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  • Timothy J. Horsley and Travis G. Parno

Finding St. Mary's Fort Using Geophysical Survey.

As geophysical techniques become more commonly integrated into archeological projects in Maryland, they are helping us to map entire sites and cultural landscapes. At the St. Mary’s City National Historic Landmark these methods have been used to rediscover St. Mary’s Fort, the location of which had eluded researchers for nearly 100 years. After English settler-colonists landed on the shore of what is now St. Mary’s River in March of 1634, historic records inform us that they constructed a fort on part of an existing settlement of the Yaocomaco, the indigenous tribe in this area. Research conducted by Historic St. Mary’s City (HSMC) since the 1980s had revealed concentrations of early seventeenth-century material at two locations, but the precise location of the first permanent European settlement in Maryland remained unknown. In the hope of resolving this, HSMC commissioned Horsley Archaeological Prospection (HAP) to undertake geophysical surveys across these two possible locations – referred to as the Traditional Site and the Mill Field Site. This work was funded and supported by the Maryland Historical Trust (MHT) and the Historic St. Mary’s City Foundation.


Given the 22-acre area of investigation, a sequential strategy was adopted that began with a large-scale reconnaissance method – magnetic susceptibility – to help identify areas of former habitation. The results indicate a significant degree of past human activity in the soil at both locations, but no obvious fort stands out. Since geophysical methods do not provide dating evidence, some of this activity could relate to the fort, Native American occupation (Yaocomaco or earlier settlements), as well as the later town of St. Mary’s City.



This magnetic susceptibility “heat map” reveals concentrations and the distribution of past human activity at two possible locations of St. Mary’s Fort.


Magnetometry was then conducted over parts of both sites to see whether a palisade, structures, and other features associated with St. Mary’s Fort might be detected. Unfortunately, no direct evidence for the fort was found at either location, but at the Mill Field Site the results revealed a brick foundation and several features related to high temperature processes, i.e., kilns or furnaces. Historic records mention that a gunsmith was based near the remains of the fort in the 1640s, which could explain the furnaces.


The magnetometer results, coupled with the fact that an earlier excavation unit had revealed a short section of a possible palisade, led the subsequent GPR survey to focus on the northeastern corner of Mill Field, and this time the technique was successful. Almost immediately the well-defined lines of a palisade were revealed, with a distinctive bastion at the southwest corner. Only a few features can be discerned with confidence within the palisade, including three or four cut-and-filled features, probably cellars. Other structures are suggested by clusters and alignments of postholes, but these are difficult to distinguish in the data. There is no evidence for ramparts or a ditch. Subsequent excavations by HSMC have confirmed many of the geophysical interpretations and recovered artifacts that firmly date the palisade to 1634.


GPR results from the Mill Field Site (left), and simplified interpretation (right).


In addition to ongoing excavations at the fort site, a new phase of MHT- grant-funded geophysical study is currently underway to expand the GPR survey across the entire Mill Field. A new, multi-channel GPR system is being employed to survey all 15 acres at high-resolution and will produce a true 3D map of the subsurface. It is hoped that this additional detail will be able to resolve smaller postholes and help to map not just the European structures, but also the long history of Native American settlement on this landscape.


A multi-channel GPR system being towed across the Mill Field

(Photo: HSMC).

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