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Site name: On-A-Slant Village

On-A-Slant Village (32MO26) is a late prehistoric/early protohistoric earthlodge village located within Fort Abraham Lincoln State Park, North Dakota. Geophysical investigations were carried out at On-A-Slant on June 25-26, 2001, by Dr. Kenneth L. Kvamme and Jo Ann Christein Kvamme of the Archeo-Imaging Lab, University of Arkansas. The purpose of the geophysical surveys was to ascertain the nature of subsurface features and the quality and clarity of results within an obvious surface depression of the village that denoted the location of a former earthlodge.

The principal area examined was a 20 x 20 m grid that encompassed the earthlodge depression. Magnetometry, electrical resistivity, and ground penetrating radar (GPR) were employed. While all data sets gave indications of the earthlodge's form, and magnetometry and GPR indicated the loci of interior and exterior cultural features, none of the data sets give convincing indications of an entryway.

Type of survey: Ground penetrating radar; Instrument: Geophysical Survey Systems, Inc., SIR-2000, withh 400 MHz antenna; Prospection depth: about 2 m; Sampling interval: 0.5 x 0.02 m (with survey wheel); Range: 40 nS; Area surveyed: 20 x 20 m in 40 transects, each separated by 0.5 m; Date of survey: 6/02.

Comments: The 40 GPR profiles were time sliced. The earthlodge floor was found at about 18 nS (TWTT), or about 90 cm below surface. GPR reveal the floor as a circular region about 10-11 m in diameter, a shape expected from the likely late dates of the occupation. The GPR data parallel many of the magnetic anomalies. The slice shown averages the amplitudes through 20 nS.

A second limited study, consisting of a single GPR traverse, was conducted adjacent to on-going excavations by the University of North Dakota. The resultant profile revealed a likely stratigraphic contact about 35 cm below the surface and indications of the wooden palisade constructed by the CCC in the 1930s.

Type of survey: Magnetic gradiometry; Instrument: Geoscan Research FM-36 fluxgate gradiometer; Prospection depth: up to 1.5 m; Sampling interval: 0.5 x 0.125 m; Area surveyed: 20 x 20 m; Date of survey: 6/02.

Comments: In contrast to GPR, magnetometry suggests that the earthlodge shape may be a rounded rectangular form (13 x 15 m), which could point to an earlier component to the site. As at other sites in the region the magnetic data express much clearer anomalies than GPR, and their accuracy is well established through archaeological testing. The magnetic data also indicate clear circular anomalies interior and exterior to the house. The magnitudes of some suggest hearths while the remainder probably point to subterranean storage pits or possibly large artifacts (like whole pots) or small middens. There is no evidence that the house burned in the magnetometry data.

Type of survey: Multi-depth Resistivity; Instrument: Geoscan Research RM-15/MPX-15; Prospection depth: 0.25-2.0 m; Sampling interval: 0.5 x 0.5 m horizontal; 0.25 m vertical; Area surveyed: 20 x 20 m; Date of survey: 6/02.

Comments: The resistivity data, recorded at 7 different probe separations or "depths," showed little variation with depth. In general, the earthlodge floor indicates relatively low resistivity while the surrounding soil portrays much higher resistivity. No interior features of the house are apparent in any of the data sets. Upon destruction or abandonment and subsequent deterioration the earth covering a lodge tends to form an outer ring about its perimeter. This overburden tends to be thinnest in the lodge interior and thickest about its margins, contributing to the formation of the commonly seen topographic depression. This spoil, in general, tends to exhibit higher relative resistivity, probably due to reduced compaction. Moreover, the central topographic depression may serve to concentrate rainfall, or moisture content, while the floor itself, if compacted, may act as a barrier serving to hold moisture above it with the result that resistivity within the surface depression is further lowered. While a circular plan is expected (see GPR), and can be argued from the data, these results also give a hint of a square/rectangular form that was suggested by the magnetic data.

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(updated: 5/02)