ILLINOIS ARCHAEOLOGICAL SURVEY

This portion of the IAS web site is devoted to the use of geophysical survey techniques in Illinois archaeology. Geophysics refers to a suite of techniques including magnetics, conductivity, electrical resistivity, and ground penetrating radar (GPR). Although geophysical techniques are not widely used in Illinois, a number of surveys (Appendix A) have been conducted here. Most readers are probably familiar with Weymouth and Wood’s study at Forts de Chartres and Kaskaskia, a relatively early application of magnetic techniques to historic archaeology (Weymouth and Woods 1984). Since the late 1980’s, Rinita Dalan has conducted a number of important geophysical investigations focused on prehistoric construction of the Cahokia mounds, plazas, and palisades (e.g., Dalan 1989, 1993, 1997). Berle Clay has conducted several highly informative geophysical surveys of late prehistoric sites in the interior uplands of southern Illinois, including the Dillow’s Ridge and Millstone Bluff sites (http://www.crai-ky.com/geophysical/). In 1995, the National Park Service’s annual training course in geophysics was held at Cahokia, introducing students to a number of different techniques (De Vore 1996). Sue Lowry has used a simple but effective resistance meter to trace the palisades at Cahokia. Michael Hargrave has conducted magnetic and resistance surveys at Cahokia and a number of late prehistoric sites in southern Illinois.

Despite the success of these efforts, Illinois archaeologists (like most professional archaeologists in the U.S.) have been slow to incorporate geophysics into their field investigations. Reasons for this conservatism include a widespread lack of familiarity with the strengths and limitations of various geophysical techniques, particularly their cost and reliability. We hope that the geophysical surveys summarized here will encourage Illinois archaeologists to consider the potential contributions of geophysics to their own research and resource management efforts. Please follow the links to other sites (Appendix B) to learn more about the role of geophysics in archaeology.

Geophysical techniques are widely used by archaeologists in Great Britain, Europe, and the Middle East. In those regions, many prehistoric sites include massive stone or brick architecture, fired roofing tiles, and metal artifacts. These materials could be detected by some of the earliest geophysical instruments, and Old World researchers quickly adopted geophysics. In contrast, most prehistoric sites in the U.S. are characterized by relatively ephemeral features such as small hearths, earth-filled postholes and pits. These low-contrast features represented difficult targets for the early instruments, and many early applications of geophysics in the U.S. were not particularly successful. Modern geophysical instruments are more sensitive than their predecessors, can store much larger data sets, and can (with some training and practice) be used effectively by nonspecialists.

Federal agencies like the National Park Service and the U. S. Army Corps of Engineers are working to integrate geophysics into Cultural Resource Management. Studies conducted by CERL (the Construction Engineering Research Laboratory, Champaign, Illinois) at a number of Army installations have demonstrated that geophysics can increase the reliability and, in some cases, reduce the costs of National Register eligibility assessments. Geophysics can also provide the basis for the non-invasive or minimally invasive investigation of sites where traditional archaeological excavation is not practical. Examples include sites of great importance that are effectively protected from adverse impacts, sites on private land where funds or permission for excavation can not be secured, and sites where human skeletal remains are highly likely to be present.

The magnetic surveys shown below were conducted using a Geoscan FM36 fluxgate gradiometer. The FM36 can measure extremely subtle disturbances to the earth's magnetic field that were caused by the creation of hearths, pits, and similar features, as well as disturbances related to modern agriculture, erosion, and bioturbations. Features can be detected in geophysical surveys because they contrast with the surrounding soils. For example, prehistoric pits are typically filled with organically rich topsoil or midden that tends to be slightly more magnetic than the subsoil. In a magnetic survey, pits (and house basins) typically appear as slightly positive anomalies. Anomalies are discrete areas in a geophysical map characterized by geophysical values distinct from those of the surrounding areas. Strongly magnetic materials such as iron objects or heavily fired hearths may be manifested in a geophysical map by a dipole (paired positive and negative) anomaly.

To conduct a gradiometer survey, one first establishes a grid of 20 by 20-meter blocks. Nonmagnetic tapes or ropes are used to define survey transects. As the surveyor carries the gradiometer along each transect (Figure 1), the automatic trigger emits an audible sound that allows the operator to distribute the data collection points evenly. To increase the chances of identifying small, low-contrast features at a prehistoric site, the geophysicist may collect up to 16 data points per square meter.

The data shown here were initially processed using Geoplot 3.0, a specialized soft ware provided by the manufacturer of the FM36. Geoplot provides a number of routines that allow one to clean up, analyze, and improve the appearance of the data. Data processed in Geoplot were imported into Surfer 7.0 to produce the maps presented here.

The magnetic survey data processing included use of a Low-Pass filter, which greatly enhances the potential to detect subtle features. The overall mean in each map is zero, plotted as 50% gray. Positive anomalies (>0) are darker and negative anomalies (<0) are lighter.

