Magnetometry at Lord Ashley

I am Jon Marcoux, an archaeologist collaborating with the folks at the Lord Ashley site. I am guest blogging today to present the reader with a little background on magnetometry, and how it relates to our investigation at the site this summer. First, however, a quick addendum to yesterday’s post. Two exciting finds were made after Katherine left the field – a white glass bead with red stripes and Indian-made pottery sherds. 

red striped bead McKee Pottery

The bead is made in the typical fashion for late 17th c., by blowing an air bubble in a mass of molten white glass, affixing red glass rods to the mass, stretching the mass until the desired diameter was achieved, breaking the now very long glass rod into segments, and tumbling the segments until smoothed. The pottery is fascinating because the potter who made it was practicing a potting tradition that was definitely not local to the Charleston area. Read more about beads and this pottery (and my ideas about where the potter might have been from) on my blog Redd Duffells and Blew Beads.

Now on to discussing magnetometry. Magnetometry is a branch of archaeological geophysics (AG) – a field of study that utilizes precise measurements of certain physical properties of soil in order to identify and define buried archaeological features (e.g., storage pits, trash-filled pits, burials, house posts). The most obvious benefit of AG is that it provides the archaeologist with a “picture” of sorts of what lies beneath the surface of the ground. This image can be used as map to direct excavations to specific features within an archaeological site – greatly reducing the amount of time spent searching for these features using traditional field methods. Archaeological geophysics has been in existence since the 1940s; however, only within the last decade have major advances in computing power and increases in the sensitivity of measuring instruments made AG a practical and cost-effective research tool. While still at a nascent stage, AG is growing in popularity among archaeologists in the southeastern U.S.. Recently, AG techniques were used to define the size and structure of man-made “shell rings” along the coast of Georgia and to identify buried trash-filled pits and house structures at the Crystal River Mound site in western Florida. Despite this recent growth, however, AG remains largely limited to projects conducted by researchers at large universities.

As the name implies, magnetometry identifies buried archaeological features by measuring magnetic fields below the surface of an archaeological site. These measurements are taken using a piece of equipment called a magnetic gradiometer. In typical use a researcher wearing the gradiometer walks over an archaeological site in evenly-spaced transects – typically 50cm-1m apart. The gradiometer records changes in magnetic fields up to 1.5 meters deep. The goal is to identify localized anomalies that represent changes in the strength (called the gradient) of the earth’s magnetic field. These anomalies are usually caused by stark differences in the composition of the soil, which would occur in a trash-filled pit or a burial, or by thermal alteration, such as in a hearth or the remains of a burned house.

mag operator

 

In January of 2012, field director Andrew Agha, Nicole Isenbarger, and I conducted our survey of a portion of the site. We established a grid of eight 20-x-20m blocks on the east side of the survey area, and a strip of four 10-x-10m blocks along the western periphery. The sampling density for all areas was established at 12.5 cm (eight readings per meter) on transects spaced 50 cm apart. This provides 1,600 data points for each 10-x-10m block and 6,400 data points for each 20-x-20m block. Ropes spaced one meter apart were used as transect guides. I covered each grid by pacing in a zig-zag pattern. The results of the magnetic gradiometer survey were processed by Dr. Marcoux using Archaeofusion software generously provided by the University of Arkansas Center for Advanced Spatial Technologies.

The results of the survey show a few interesting things. First, is the effect metal has on the instrument. The large mass of regularly spaced black and white “blobs” on the map represent reading of the nails used in the 2011 field school. These were kept in the ground to be able to re-establish the excavation grid this year. The entire western edge of the survey area was drowned out by the metal fence at the edge of the field. A couple of very large metal object were also recorded and appear as big black and white “blobs.” There are some other areas of interest, but to keep this post short I will discuss the obvious feature. This is the anomaly composed of two intersecting dark lines – one running southwest-northeast and one running southeast-northwest- that meet in the western portion of the study area. This anomaly suggests a linear feature containing organically rich soil (manifested as soil with increased magnetism) cutting though less magnetic subsoil – sounds like a moat to us!

New Picture (10)

 

At the end of the day today we came down on what might be the top of the moat in the unit I am supervising…we will clean it up and photograph it tomorrow so stay tuned!

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One Response to Magnetometry at Lord Ashley

  1. Pingback: Ceramics and social interaction in the Lowcountry. « The Human Family

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