The following is an excerpt from: (2014) Preliminary Report Related to Possible Unmarked Burial Locations the Hamburg Cemetery, Hamburg, Illinois. Report submitted to Dr. Jason L. King, Center for American Archeology, Kampsville, Illinois.
Over the course of two weeks (June 9-19, 2014) students from the Arizona State University Field School (ASUFS), hosted at the Center for American Archeology (CAA) in Kampsville, Illinois, conducted a survey of both the historic and prehistoric components at the Hamburg Cemetery in Hamburg, Illinois. The purpose of the survey was two-fold: (1) to evaluate the extent of the prehistoric subsurface anomalies associated with the bluff ridge mounds located within the cemetery grounds; (2) survey two locations of the cemetery, identified by the Hamburg Cemetery Board, to evaluate evidence of unmarked burials in these areas.
SURVEY AREA AND METHODS USED
The Hamburg Cemetery is located at the top of an elevated landform situated above the Mississippi River. The location contains numerous historic and contemporary burials and associated stones. Additionally, the cemetery is an archaeological site with the additional presence of at least eight prehistoric constructed earthen mounds. The mounds are all located along the western edge of the landform and overlooking the Mississippi River valley. The survey area was established to gain “full coverage” of both the prehistoric mound locations and the historic cemetery area. As part of the survey, students mapped surface stones and the locations of the possible unmarked burials as marked (with spray paint) by the Hamburg Cemetery Board. Vegetation at the time of survey was mowed grass, which made for easy mapping and survey. The only obstructions were the surface headstones and a few large shade trees.
In addition to the mapping efforts, students collected data using magnetometry and resistivity methods. Magnetic gradiometry is measured as the net effect of magnetic variations in the shallow subsurface soil matrix (approximately 1-2 meters). The measurement of magnetic gradiometry is conducted using a passive approach, in that the instrumentation measures the naturally occurring magnetic field without emitting a magnetic field, pulse, or current into the subsurface. While earlier instrumentation measured the total magnetic field, which required the manual differencing of diurnal changes measured by a remote sensor, magnetic gradiometers do not report the total magnetic field strength. Instead, magnetic gradiometers calculate the real-time difference of the two vertically separated sensors as the survey is conducted and measurements are simultaneously being recorded. This difference yields a vertical gradient measurement of the magnetic field free of diurnal variations. The vertical gradient is measured and reported in nanoteslas (nT; 10-9 Tesla).
Electrical resistance surveys measure the level of resistance (R) in the subsurface by injecting a current (I) into the ground using a low voltage (V) resistance meter. The ratio of current to voltage defines resistance and is expressed by Ohm’s Law, mathematically stating that R=I/V. While resistance is an electrical quality, resistivity is the actual specific property of the material. Its conversion allows for the “resistance of different materials to be compared in a standardized way” (Clark 1996:27). Electrical resistivity is measured and reported in ohm-meters.
Variations in resistance measurements are based on the principle that geological features hold different materials and different amounts of moisture. Both exhibit varying levels of resistance to an electrical current, and the latter has a particularly large effect in archaeological sites. These varying levels of resistance can be influenced through anthropogenic processes that alter the compaction of the soil resulting in a change of soil moisture properties. A measurement of high resistance (low conductivity) might represent a shallow subsurface of compressed soil matrix such as a house floor where porosity is decreased and evaporation is elevated. In contrast, a measurement of low resistance (high conductivity) might represent a storage pit or house depression where moisture has accumulated into a more porous soil matrix and is less likely to evaporate. Certain materials, like stone or sand, are known to exhibit high electrical resistance.
MAGNETOMETRY AND RESISTIVITY SURVEY AREA
The magnetometry survey results reveal numerous high magnetic subsurface anomalies as high-low dipole readings directly associated with the mapped surface stones throughout the cemetery. The anomalies associated with surface stones are related to several possible metallic features: (1) rebar within the headstone foundations and numerous metal ornamental decorations on the surface; (2) the presence of buried metal caskets or metal hardware; (3) a variety of metal objects buried with individuals; (4) all of these. On the eastern portion of the cemetery additional high magnetic readings arranged in east-west rows are associated with buried metal poles, the tops of which are visible at the ground level and are used as corner markers for cemetery organization.
