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NCPTT, Washington University at St. Louis (Dr. T.R. Kidder), the University of Minnesota’s Institute for Rock Magnetism (Dr. Peter Solheid and Dr. Subir Banerjee), and the Louisiana Division of Archaeology (Dr. Joe Saunders) have teamed up to explore how soil formation might be used to determine the relative age of prehistoric earthworks.

Poverty Point

Poverty Point

Line drawing of Poverty Point.
(Graphic: Gibson 1996)

Sediments used to build earthworks break down into identifiable soil horizons over time. Previous and on-going studies show that older earthworks have a more developed soil sequence with a well defined and thick accumulation of clay. Thicker clay accumulations at greater depth indicate a longer passage of time from the initial construction. Comparing the depth of clay accumulations between earthworks may allow the creation of a relative history of earthwork construction, a dating technique that becomes more robust when combined with radiometric dating of organic material within the soil sequence. Because it is possible to study clay accumulation in small diameter solid soil cores, this method offers the possibility of a rapid and relatively inexpensive means of assessing site chronologies that is also minimally damaging to archaeological resources.

Such a study has been hypothetically possible for many years, but the promise of this method is limited because of three factors:

  1. minimal investigation of variables possibly affecting clay translocation processes,
  2. extant hydrometer sediment analytical techniques require too much time to make translocation studies worth pursuing, and
  3. the technique usually results in the sacrifice of the entire core.

Shepard's Classification graphic

Shepard’s Classification graphic

Classification of soil by composition of sand, silt, and clay.
(Graphic: Shepard 1954 in USGS 2003)

This research collaboration will employ a geological and commercial technique never applied for sediment study in archaeology: laser diffraction particle size analysis. Using a laser diffraction particle size analyzer (LDPSA) to categorize sediment size represents a marked technological advance, because, compared to the traditional hydrometer method, the LDPSA process is roughly 90 times faster (circa 30 samples processed every 8 hours) and sample sizes can be as much as 92.3 percent smaller (5 grams, rather than 65 grams). In short, LDPSA makes practical for the first time an effort to develop a new technique for the relative age assessment of earthworks through translocation studies.

Soil cores from non-archaeological contexts in the vicinity of the Poverty Point earthworks, a World Heritage site in northeast Louisiana, are currently undergoing magnetic and laser diffraction particle size analyses to determine if the proposed research is viable. If so, the project will seek permission to continue investigating in archaeological contexts at the site itself.

1996 Poverty Point: A Terminal Archaic Culture of the Lower Mississippi Valley. 2nd edition. Anthropological Study Series No. 7. Department of Culture, Recreation and Tourism, Office of Cultural Development, Division of Archaeology, Baton Rouge.

Shepard, F.P. 1954 Nomenclature Based on Sand-Silt-Clay Ratios. Journal of Sedimentary Petrology 24:151-158.

U.S. Department of the Interior, U.S. Geological Survey 2003 Surficial Sediment Data from the Gulf of Maine, Georges Bank, and Vicinity: A GIS Compilation. U.S.G.S. Open-File Report 03-001. Available on the World Wide Web at: (Last updated on August 14, 2003).

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