North American Database of Archaeological Geophysics
North American Database of Archaeological Geophysics
Abstract/Summary:
Project Name: 41CN95, TX;
Reference: Frederick, C. D. and J. T. Abbott (1992). Magnetic Prospection of Prehistoric Sites in an Alluvial Environment: Examples From NW and West-Central Texas. Journal of Field Archaeology 19(2):139-153. Reproduced from Journal of Field Archaeology with the permission of the Trustees of Boston University. All rights reserved.
Portable proton magnetometers were employed at a number of sites dating to the Archaic and Late Prehistoric periods at several different localities in NW and west-central Texas in an attempt to locate subsurface prehistoric features for excavation. Eight magnetic surveys were performed, resulting in the identification and recovery of several fired features, clusters of ceramics, and numerous metal objects of historical age. The results demonstrate that thermoremanent magnetization of fired artifacts and features in this environment is often strong enough to allow their detection by magnetic prospection, and suggest that this technique is a valuable approach for maximizing data recovery during testing and mitigation of shallowly-buried alluvial sites in the region. Negative results, however, do not necessarily mean that features are absent; therefore, the method should be considered an adjunct to, rather than a substitute for, traditional testing procedures.
Site 41CN95 is a large prehistoric site containing Archaic and Late Prehistoric components situated on a terrace overlooking the confluence of the Colorado River and Elm Creek in Coleman County. The site is situated at a confluence of the Colorado River and a major tributary, and consists of a late Holocene alluvial fill lapped onto an early to middle Holocene alluvial unit. At this site, the more recent of the two fills was targeted in an attempt to detect features dating to the Late Prehistoric occupation. One magnetic prospection block measuring 18 m x 25 m was established and surveyed at the site. A number of anomalies were identified in the resulting plot. Two relatively large anomalies present in the ENE corner and the NW side of the magnetometer grid were interpreted as historical metal and geological variation, respectively, and were not recommended for testing, but two small, discrete dipoles present in the south central and WSW portions of the grid were considered quite promising, and hand investigation was recommended. Testing of these two anomalies revealed the presence of a small, rock-lined hearth at a depth of approximately 50 cm below anomaly A and a single, large, possibly burned, limestone clast at a depth of approximately 50 cm below anomaly B.
Site 41GR323 is located in the alluvial valley of Grape Creek, a major tributary of the Double Mountain Fork in the project area. It is situated on late Holocene sediments consisting of interfingering alluvial terrace and alluvial/colluvial fan deposits. Surface reconnaissance and shovel tests demonstrated that the site contains Late Prehistoric materials at a shallow depth. The prospection grid was situated entirely on alluvial/colluvial fan deposits. Although several monopolar anomalies were identified and tested, no discrete dipoles were detected and no prehistoric features were recovered.
Site 41HF1 consists of a shallowly buried, Late Prehistoric component present within a colluvial slope/alluvial terrace adjacent to Palo Duro Creek. Two grids were established along the margin of a colluvial slope/alluvial terrace adjacent to the canyon wall, while a third was positioned on an alluvial terrace that is locally buried by a thin mantle of eolian sand that thins northward. Nine anomalies, considered to be possible prehistoric, thermally altered features were identified on the magnetic field plots of these three grids, but shovel excavation subsequent to evaluation with a metal detector ascertained that most were attributable to historical metal. Excavation of two of the anomalies did yield prehistoric sources, which consisted of burned rock and two ceramic sherds in the first instance and two ceramic sherds in the latter. Another anomaly located within grid 3 yielded burned rock, lithic debitage, and a pin flag wire.
Site 41HF8 is a multicomponent site situated upon and within alluvial-colluvial deposits adjacent to Palo Duro Creek. One 50 m x 25 m grid block was surveyed at the site. Eight magnetic anomalies were identified in the magnetic plot, but only one of these was associated with a thermally-altered prehistoric feature. This feature was an intact rock hearth buried 20 cm beneath the ground surface. Of the other seven anomalies identified at this site, three were associated with metal objects, one was apparently the result of a concentration of organic matter at the former location of a large tree, and three were unexplained despite excavation of the upper meter of the site matrix.
Site 41KT53 is located in the alluvial valley of Grape Creek, a major tributary of the Double Mountain Fork in the project area. It is situated on late Holocene sediments consisting of interfingering alluvial terrace and alluvial/colluvial fan deposits. Surface reconnaissance and shovel tests demonstrated that the site contains Late Prehistoric materials at a shallow depth. Two grids were surveyed at the site. Grid 1 straddled the interface between the alluvial/colluvial fan deposits and the alluvial terrace. Grid 2 was situated entirely on the alluvial terrace. Grid one yielded one concentration of ceramic sherds that was contained in stream alluvium. The results from Grid 2 were quite good, revealing a number of anomalies of various sizes and configurations. Two large positive anomalies were detected in the NW and NE corners, and a NW-SE trending dipole occupied the SE quadrant. Additionally, several small discrete dipoles were identified in the north-central, south-central, and western sections of the grid. Excavation units were placed in a number of these locations in an attempt to identify the source of the anomalies.
Site 41RN169 is a large prehistoric site containing Archaic and Late Prehistoric components situated on a terrace and bedrock bench overlooking Rocky Branch at its confluence with the Colorado River in Runnels County. One 30 x 30 m magnetic prospection grid was established and surveyed at the site. The grid was located on the northern half of the site on a smooth, gently sloping surface composed of early to middle Holocene alluvium overlain by upland slopewash. No features or artifacts were present on the surface within the grid, although sheetwash erosion of the same surface approximately 25 m away had revealed an almost continuous lag of burned rock, debitage, and shell. A number of relatively small, discrete dipolar anomalies were detected and interpreted as possible burned features. During the testing phase, an excavation unit was placed to investigate the strongest of these anomalies. This revealed that the anomaly was caused by a significant concentration of burned rock located 20-40 cm below the surface. Unfortunately, testing of the other target anomalies on this grid was never undertaken.