The resistivity survey results record variations in soil resistance related to changes in porosity associated with the burial pits and other landscape modifications. Similar to the magnetic results, many resistivity anomalies are clustered in association with the surface stones. Of interest is the presence of long linear anomalies that extend throughout the site, particularly on the eastern portion. The linear arrangement suggests human landform modification over time associated with cemetery maintenance, such as the movement of soil or possibly a row of trees that were planted and no are longer at the site.
POSSIBLE UNMARKED BURIAL LOCATIONS
As part of the survey, the Hamburg Cemetery Board outlined two areas on the surface with spray paint. These areas were surveyed for evidence of possible unmarked burials. In the northern portion of the cemetery, two adjacent locations were surveyed using magnetic gradiometry and resistivity. The magnetic gradiometry results demonstrate a lack of magnetic anomalies associated with the possible burial areas. The resistivity results also demonstrate a lack of anomalies directly associated with the possible burial areas.
While this might indicate that there are no burials present in these two areas, a conservative interpretation suggests the following: (1) magnetometry results indicate that there are no subsurface metallic objects. This could indicate that there are no burials, or that there are burials that pre-date the use of metal caskets and hardware (stones nearby with little or no associated metal anomalies); (2) resistivity results indicate soil resistivity levels that are similar to areas without burials. This could indicate that there have not been pits dug (thus changing the soil porosity) in these areas, or that there are burials that are very old with their age allowing the soil chemistry to homogenize over a long period of time.
The lack of large magnetic anomalies in the two areas in the north portion of the cemetery, so clearly present with known burials, suggests that there might not be burials in these two locations. However, as these interpretations are preliminary and two proximate stones show little subsurface metal debris, it is suggested that these area be further tested with probing as an additional dataset to corroborate survey results.
The second possible unmarked burial is located in the south portion of the cemetery and was outlined as two contiguous areas. The area was surveyed using magnetic gradiometry and resistivity. The magnetic gradiometry data reveal numerous high magnetic anomalies within the unmarked burial area. This portion of the cemetery is fairly dense with numerous surface stones and known burials in close proximity. The high magnetic anomalies within the unmarked burial area are likely related to known burials, including a stone marker directly north of the unmarked burial area. These known burials might be intrusive into the unknown burial area. The high level of metal debris in this area prohibits any additional interpretations. The resistivity survey demonstrates the presence of variable soil resistivity within the unmarked burial area, similar to the soil resistivity readings associated with known burials. As with the magnetic gradiometry, these anomalies could be related to known burials and the movement of soil. The density of known burials and their associated soil modifications prohibits any conclusive interpretations in this area.
As the southern portion interpretations are inconclusive, it is suggested that this area be further tested with probing as an additional dataset to corroborate survey results.
In the northern portion of the cemetery, a lack of magnetic and resistivity anomalies in the possible unmarked burial area suggests that there are no burials present or a burial lacking metal debris. A variety of cultural and natural factors could influence the possibility of the presence of burials. In the southern portion of the cemetery, there are several magnetic and resistivity anomalies in the possible unmarked burial area that suggests the presence of either unmarked burials or that more recent burials overlap into these areas. Interpretation of data in both areas is preliminary and it is suggested that further testing with a probe be conducted to corroborate survey results.
For more information:
1996 Seeing Beneath the Soil: Prospecting Methods in Archaeology (New Edition). Revised Edition. B. T. Batsford Ltd., London
Kvamme, Kenneth L.
2006 Magnetometry: Nature’s Gift to Archaeology. In Remote Sensing in Archaeology, An Explicitly North American Perspective, edited by Jay K. Johnson, pp. 205-233. University of Alabama Press, Tuscaloosa, Alabama.
McKinnon, Duncan P., and Bryan S. Haley
2017 Archaeological Remote Sensing in North America: Innovative Techniques for Anthropological Applications, edited by Duncan P. McKinnon and Bryan S. Haley. University of Alabama Press.
2006 Resistivity Survey. In Remote Sensing in Archaeology: An Explicitly North American Perspective, edited by Jay K. Johnson, pp. 109-129. University of Alabama Press, Tuscaloosa, Alabama.