The following is an excerpt from: (2016) Preliminary Results from Geophysical Survey at the Fingerhut Site and Mound 47/West Palisade Locality at Cahokia, Illinois. Report submitted to Dr. John E. Kelly, Washington University in St. Louis.
On June 17-24, 2016 students from the Arizona State University Field School (ASUFS) and staff from the Center for American Archeology (CAA) in Kampsville, Illinois, conducted magnetic gradiometry and resistivity surveys at the Fingerhut Site and the Mound 47/West Palisade Locality in Collinsville, IL. Both sites are situated within “central Cahokia”. A team of five ASUFS students, under the instruction and assistance of Dr. Duncan P. McKinnon and field assistant Amanda Wissler, collected data in both survey areas.
ARCHAEOGEOPHYSICAL PRINCIPLES AND INTERPRETIVE FRAMEWORK
Principles that define the use of archaeogeophysical (also “geophysics”) methods have been thoroughly discussed and summarized in a variety of publications. A short summary of magnetic gradiometry and electrical resistance principles and are discussed below. A summary of the interpretive framework is also discussed.
Magnetic Gradiometry
Allowing for an effective combination of rapid data acquisition, high-density sampling, and low processing time, magnetic gradiometry is a highly productive geophysical method in terms of understanding spatial relationships between anomalies across a large area. 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. 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 (nT; 10-9 Tesla) of the magnetic field free of diurnal variations. The two primary types of magnetism relevant to archaeological applications are induced and remanent magnetism. The combinations of these sources of magnetism constitute the net effect of measured magnetic variations in the subsurface soil matrix.
The minute magnetic variations that soils, sediments, and rocks have on Earth’s magnetic field are known as induced magnetism. This is because they do not maintain their own magnetic field but exist within Earth’s magnetic field. If the effects of induced magnetism are strong enough compared to the surrounding soil matrix, contrasting features can be identified in the geophysical data. Generally, the identification of induced magnetism features is a result of magnetically enriched topsoil being modified. For example, disturbances such as the digging of borrow pits or trenches, construction of mounds through the accumulation of soil, soil compaction and dissipation on house floors, or soil dispersion and erosion on trails are some cultural processes can result in induced magnetic contrasts.
Remanent magnetism is produced when an object maintains its own magnetic field independent from Earth’s magnetic field. This occurs when objects have been thermally altered, creating a magnetic state called thermoremanent magnetism. Iron oxides in the soils, clays, and rocks contain magnetic domains that are randomly situated and annul the combined strength of their magnetic signature. When the iron oxides are heated to high temperatures (around 600° Celsius), the magnetic domains align to Earth’s magnetic field and upon cooling remain “frozen” in that direction. The result is a concentration of magnetic domains pointing similarly and generating a higher magnetic field that can be measured and recorded. Generally, the identification of thermoremanent magnetism is the result of the firing of objects associated with human activities, such as the burning of a structure at high heat, a continually burning fire pit or hearth, and the disposal of pieces of fired clay in large amounts. The magnetic gradiometer used in these surveys is a Bartington Grad 601-2 Dual Sensor Gradiometer.
Electrical Resistance
Subsurface materials vary in their ability to conduct electricity. 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 (Somers 2006). 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. 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. These varying levels of resistance can be influenced through anthropogenic (and natural) processes that alter the compaction of the soil resulting in a change of soil moisture properties. For example, 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. The Geoscan Research Advanced RM85 with MPX15 Multiplexer was used for these resistance surveys. A twin-probe array configuration was utilized with mobile probe spacing at 0.5 meters.
Data from the Fingerhut and Mound 47/West Palisade surveys has been examined using a combination of inductive and deductive theoretical approaches. An inductive approach has roots in satellite and aerial image interpretation with the recognition that geometric shapes, relative dimensions, and systematic repetitions of objects can form interpretable patterns that are frequently anthropogenic in origin. A deductive approach utilizes known physical properties of the subsurface matrix to explain how instrument sensors might respond. For example, thermoremanent magnetism is the result of highly heated burning events, which can produce an anomaly composed of stronger magnetic values where anomalies of medium to high magnetic value may be deduced as being generated as a result of a continuously used hearth or the ritualized burning of a structure. A soil matrix that has been magnetically enriched through pedogenesis (induced magnetism and magnetic susceptibility) can also produce anomalies containing stronger magnetic values than those in the surrounding matrix. Thus, several low to medium magnetic signatures identified within or around a structure may also be deduced as being constructed pits. Highest magnetic values are typically related to ferrous metal debris buried close to the surface, which can generate anomalies of extreme magnitude.
THE FINGERHUT SITE
The Fingerhut site (Survey Area 1) is located 1.5 miles (2.41 km) west of Monks Mound (Mound 38) along Collinsville Road and behind the Fingerhut archaeological field house. The site is situated between Jondro Mound (Mound 78) to the south and the former Powell Mound (Mound 86) to the north. The Indian Mound Golf Course is located directly behind the site where numerous golf balls regularly become a part of the Fingerhut landscape (we collected over 15!). A total of 19 complete 20 x 20 m grids and 14 partial grids (approximately 1 hectare) were surveyed using magnetic gradiometry. A total of 19 complete 20 x 20 m grids and six partial grids (a little less than a hectare) were surveyed using electrical resistance. The surveys were conducted June 17-22, 2016.
Obstacles were minimal, which allowed for good coverage in a short amount of time. Most of the site is situated in an open field with overgrown bushes and trees along the fence line. Some obstacles existed around portions of the field house and outbuildings, which made for a variety of challenges, yet important for the students to experience. Given the history of historic and recent use of the landscape and a significant amount of metallic debris located throughout the site, resistivity data are the most informative results.
