by Lynne P. Sullivan, McClung Museum, University of Tennessee
Archaeologists use both absolute and relative dating methods to find out the ages of things. Absolute dating assigns an actual age to something rather than simply establishing that it is older or younger relative to another item. Archaeomagnetic dating is an absolute dating technique that relies on the fact that the earth’s magnetic field (the “North Pole”) changes in direction and intensity through time, and that certain archaeological features preserve evidence of the pole’s location at specific times. For example, a hearth made from clay is an excellent source of archaeomagnetic information because clay typically contains iron minerals that, when fired, become remagnetized parallel with the earth’s poles. Through painstaking sample collection and measuring techniques, the alignment of the mineral particles can be matched to a known past location of the earth’s magnetic field.
Archaeomagnetic dating was in development during the mid-1970s when, before the impoundment of the Tennessee Valley Authority’s Tellico Reservoir, the University of Tennessee conducted extensive excavations in the Little Tennessee River Valley. One excavated site, Toqua, was a large Mississippian town that contained the remnants of many buildings with fired clay hearths. Although 62 samples were taken from Toqua for archaeomagnetic dating, the data from these samples were never fully interpreted, but were kept on file at the McClung Museum. At the request of Lynne P. Sullivan, the McClung Museum’s Curator of Archaeology, Jeffrey Eighmy, Director of Colorado State University’s Archaeomagnetic Dating Laboratory, and his graduate student, Stacey Lengyel, recently interpreted the samples from the Toqua site. Measurements on 53 of the samples were accurate enough to use for dating.
Archaeomagentic dating has been used with considerable success in the southwestern United States, but a problem with using this technique in eastern North America is that the ancient movements of the earth’s magnetic field are not well understood from this region. Lengyel and Eighmy plotted the measurements from the Toqua samples against two possible curves. The results not only provide dates for Toqua, but also indicate that one of the curves, known as MCCV190 (Figure 1), is more accurate than the other.
Figure 1. Movement of the earth’s north magnetic pole between AD 1150 and 1550, as estimated on curve MCCV190.
Figure 2. Plot of mean dates [black dots] for Toqua site buildings with multiple dated samples. Circles around the means indicate ranges in which statistical confidence limits of the dates are 95 percent.
On the MCCV190 curve, all but four of the Toqua samples dated between AD 1200 and 1500, the expected range of occupation for the site. The precision of the dates on individual samples ranges from 75 to 350 years. Many of the dated samples are from hearths in buildings on various levels in the large platform mound at the site. This mound supported a succession of public buildings. Other samples are from hearths in buildings that served as dwelling houses. The dates from these various hearths place individual buildings in time, making it possible to determine which of the dated buildings are older and which more recent (Figure 2).
In some cases, hearths were refurbished and multiple samples were taken from a succession of hearths in one building. The resulting series of dates can show whether the building was used for a long or short period of time.
Dates from the hearths also can be used to help determine the ages of artifacts found in the buildings. For example, Sullivan plans to use the archaeomagnetic dates from Toqua to assist a study she is doing on changes in pottery styles between AD 1200 and 1600.
The interpretation of the archaeomagnetic dates from the Toqua site was funded by a grant to Sullivan from the National Science Foundation.
More detailed information on the archaeomagnetic dates from Toqua can be found in:
Lengyel, Stacey N., Jeffery L. Eighmy, & L. P. Sullivan. 1999. On the Potential of Archaeomagnetic Dating in the Midcontinent Region of North America: Toqua Site Results, Southeastern Archaeology, 18(2):156-171.