series of sandstone dikes with a carbonate cement and some unusual material constituents. Geoarchaeological methods, including magnetic and electrical resistivity surveys, and Mössbauer studies were used in identifying the precise location of the rockwall. This was possible because of strongly magnetic secondary minerals produced within the wall by weathering of two primary mineral constituents, siderite and marcasite. the resulting natural magnetic anomaly pattern made it possible to pinpoint the wall's location. This pattern closely resembles those identified in association with historic or prehistoric cultural activity. Our observations may help explain why some archeological excavations, at sites associated with well defined magnetic anomalies, do not yield clear sources for those anomalies, i.e., buried cultural material. These anomalies may result from weathering or from other natural processes, and in some cases interpretation of such ambiguities can be resolved using mineralogical and geochemical data.
B. B. Ellwood et al., "The Rockwall in Rockwall, Texas: A Study of Unusual Natural Magnetic Effects in Geoarcheological Surveys Produced by Mineral Oxidation," Geoarchaeology, vol. 4, no. 2, pp. 103-118, John Wiley & Sons, Jan 1989.
The definitive version is available at http://dx.doi.org/10.1002/gea.3340040202
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