Late prehistoric (terminal Late Woodland, Mississippian) settlements in the Midwestern and southeastern U.S. are often excellent candidates for geophysical investigation. Many settlements are highly structured and exhibit a wide range of feature types. Household feature complexes are clustered around open courtyards at some sites. Large settlements are often organized around plazas and mounds.

Pfeffer Site (11S204) is one of several early Mississippian settlements investigated by Dr. Tim Pauketat (University of Illinois at Urbana/Champaign) and Susan Alt (UIUC) as part of the NSF-funded Richland Archaeological Project. Pfeffer is the second largest of four Mississippian mound centers in the central Silver Creek valley. Investigations in 2000 included controlled surface collections, mechanized stripping, and the hand excavation of 16 structures and 112 pits. The single-post and wall trench structures and many of the pits date to the Lohmann (A.D. 1050-1100) and Stirling (A.D. 1100-1200) phases.

Prior to the UIUC excavations, Dr. Michael Hargrave (ERDC/CERL) conducted a magnetic field gradient survey at Pfeffer. In the accompanying map (Figure 2), the magnetic data are shown in gray tones. The small black areas are positive magnetic anomalies. The white line shows the limits of mechanized stripping of the plow zone. Structures are shown as red rectangles and the small yellow circles are pits. One can see that many of the magnetic anomalies turned out to be associated with pits. Magnetic anomalies suggest that additional pits are located in the central portion of the survey area where no excavation occurred. Figure 3 shows a close-up of anomalies associated with a structure complex and several pits.

Grossmann Site is an early Mississippian period settlement located in the uplands about 18 km east of Cahokia. In 1998, ITARP archaeologists excavated 17 structures located in the right-of-way of a new road. In 1999, ITARP conducted a controlled surface collection of artifacts across the remainder of the site. Dr. Michael Hargrave (ERDC/CERL) conducted a magnetic survey of the site in 2001. Dr. Tim Pauketat (UIUC) and Susan Alt (UIUC) directed large-scale excavations at Grossmann, under the aegis of the University of Illinois Field School in Archaeology and the Richland Archaeological Project. An area of 3,188 m² was mechanically stripped, exposing 42 Mississippian period structure complexes (55 building episodes) and 58 other features (pits, post pits, truss trenches, hearths, etc.). Grossmann is an unusual site in many ways. Numerous structures are closely packed into a relatively small area. By regional standards, the structures tend to be quite large. One feature included a cache of 70 ground stone celts.

The magnetic gradient survey at Grossmann was quite successful (Figure 4). About 60% of the structures and 40% of the other features were detected (Figure 5). Less than 20% of the anomalies were false positives (i.e., anomalies that were not associated with prehistoric features).

Magnetic lab tests of soil and artifact samples conducted by David Maki (Archaeo-Physics) helped explain factors that influenced feature detection and false positives. Features with larger volumes and evidence for burning, and structures with basins were more likely to be detected. Some of the false positives may relate to the remains of features preserved in the plow zone. Based on the ratio of anomalies to detected and undetected structures, we estimated that an additional 49 structures are present outside the excavated area.

Cypress Citadel (11JS76) is a large, flat-topped upland ridge formation located near Cypress, Johnson County, Illinois. Defined on all sides by cliffs and very steep slopes, the landform is reminiscent of mesas found in the southwestern U.S. The bluff top area is covered in dense second growth timber but appears to have never been plowed.

Because of the site's defensive potential, Cypress Citadel has often been grouped with the stone fort sites found elsewhere in southern Illinois. Artifacts on the bluff top attest to an intensive occupation dating to the later portion of the Late Woodland (Lewis Phase) period (ca. AD 600 to 850). Unlike the stone forts, Cypress Citadel exhibits no evidence of Native American defensive architecture. A number of rock mounds or cairns are distributed around the bluff base, however, and several occur along the edge of the bluff top.

Dr. Michael Hargrave (ERDC/CERL) conducted a magnetic field gradient survey of ten 20 by 20-meter blocks scattered across the Cypress Citadel bluff top. Numerous anomalies were identified, some obviously related to looter holes, trees, etc. In 2001 the Southern Illinois University at Carbondale Field School in Archaeology directed by Dr. Brian Butler and Mr. Mark Wagner investigated several of the magnetic anomalies. One of these (the large anomaly shown in Figure 6) proved to be a roughly rectangular house basin that measured 5.6 by 5 meters. Late Woodland structures are rarely found in southern Illinois, and this one represents only the second Lewis phase structure to be identified.

In 2002, the SIUC Field School will return to Cypress Citadel to investigate additional magnetic anomalies and other areas. The excavation data should provide an excellent opportunity to determine the types of features that were detected in the geophysical survey, as well as those that were not detected.

http://www.cast.uark.edu/~kkvamme/geop/geop.htm

http://www.cast.uark.edu/nadag/ (Sponsored by the National Center for Preservation Technology and Training, National Park Service).