North American Database of Archaeological Geophysics
North American Database of Archaeological Geophysics
Abstract/Summary:
Project Name: Toltec Mounds Archeological State Park (3LN42), AR (2);
Reference: Lockhart, J.J. (2001). Exploratory Archaeogeophysics at Toltec Mounds Archaeological State Park (3LN42), Mound G. Internal Arkansas Archeological Survey project report on file, Arkansas Archeological Survey, Fayetteville.
An exploratory archaeogeophysical survey was carried out at Toltec Mounds Archeological State Park (3LN42) 15 miles southeast of Little Rock, Arkansas. The limited research plan was designed to achieve several goals, including the testing of multiple archaeogeophysical technologies at Toltec, as well as contributing to the understanding of the features and extents of Mound G. The four techniques used during the survey were electrical resistance, electromagnetic conductivity, magnetic susceptibility, and magnetic gradiometry. Procedures and initial results are described in this report.
Mound G, as it appears today is a barely perceptible rise at the geographic center of the Toltec site. It was one of 10 mounds on the site described as "much reduced by the plow" in a late 19th century eyewitness account. Indeed, early plats and maps show a house and outbuildings at the location, but no mound. In addition, the mound was further leveled in the mid-1960s by pushing material from west to east; with the excess soil (mound fill) being used as fill for leveling a prehistoric borrow near Mound B on the site. Consequently, Mound G's current surface expression is less than 50 cm in height. When in use by the people that built it between 600 and 1000 A.D., however, Mound G is estimated to have been between 1-3 m in height and perhaps as much as 110 m in length, which would have made it the longest mound of the Toltec complex.
A Geoscan Research RM-15 electrical resistance meter with a MPX-15 multiplexer allowed acquisition of resistance data at six depths. This survey was performed first in order to (1) identify the extents of a suspected circular feature identified by Rolingson in a 1985 excavation in that grid, and (2) determine the optimal survey depth for future grids. Data at depths of 0.25, 0.50, 0.75, 1, 1.25 and 1.5 m were collected throughout the 400 sq m of this grid. Extrapolating from the portion of the feature visible in a test trench excavated 15 years earlier, Dr. Rolingson had estimated that the circular feature was approximately 6 m in diameter. The resistivity data at 25 and 75 cm depths indeed showed the test trenches and an indication of a circular anomaly approximately 5-6 m in diameter positioned as hypothesized.
Two Geoscan Research RM-15 resistance meters with multiplexers allowed the acquisition of up to four measurements for each placement of the instrument. Depth of prospection was uniformly set to 50 cm, which appeared to yield an adequate response based on the multiple depth study results. The resistance data exposed seemingly patterned, rectilinear anomalies and arcing features. The distinct north-south boundary between high and low contrast zones may indicate the edge of Mound G or the area over which Mound G deposits were spread when it was leveled. It is hypothesized that the extra thickness of this overburden, perhaps as much as a meter, causes the blurring or lack of contrast in the anomalies that can be seen to the right of this boundary. In other words, the various arcing and rectilinear patterns may be cultural features below the mound which tend to be obscured by the mound's overburden because the thicker deposits increase prospecting depth to these features (in general less detail is achieved in deeper prospecting).
To investigate this possibility the resistivity data were processed by a variance balancing method which, in effect, is equivalent to the removal of the mound's overburden in the eastern two-thirds of the data set. The result shows, with good clarity, numerous rectilinear and curvilinear patterns in the data. It seems clear that these patterns cannot derive from natural processes, but must be cultural in origin. It is also unlikely that they derive from 19th century agricultural practices because they appear to exist below the Mound G overburden. Their interpretation will require evaluation based on local and regional archaeological expertise and, most probably, limited archaeological testing.
Magnetic Susceptibility (MS) data were collected by using the in-phase component of the Geonics, Ltd., EM-38B. Although the area covered was smaller than that achieved in the resistivity survey, several comparisons can be made. For example, there is good correspondence between the data sets, particularly the independent conformation of certain east-west trending anomalies. Because the magnetic susceptibility of soils tends to be enhanced by anthropogenic factors this correspondence supports the interpretation of culturally generated anomalies.
Collected simultaneously with magnetic susceptibility, the quadrature (out-of-phase) component of the Geonics, Ltd., EM-38B acquires electromagnetic conductivity (EM) data. EM methods are sensitive to metals and the data reflect that sensitivity, particularly in grid 19. An 1882 plat of the Knapp Mounds site by H. J. Lewis indicates a house and outbuildings in the location of Mound G, and the high concentration of metal in grid 19 may prove useful in locating the remains of those historic structures.
A Geoscan Research FM-36 magnetic fluxgate gradiometer shows the locations of the 1985 trench excavations due to the disturbance of the soil and residual metals left in the excavations. In grid 19 the gradiometry corroborates and further defines the large field of ferrous metal artifacts, also identified in the electromagnetic conductivity and magnetic susceptibility images, and possibly indicating the locus of the reported historic structures.