Tuesday, February 7, 2012

Plains Archaeology: Dating Methods

Knight, George C., and James D. Keyser. 1983 A Mathematical Technique for Dating Projectile Points Common to the Northwestern Plains. The Plains Anthropologist 28(101):199-207.

            Plains archaeologists often encounter lithic projectile points dating from the Archaic and Late Prehistoric periods that are not in their original stratigraphic layer.  This makes dating these stone points using radiometric or other dating methods impossible.  However, using statistics, Knight and Keyser have been successful in providing a method to determining the approximate date of certain lithic tools, specifically projectile points.  Although their article only describes differentiating between two time periods in a subsection of the plains with a very specific subset of tools, their high success rate shows that their methods should be applicable to other regions and time periods.

            In order to calibrate their statistical formulae, the authors needed a large sample size of projectile points with known radiometric dates.  Fortunately, this data is not scarce.  The specific statistical method they used is called discriminant analysis (DA), which specifically applies to the testing of an object, and placing it in two distinct categories using statistical significance.  Using abundant student help, these researchers measured thousands of projectile points from the time periods, and derived functions that described the various notch widths, heights and lengths.  Using an adequate number of digits to reduce error, the authors were able to achieve a 96% rate in identifying unknown projectile points.  The authors, however, did not stop there.  They realized that projectile points are often incomplete.  Knowing this, they provided further statistical analyses to help determine the period of the respective projectile points using only a single variable, in their case the neck width.

            The method of determining the time period of a given projectile point using the method given by Knight and Keyser proved to be quite simple.  In the article, two equations are given.  Simply inputting the measured characteristics of a projectile point into both equations gives two values.  The equation that gives a larger value is the correct one.  Using this relatively simple method expanded to other locations and time periods, tools can be more effectively dated suffice the sample size that is radiometrically dated is adequate.

Caldwell, Warren W., and Lynn M. Snyder. 1983 Dendrochronology in Plains Prehistory: An Assessment. The Plains Anthropologist 28(99):33-40.

            It is a well-known fact that absolute dating methods are far more desirable than relative dating systems in archaeological excavations.  Dendrochronology, or tree-ring dating, is one such absolute dating method.  Although this system gets much more use in the wooded forests of the world, it still has some utility on the plains, where limited tree growth restricts the overall number of trees that have been preserved that are able to be dated.  The plains also receive a medium amount of moisture, which means that the preservation processes that are much more likely to occur in extreme moisture and extreme lack of moisture almost never occur on the plains.  In addition, many tree species on the plains are unsuitable for dating, for various reasons including inconsistent growth seasons, and erratic development.  Despite these setbacks, dendrochronology was a growing field at the time of this articles writing, although it was quickly being surpassed by radiometric methods.  Archaeologists of the plains were willing to use dendrochronology, regardless of the veracity of the results.  Since tree ring dating is extremely difficult to check properly, archaeologists often asserted incorrect dates not realizing their errors. Their major flaw was using master tree ring lists from a distance away that they would not apply at the location of the excavation.

            Realizing the errors that had been made, several groups, such as the Missouri Basin Chronology Program, attempted to create master tree ring lists for all of the specific region on which they were focusing, and then clearly defining accuracy based on distance from the wood sources.  Radiocarbon dating was also used to supplement this data.  The dates were found to be inconsistent, however, and dendrochronology was discarded.

            Caldwell and Snyder attempt to show why this apparent inconsistency occurred across virtually all tested samples.  On average, the pair showed a discrepancy in the oldest houses from between 600-850 years.  This huge inconsistency calls into question not only dendrochronology, but also radiocarbon dates for plains tree samples.  To try to determine why, the authors carefully took recent samples of relatively known age and dated them.  The inconsistencies found previously did not resolve themselves completely.  This would mean that the entire timeline of the Plains would be inaccurate.  It is likely that a more recent article has tried to resolve this issue, but I was not able to locate it.

1 comment:

  1. I am disappointed to hear of the inaccuracy of dendrochronology in the Plains. In Liebmann’s article, the dating of the Big Horn Medicine Wheel of Wyoming has been obtained through dendrochronology. Although methods such as determining chronology by soil deposition, using hydration analysis for flakes found at the site, and attempting to ascertain a date from ethnographic accounts have been attempted, none have provided a reliable date. The only dating method that has worked is that of dendrochronology. The sample was taken from wood found in the western cairn of the wheel and was dated AD 1760. It is unfortunate that inaccuracies of up to 600-850 years have been found. This ambiguity only adds to the uncertainties of the Big Horn Medicine Wheel.

    Liebmann, Matthew.
    2002 Demystifying the Big Horn Medicine Wheel: A Contextual Analysis of Meaning, Symbolism, and Function. The Plains Anthropologist 47: 61-71.