Results document both historic and possible prehistoric subsurface features. Historic features include a buried historic basement, evidence of modern disturbance in the back yard of the Fingerhut house, and possible evidence of historic plowing or landscape modification. Evidence of historic and recent land use is most apparent on the eastern extent of the site, where early twentieth-century aerial imagery documents former buildings in this area.
Prehistoric features include five possible structures with a centrally located anomaly interpreted as a hearth. Throughout the site are several grouped anomalies that are interpreted as possible pits or middens. To the south, numerous large anomalies seemingly trend parallel to each other and across much of the site. Given the density, relatedness, and size, they might represent prehistoric pits or midden features, which might also suggest a non-residential area. Bareis and Porter (1965) report that 53 graves with 46 individuals in burial pits were excavated from the “Fingerhut Cemetery”, although the precise location of the cemetery is not illustrated. The large anomalies could represent disturbed soil from those burial pit excavations, possible undisturbed burial pits, or likely a combination of both. Conversely, the linear trending and similar space between each group might also suggest they represent historic plowing and other landscape modification, such as former irrigation ditches or trenches.
To the north, a small cluster of anomalies is interpreted as a group of possible structural remains and associated pits. Three possible structures are estimated around 5 x 5 m in size. A higher resistance anomaly is present in each of the possible structures that might represent a central hearth or other centralized feature.
To the west, several grids contain linear high resistance anomalies that terminate at the grid edge. The location and abrupt transition from adjacent grids suggest that these anomalies are the result of operator error when moving and recalibrating the remote probes. Some anomaly patterns are highlighted within this area. However, the specific area should be recollected to isolate possible operator error.
MOUND 47/WEST PALISADE LOCALITY
The second survey area is located adjacent to the northwest corner of the Mound 48, a large platform mound, possible residence of the ‘famous’ Trappists Monks, and location of historic farmhouse in the early twentieth-century. Throughout the mid-twentieth century (1949-1983), this area was the location of Mounds Drive-In (later the Falcon Drive-in) until the theater closed and became a picnic area within the park. The survey area was situated over parts of Mound 47 and the possible location of a portion of the west palisade wall. Interestingly, men killed during a mid-nineteenth century sawmill explosion (on Mound 39) may have been buried in Mound 47.
A total of 17 20 x 20 m grids were surveyed using magnetic gradiometry and 14 complete 20 x 20 m grids were surveyed using electrical resistance. The surveys were conducted June 23-24, 2016 on the last two days of field survey instruction. A paved road leaving the picnic area bisects the survey area. Topography north of the road was a somewhat undulated surface slightly higher than the south where the landscape “dropped-off” toward Mound 48. The area north of the road is much more disturbed. Interpretations are organized into three interpretive subheadings: off-mound structures, Mound 47, and palisade evidence. Resistivity data are interpreted here.
Off-Mound Structures
Six rectangular off-mound structure remains are suggested along the western base of Mound 48. The high resistivity results document a compacted soil matrix that is suggestive of house floors. The rectilinear shape further supports the suggested house floors. Structures 1 and 2 are similar in size at approximately 5 x 5 m in area. A larger Structure 3 to the south is approximately 5 x 7 m in area. Structure 4 is west of 1-3 and is approximately 2.5 x 2.5 m in size. Two additional possible Structures 5 and 6 are only partially surveyed. They are estimated around 5 x 5 m in size.
Several additional high resistance anomalies, some quite large, are distributed throughout the survey area. Given the proximity to several proposed structures, they could represent prehistoric pits. However, with the long history of land modification and use in this area any further interpretation will require ground testing. A highly disturbed area is located in the westernmost survey area and certainly represents land modification related to road construction and maintenance.
The survey area was located over a portion of Mound 47, now buried under the paved park exit. Mound 47 is documented in several maps as a small mound likely less than 3 m in height. The mound is apparent in several historic aerial photographs. For example, a 1933 Lt. Dache Reeves aerial was taken before significant urbanization of the area and records the location and approximate size of the mound at 55 m E-W and 68 m N-S (Figure 6a). Using the Dache Reeves aerial imagery as a guide, the Mound 47 boundary was traced and overlain on the resistivity data. No corresponding boundary is readily apparent in the resistivity (or magnetometry) data, although several high resistance anomalies are located within the mound extent. These could be related to mound construction or activity (including the possibility of burials from a mid-nineteenth century sawmill explosion).
Palisade Evidence
Projected portions of the palisade wall suggest the western palisade was located to the west of Mound 48 and east of Mound 47. The approximate distance between the bases of Mound 47 and 48 is 40 meters.
The identification of palisade walls using geophysics has been successful in other portions of the site. For example, resistivity surveys south of Mound 48 focused on mapping the relationship of a low purposefully constructed ridge and the western palisade wall. In these areas, high resistivity results were the result of ridge fill. The associated trenches (from where the ridge fill was excavated) are defined as low resistivity anomalies. Portions of the west/south palisade around the Twin Mounds have been surveyed using electromagnetic induction (EM) methods.
The resistivity survey in the Mound 47/West Palisade locality reveals a linear low anomaly with a parallel high resistivity anomaly. Using the previous resistivity surveys as a model, the alternating high and low values might represent a ridge (high) and trench (low) portion of the palisade wall in this area. The possible palisade is situated at the base of Mound 48. Structures 1-3 are located within the wall and Structures 4-6 are outside. Mound 47 is approximately 30 m to the west of the interpreted palisade wall.
The Fingerhut and Mound 47/West Palisade areas preliminary results discussed here are tentative, yet informative as a set of data to test. In the case of Fingerhut, it is suggested that certain areas be resurveyed with resistivity. At Mound 47, an expanded survey area will allow for a “bigger picture” to more further define feature relationships and the nature of subsurface deposits.
For more information:
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