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An Example of TPQ and Seriation Analysis
Terminus Post Quem or TPQ analysis is a way to date layers of soil by identifying the most recent artifact from that stratum. For example, say you dug a layer of soil and the following artifacts were unearthed: a white salt glazed stoneware ceramic sherd (1720); a machine cut nail (1790); and a Styrofoam cup fragment (1962). The dates listed are the earliest possible date of manufacture for that artifact. By looking at the dates, you know this layer contains artifacts from the 18th and 20th centuries. The presence of the Styrofoam cup tells archaeologists that this layer could not have been deposited before 1962, when this synthetic material was first produced. The layer could have been deposited anytime after 1962, but not before.
Not all artifacts are deposited at the same frequency in each layer. For example, five successive layers of soil may have various amounts of machine-cut nails, say -- 2, 5, 20, 15, 1 -- from the bottom layer to top. Now, this may simply reflect random use and discard of machine-cut nails BUT it might indicate a change in past behavior that has created a meaningful pattern. Our job as archaeologists is to see if we can detect a meaningful pattern.
Seriation is a way to look at the changing frequency of artifacts deposited in the layers of soil allowing these meaningful patterns to be found. If we convert the numbers of nails to percents and arrange them in stratigraphic order on a graph, we arrive at a standard seriation curve. Stratigraphic order means the sequence in which the layers were deposited, or the earliest layer is on the bottom and the most recent layer is on the top. Archaeologists dig in reverse stratigraphic order, excavating the most recent layer first and call it Layer A.
If we were to identify TPQ dates for the different layers, a hypothesis of deposition can be formed (Figure 1). Say Layer E has a date of 1800, Layer C has a date of 1835, and Layer A has a date of 1885. We already know that machine-cut nails have a TPQ 1790. By performing research into changing nail technologies, we know that cut nails replace earlier hand-wrought nails around 1790. Additionally, wire nails become common on archaeological sites around 1885. In Layer E, the low cut nail count could relate to the initial use of wire nails. The nails are most frequency used and discarded in Layer C, but decrease in frequency from here up to Layer A (or from 1835 through 1885). Therefore, the change in frequency of cut nail deposition could reflect the changes in nail production technology in the 19th century. Though this is a simplistic view of site analysis, it shows one example of the combined use of TPQ and seriation analysis combined with documentary evidence. Click here to explore how these analytical methods helped us figure out whose trash was in the South Grove.