The Mound R Project: Ground Penetrating Radar and Core Sampling of an Earthen Structure at the Moundville Site

Matthew D. Gage

Abstract

During the fall and winter of 1997, the University of Alabama Museums, Office of Archaeological Research conducted a ground penetrating radar (GPR) survey and core sampling of Mound R at the Moundville Site (1Tu500). The project was intended to gather data related to the internal structure of the mound using the least destructive means available. As a consequence of the field investigations the entire summit of the Mound was imaged and twelve coring location were sampled. The information provided by this survey has shed new light on construction sequences--information which previously was only available through intensive excavation.

Introduction

The University of Alabama Museums, Office of Archaeological Research's (OAR) investigation of Mound R began in October of 1997. Funding for the project was provided by the Energen Corporation of Alabama. GPR imaging was employed in an effort to locate various anomalies contained within the mound. These anomalies were later sampled in an effort to identify them and to examine the internal matrix of the structure.

Figure 1. Mound R of the Moundville Site in Tuscaloosa County, Alabama.

Mound R is a large, flat topped, pyramidal, earthen structure which covers approximately 6,375 m² (20,910 ft²) at the base and slightly less on the summit. It is the third largest of 29 mounds built during the Mississippian stage occupation of the Moundville Site in western Alabama (Figure 1). Mound R was utilized as a domiciliary platform with wooden framed structures built on the summit. The surface of the mound provided ample room for GPR survey equipment and a tandem axle drilling rig utilized for the coring phase of the project.

Principles of GPR

GPR is a noninvasive investigative technique which provides a continuous profile of subsurface conditions for the purpose of distinguishing differences in soil. A radar image is achieved by transmitting low frequency electromagnetic energy (120 Mhz to 500 Mhz) into the ground and then recording the amount of reflected energy. The reflectance of these media is due to the differences in the material's dielectric constant which accounts for its ability to conduct energy waves as they pass through. The differences in reflection may be the result of various episodes of mound construction which utilized differing soils, or the presence of anomalies such as large pit features, hearths, burned structures, or buried living areas. OAR currently utilizes a SIR 8 or Subsurface Interface Radar System unit for GPR surveys. The unit produces a profile which is graphically displayed on a monitor while simultaneously being generated on a strip chart printout.

Figure 2. Topographic Map of Mound R with Core Sample Locations Plotted on the Summit.

Survey Methodology

In an effort to image the entire surface of the mound summit, GPR transects were spaced at 1 m (3.28 ft) intervals. The images produced were then matched together to show the location of the numerous subsurface anomalies. These were then plotted on a plan view map of the mound. This map provided the basis for determining the location of six anomalies intended for core sampling. In addition, six core sample locations were spaced evenly around the mound's summit to provide a broad set of data for determining construction episodes and buried mound summits (Figure 2).

Mound R, like all of the mounds at the site, was built in multiple episodes of construction, occupation, and abandonment. In many instances, structures built on the various summits were burned and then buried by later additions. These structures as well as the various stages of construction were the focus of OAR's project.

Anomalies and House Structures

Of particular interest to the crew was a large anomaly which appeared in the northwest corner of the mound at a depth of approximately 1.5 m (4.92 ft) below the surface. The anomaly continued across a relatively large area [approximately 30 m (98.4 ft) by 27 m (88.56 ft)] and appeared as a distinct strata on the GPR image. In contrast to the relatively consistent strata which appeared at that depth, the anomaly showed multiple layers elsewhere in the mound. These layers extended for less the 0.2 m (0.65 ft) within the confines of the strata (Figure 3).

Figure 3. GPR Image of Northwest Anomaly.

Figure 4. Core Sample 3 Showing Charcoal and Fired Clay Zone Matching the GPR Anomaly.

Four core samples (3, 9, 10, and 12) were placed within the area of the anomaly. Within each of the cores a zone of charcoal and fired clay underlain by heated sandy clay (Figure 4) was found at the same depth as the anomaly recognized in the GPR images. The charcoal and heated sandy clay are the result of a structure which once occupied the northwest portion of the mound's summit.

 Mississippian Architecture

Excavations conducted at Mississippian stage sites throughout the southeastern United States, and particularly at Moundville, have shown rectangular house structures to have been built with an internal frame of posts and rafters. Wattle and daub was used to cover the exterior walls and thatch was placed on the roof. As hearths were often located within these structures fire must have been a common threat. Ethnographic accounts by Spanish explorers in and around the southern portion of the Mississippi River Valley mention the intentional burning of structures built on the summits of similar mounds. These structures were then covered over with soil and another structure built on top.

The information provided by the GPR and core samples is consistent with such a sequence of construction, destruction, and re-building. Evidence shows that a structure was built in the northwest quadrant on what was once a mound summit (Figure 5). The structure was completely burned, then buried by later episodes of additional mound building.

Figure 5. Line Drawing Outlining the Structure which Once Occupied the Northwest Corner of Mound R.

 Discussion

The information gathered from Mound R has been invaluable in examining its internal structure. By imaging, mapping, and then coring anomalies located by GPR investigations, OAR has been able to use relatively non-invasive techniques to create a virtual blue-print of the internal construction of the mound.

The technology employed by OAR in the Mound R Project is also applicable to archaeological sites located in a vast variety of environments. If variations in the soil matrix exist as a result of anthropogenic modifications, and given that those anomalies provide a deviation in the reflection of radar energy, then GPR surveys will provide invaluable information to the archaeologists. Locating subsurface features may potentially cut down on both excavation time and labor expenditures, as well as aid in locating potentially significant features which would otherwise be missed by most commonly employed sampling strategies.