Surveying Steel

Newly Discovered Ancient Earthworks at the Steel Group

2008 article by Dr. Jarrod Burks, HEC President

In 2006, I had the good fortune of being contacted by a landowner in Ross County, Ohio who owned one of the more modest earthwork sites depicted in Squier and Davis’s 1848 volume. Though not named in their volume, this site is now known as the Steel Group. In a famous map in their book, Squier and Davis (1848) show a twelve-mile section of the Scioto River Valley around the town of Chillicothe, an area famous for its many earthworks, and squeezed in on the left hand side of the map are the two small, apparently circular, enclosures of the Steel Group. Having worked for some years at the nearby national park that contains earthworks, I was well aware of the circles at the Steel Group and, since moving on to the private sector, had wondered how I might gain access to the site to conduct a geophysical survey. The unexpected phone call and invitation to survey the earthworks was seemingly providential!
Dr. Jarrod Burks conducts magnetometry survey at Steel Group.

Dr. Jarrod Burks conducts a magnetometry survey at Steel Group.

At this point I should mention that I am an archaeologist, trained in the traditional ways of trowel-and-shovel archaeology. While in graduate school at Ohio State University our Anthropology department had the good fortune to purchase a magnetometer (the FM 36 fluxgate gradiometer made by Geoscan Research). Unfortunately there was nobody in our department to teach us how to use this instrument, so a group of us grad students taught ourselves. From these early experiences with geophysics I was so convinced of the importance of integrating geophysical survey into archaeology projects on a regular basis that I talked a friend of mine into buying a magnetometer for his contract archaeology business, with the promise that I would come work for him after graduation (contract archaeologists work in the private sector and help clients comply with federal and state laws that require archaeological study be done ahead of certain development projects). I now conduct geophysical surveys (magnetic gradient, electrical resistance, and radar, primarily) on all kinds of work-related archaeology sites, and in my spare time I seek out earthwork sites in need of survey.
1934 aerial photograph of the Steel Group by Dashe Reeves.

1934 aerial photograph of the Steel Group by Dashe Reeves.

In the summer of 2007 an archaeology research group I am part of rented a helicopter and pilot for a couple hours to take aerial photographs of our excavations at a prehistoric settlement south of Chillicothe. On the way back from taking my photographs I asked the pilot to swing by the Steel Group site, knowing that I would be beginning my magnetometer survey there later in the year and that some aerial photographs might be useful. Much to my delight, the field in which the Steel Group earthworks are located was planted in wheat that year, and, being early June, the wheat was just starting to turn brown. This is perhaps the most ideal time and crop cover for seeing earthworks in agricultural fields since the wheat growing over the ditches of the earthworks remains green as all of the other wheat first starts to turn brown. The photo in Figure 3 is one of the aerial shots of the Steel Group I was able to snap as we banked by the site in the helicopter. The two enclosures shown on the Squire and Davis map are clearly visible, and I have marked them with the numbers 1 and 2. Also evident are a number of what look to be additional enclosures, previously undocumented, and these I have marked with little blue arrows. To say that I was surprised to see all of the new possible enclosures in the photographs is an understatement. It is not every day that an archaeologists finds so many new and undocumented earthworks, especially not in Ross County, Ohio where archaeologists and scholars have been studying the earthworks for nearly 200 years.
LiDAR image of Steel Group with newly discovered earthworks drawn in place.

LiDAR image of Steel Group with newly discovered earthworks drawn in place.

In October of 2007 I began the magnetic survey at Steel Group using a fluxgate gradiometer—the FM256 model made by Geoscan Research. This instrument only records the magnetic gradient (i.e., not the total field) between its two sensors, which are spaced 50 cm apart. Because of the size of the sites we usually study and the way in which the geophysical instruments are set up to collect data, archaeologists in the U.S. tend to collect geophysical data in 20×20 meter blocks, which are then stitched together in a software package and the whole of the dataset processed through a variety of algorithms. At Steel Group I covered 240 blocks, or about 24 acres. The instrument was set up to collect eight readings per meter along transects spaced one meter apart. At this data density I can cover about three acres per day, including setting up the block corners with a laser transit. Since I may want to conduct some excavations at a future date, it was important to collect the magnetic data in a very controlled fashion and have it all tied in to semi-permanent mapping points that can be used to re-establish the survey grid in a very accurate way— using the laser transit makes this easy. While geophysical survey is fast, archaeological excavation is not, so it is important to dig in the right place and with excavation units that are as small as possible to get the job done.
Interpretation of Steel Group magnetometry survey results

Interpretation of Steel Group magnetometry survey results

Squier and Davis mapped just two enclosures at this site in 1845. In fact, there are at least ten embankment and ditch enclosures apparent in the magnetic data collected to date.  Each of the enclosures appears in the data as a dark inner feature surrounded by a lighter, kind of fuzzy area. The dark inner portions of these enclosures are the ditches. Since the ditches have filled back in with topsoil and some of the original embankment fill, they appear as positive magnetic anomalies—the ditches are more magnetic than the surrounding soil. This is not too surprising since topsoil is magnetically enhanced and when it occurs in thicker deposits, either in ditches below surface or as plow ridges at the surface, it creates positive magnetic anomalies. The lighter colored, fuzzy areas surrounding the in-filled ditches are the remains of the embankment walls. Since these once above-ground features are now so flattened that they are not evident at the surface, why are they even present in the magnetic data? One possibility is that the ground on which the embankments was to be constructed was first stripped of its topsoil; and then the soil for the embankments was piled directly onto the subsoil. The lack of topsoil would cause the area of the embankments to appear as less magnetic in the survey data. Another possibility is that the soil used to create the embankments was clay, and clay is typically less magnetic than topsoil. Though plowed flat, this clay is still present in the plow layer and is making it less magnetic. More than likely, it is a combination of both of these explanations—the topsoil was removed first and then clay was piled up to form the embankments. Together, these factors are what make the embankments appear less magnetic in magnetic gradient survey data.
3D LiDAR of Junction and Steel Earthworks area.

3D LiDAR of Junction and Steel Earthworks area.

As an archaeologist who studies prehistoric earthworks in Ohio, I will have to admit that every time I see the Steel Group results I can’t help but think to myself what else might be out there, just waiting for a magnetometer or electrical resistance meter to find it? Some months before surveying at Steel Group I was helping a friend survey a prehistoric village site near Columbus, Ohio that is about 800 years old. As we were detecting the magnetic signatures of buried trash pits and cooking pits, we accidentally detected a ditch and embankment enclosure at the edge of the village. This was an earthwork that nobody knew was even there—it had never been seen in any aerial photos and it had eluded the nineteenth and early twentieth century earthwork mappers. How many more unknown earthworks could there be in Ohio?
I once thought that we are incredibly lucky that people in the 1800s were interested enough to map the earthworks, because today most of the earthworks have been so plowed down that they are invisible and we can no longer map them through conventional means. Without those nineteenth century maps, we would be unaware of the locations and shapes of at least 90% of Ohio’s earthworks since they are no longer visible. Now, with the help of speedy (i.e., digital) near surface geophysical survey instruments and powerful data processing software, the results from the Steel Group survey show us that the process of mapping Ohio’s earthworks has only just begun. It is time to trade in the surveyors chains and compasses for the beeping and clicking of today’s geophysical instruments. For the sake of my knees and back, I just hope that tomorrow’s more affordable instruments are self-propelled….

 

Find out more about the Steel Earthworks Conservation Campaign:

The Conservation Plan for the Steel Group

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