Archaeo-Physics, LLC - geophysical survey and subsurface mapping

case study

Investigations at the "East of A" site (41OC29)- Buried City locality

Ochiltree County, Texas

In March of 2000 Archaeo-Physics was contracted by Courson Archaeological Research (CAR) to conduct a shallow subsurface geophysical survey at the "East of A" site (41OC29), located in Ochiltree County, Texas. This site is part of a large aggregation of Plains Village period (A.D. 1200-1500) sites on Wolf Creek collectively referred to as the Buried City locality. Click here to view a map of the locality in a pop-up window.

The Buried City locality is one of the most investigated archaeological complexes in Texas history, with archaeological investigations conducted in 1907, 1931, 1966, 1985, and from 2003 to present. A thorough summary of the Buried City locality and the archaeological investigations that have been conducted there is presented by Texas Beyond History and may be viewed by clicking here.

The locality is thought to contain approximately 100 stone slab house features occupying elevated terraces above Wolf Creek. A reconstructed settlement plan from the locality may be viewed in a pop-up window here. The locality is perhaps best known for its stone slab architecture and for this reason its inhabitants were often assumed to be closely related to the Antelope Creek phase peoples living to the SW and NW who also occupied stone slab houses. The Buried City stone slab houses were the focus of nearly all archaeological investigations conducted prior to 2003, perhaps due to the relative ease with which these features can be located (i.e., the stone alignments are often visible on the ground surface). However the material culture of the Buried City peoples more closely resembles that of the subterranean pithouse dwelling Odessa phase peoples living directly adjacent to the northeast. One of the primary objectives of the geophysical investigation was to test whether the Buried City peoples utilized subterranean pit houses in addition to the stone slab structures this locality is best known for.

The geophysical investigation consisted of magnetic field gradient, electrical resistance, and to a limited extent, ground penetrating radar (GPR) survey. Numerous geophysical anomalies were identified as possible buried archaeological features. In March of 2001 a selection of these anomalies were tested by a process often referred to as "ground truthing." The ground truthing methods consisted of systematic close interval hand cores centered on geophysical anomalies of interest. Small-scale hand excavations ranging is size from 50 cm x 50 cm shovel tests to 50 cm x 200 cm hand dug trenches were employed at several anomalies as well. The primary objective of the ground truthing program was to determine whether the source of these geophysical anomalies were cultural features or not. Once the source of an anomaly was determined to be cultural in origin, a second goal was to gather the information necessary for assessing the horizontal and vertical extent of the cultural feature in question. Based on this information it is sometimes possible to determine the type of feature that is represented.

The geophysical investigation and subsequent ground truthing program resulted in the discovery of several probable subterranean pithouses. Two of these features were targeted by traditional block excavations during the 2003 University of Oklahoma field school sponsored by CAR. Both of these block excavations documented pithouse features, confirming the presence of this type of living structure at the Buried City locality.

The results of the geophysical investigation, ground truthing, and block excavations are summarized in the interactive graphic below. Additional details concerning the geophysical methodologies and block excavation results can be found below. Viewers are encouraged to explore the interactive graphic using their mouse. Internet Explorer users may need to activate ACTIVE X controls.


41OC29 Interactive Data Exploration Tool


Geophysical Investigation Methodology

The geophysical investigation consisted of magnetic field gradient, electrical resistance, and ground penetrating radar (GPR) survey over a sizeable area with the potential of containing buried and intact cultural resources. Data were collected in 30 x 30 m grid squares.

Magnetic Field Gradient Survey

Magnetic survey was completed over 13,500 square meters of the site. The survey was completed using a Geoscan Research FM 256 fluxgate gradiometer. The fluxgate gradiometer records the vertical gradient of the earth's magnetic field at a point near the surface of the earth. Data were collected in linear transects spaced 50 cm apart. A measurement of the vertical gradient was recorded every 12.5 cm along each transect, resulting in an overall data sample density of 16 samples per square meter.

Electrical Resistance Survey

Resistance survey was completed over 15,300 square meters of the site. The survey was completed using a Geoscan Research RM 15 electrical resistance meter in twin-probe configuration. The distance between the mobile probes was 50 cm and the distance between the remote probes was 100 cm. A potential of 40 V was used to apply a 1 mA current. Data were collected in linear transects spaced at 1 m, with 2 samples per linear meter collected along each transect. This resulted in an overall data sample density of 2 samples per square meter.

Ground Penetrating Radar

GPR survey was completed over one 20 x 20 m square grid (400 square meters). The survey was completed using a Sensors & Software pulseEKKO 1000 GPR system operating at a center frequency of 450 MHz. Data were collected in linear transects spaced at 50 cm, with a GPR trace collected every 5 cm along each transect. This resulted in an overall data sample density of 40 GPR traces per square meter. The resulting 3-D GPR data set can be explored in the interactive data exploration tool provided above.


Ground Truthing Methodology

From March 9-13, 2001 a crew of six archaeologists conducted ground truthing of a sampling of those geophysical anomalies identified as possible cultural features.

The locations of 26 anomalies were marked on the ground surface with a wooden stake using an electronic total station. These anomalies were then tested by systematic coring using Oakfield hand probes with a 2.54 cm (1 inch) barrel by teams of two field technicians (i.e., one corer and one recorder). Over 450 hand cores were collected during ground truthing. Coring strategies varied depending on the nature of each anomaly, although cores were minimally excavated in linear transects perpendicular to anomalies that were linear in shape and + shaped transects centered on point target anomalies. The distance between individual cores varied from 1 m to 25 cm.

Several anomalies which were cultural in origin, but whose overall size and shape could not be determined through coring were selected for more extensive excavation by shovel tests, test units, or hand dug trenches. The size of these hand excavations varied from 50 cm x 50 cm to 50 cm x 200 cm. These units were excavated in arbitrary 10 cm levels using shovels and trowels. Shovel tests and test units were screened through 6 mm mesh and cultural materials were collected. Sediments excavated from trenches were not screened.

A total of 26 geophysical anomalies were tested during ground truthing, with 18 of these confirmed as buried archaeological features. Although in some cases it was possible to guess the function of the buried features from this limited invasive testing, in most cases more extensive excavations were required to definitively identify the buried resources.


aerial view of 2003 block excavations in progress

2003 blocks excavations in progress at Buried City

Block Excavations During the 2003 Field Program

During the summer of 2003 CAR research archaeologists and University of Oklahoma archaeologist Dr. Susan Vehik conducted block excavations at the "East of A" site. Two excavation blocks were placed over geophysical anomalies that hand coring results suggested may represent buried subterranean pit houses.

Both excavation blocks revealed deeply buried pit houses that differed considerably from the stone slab architecture the Buried City locality is known for. Three calibrated radiocarbon dates were obtained from samples collected from the floors of these houses. These dates ranged from A.D. 1280 +/- 50 to A.D. 1300 +/- 40. A fourth radiocarbon date was obtained from charred wood recovered from the burned structure encountered in Shovel Test #6. This sample returned calibrated dates of A.D. 1310, 1370, and 1380 (the calibration curve crossed three possible dates).

 


Conclusions

Analysis of these data is ongoing, but the research program sponsored by CAR has unequivocally shown the Buried City peoples to have occuppied subterranean pit houses. More research is necessary to determine whether the pit houses pre-date the stone slab architecture, or if these two types of structures were inhabited concurrently.

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