Archaeo-Physics, LLC - geophysical survey and subsurface mapping

Case Study

Historic Scots-Highlander Farmsteads (Fort Bragg, North Carolina)

Archaeo-Physics, LLC was contracted by TRC Garrow (TRC) to conduct a geophysical investigation at several historic archaeological sites located on Fort Bragg, North Carolina.  The geophysical investigation was conducted in support of a larger National Register eligibility assessment of eight archaelogical sites at Fort Bragg.

This case study will provide an overview the results from the geophysical investigation, as well as relate these data to the archaeological data acquired during the eligibility assessment. This combined presentation of geophysical and traditional archaeological data was made possible due to the collaborative efforts of Archaeo-Physics, LLC, the Fort Bragg Cultural Resource Department, and the Diachronic Research Foundation.

The geophysical investigation utilized two well-established survey methods known as magnetic gradiometry and electrical resistance. Two new experimental methods were also applied. These new imaging methods are based on an analysis of the magnetic mineral properties of soil samples obtained during shovel testing. A description of the shovel test imaging methodology can be viewed by clicking here.

Results from two historic farmsteads are presented below. Both sites were occupied by descendents of Scottish Highlander immigrants who settled in the “sand hills” of North Carolina's upper Cape Fear valley in the mid 18th century.  Many of the Highlanders survived by subsistence farming while supplementing their incomes by producing naval stores (for example, pine sap based tar, turpentine, and pitch) which had become an important regional industry in the 18th century. 

The first site investigated was the McLauchlin-Holt farmstead (31HK1839). The McLauchlin-Holt farmstead was occupied between 1877 and 1918, however it is located on land that devolves from the McKay family. Alexander McKay was one of the earliest Scottish settlers in this region. His home and tavern formed the nucleus of what became known as the small rural settlement of Argyle. In 1756 the Long Street Presbyterian Church was organized in the home of Mr. McKay. In 1765 a separate building was built - probably a simple log structure. The southern portion of the survey area at the McLauchlin-Holt farmstead is a suspected location of these 18th century structures. One of the objectives of the geophysical investigation was to search for evidence of the original Long Street Church and/or Alexander McKay's home and tavern.

The second site investigated was the Archibald McNeil farmstead. Archaeological and documentary research of this Highlander farmstead is currently underway - more information will be presented here as it becomes available.

The results from these two Highlander farmsteads are presented using an interactive data exploration tool (see below). The presentation of results from the McLauchlin-Holt farmstead and possible location of the original Long Street Church (collectively known 31HK1839) are considerably more detailed as more data is currently available from this site.

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The above analysis tool provides viewers an opportunity to explore these archaeological data in considerable detail. Viewers are encouraged to:

1. Explore the historical context of the sites.

2. View and compare geophysical images from several different methods, including magnetic field gradient, electrical resistance, and two new imaging methods based on an analysis of soil samples obtained during shovel testing (31HK1839 only). This analysis quantified the soil's magnetic mineral concentration and grain size by measuring several common environmental magnetism parameters.

3. Compare the geophysical survey results with a plan map of surface features at the site (31HK1839 only).

4. View detailed or "zoomed-in" imagery from the McLauchlin-Holt farmstead (northern portion of 31HK1839) and the original Long Street Church search area (southern portion of 31HK1839).

5. Compare artifact distribution contours with the geophysical imagery. The artifact distribution contours are based on 10-meter interval shovel testing results (31HK1839 only).

6. View correlations between the electrical resistance and magnetic gradiometry anomalies by utilizing transparent images (see detailed view of the southern portion of 31HK1839)

Summary and Discussion

Geophysical investigations at two Scottish Highlander settlements at Fort Bragg have successfully mapped the locations of archaeological features and landscape patterning associated with the occupation of these sites.

Distinct patterning in the southern portion of 31HK1839 may represent the locations of 18th century features. This patterning is visible in both the electrical resistance and magnetic field gradient results. Correlations between the two data sets can be examined by navigating to the zoomed view of the Original Long Street Church Search Area and applying a transparency to the magnetic imagery. A systematic testing strategy is currently under development. The objective of this testing program is to determine whether these anomalies represent archaeological evidence of the original Long Strong Presbyterian Church (ca. 1765) and/or the home and tavern of Alexander McKay (ca. 1756).

In the northern portion of 31HK1839 the geophysical survey mapped the locations of several structures and landscape modifications associated with this 19th century farmstead. One apparent structure is partially defined by a lightning induced remanent magnetization (LIRM) anomaly. The LIRM anomaly appears to mark the point where a large electrical current entered the soil through a lightning rod mounted to a structure. Another interesting anomaly is defined partially in the resistance data, partially in the magnetic data, and appears to represent a “worm fence.”

Perhaps the most intriguing geophysical anomaly observed during the investigation is the large-scale rectilinear patterning visible in the Soil Magnetic Mineral Grain Size imagery. This patterning was identified during a soil magnetism analysis of samples collected during shovel testing of the site. The patterning is thought to represent the distribution of very fine grained magnetic minerals precipitated by iron reducing bacteria. These bacteria utilize organic matter as a food source and precipitate fine grained magnetite as a byproduct of bacterial respiration. Animal pens with lots of manure, midden deposits, or regularly fertilized gardens or fields could all provide the conditions necessary to locally increase the population of these iron-reducing bacteria. A more complete discussion of iron reducing bacteria can be viewed by clicking here.

The analysis of soil samples collected during systemic close interval shovel testing provided valuable information concerning cultural landscape patterning and the distribution of anthropogenic soils at 31HK1839. The method was not adversely affected by the large amounts of metal trash and debris associated with more than 80 years of military training at the site. This metal "signal clutter" did have an adverse affect upon the magnetic field gradient imagery.

The results of the experimental soil magnetic mineral analysis were encouraging, and future application of the method is recommended, especially at sites where metal debris has comprimised the effectiveness of more common survey methods. Widespread application of "shovel test imaging" could allow researchers to extract more information from this most common of North American archaeological evaluation methods.

References

Celeste, Ray (2001). Highland Heritage: Scottish Americans in the American South. The University of North Carolina Press, Chapel Hill, North Carolina.

Dobson, David ( 1994). Scottish Emigration to Colonial America 1607-1785. The University of Georgia Press, Athens, Georgia.

Magnusson, Magnus (2000). Scotland—The Story of a Nation. Grove Press, New York, NY. Retrieved from www.wikipedia.com on May 1st, 2008

Meyer, D. (1961). The Highland Scots of North Carolina 1732–1776. Chapel Hill, NC: University of North Carolina Press.

The Preservation Post (2007). Fort Bragg Cultural Resource Program, Volume 5, Issue 1. http://www.bragg.army.mil/culturalresources/pdfs/Preservationpost2007.pdf

Steen, Carl (2008). The Long Street / Argyle Community: NRHP Eligibility Evaluation at Four Archaeological Sites on Fort Bragg, Hoke County, North Carolina. DACA42-02-D-0010-0004. Diachronic Research Foundation, Columbia, SC.

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