How Archaeologists Tell Time

Historical archaeologists date their sites using a variety of methods, each with its strengths and weaknesses.  Here’s a roadmap that explains how we dated the midden feature itself, its layers and phases, and developed the site chronology incorporating documented events found in the archaeological record.

Mean Ceramic Dating.  Historical archaeologists most commonly utilize documented changes in ceramic ware type and decoration to date layers, features, and archaeological sites.  One way ceramics are used to calculate the average date of a site’s occupation is to find a mean of all the midpoints of the periods of manufacture for the excavated ceramic sherds.  The midpoints are weighted by the frequency of sherds of different types.  The mean ceramic date for the South Grove Midden is 1746.

Tell Time - figure1 

Pipe Stem Dating.  Historical archaeologists also rely on pipe stems to estimate when a site was occupied.  Dating sites using the principle that the bore diameter of a pipe stem regularly decreases over time has been around since the 1950s and has been subjected to statistical analyses and further refined.  For more detailed information on the dating methodologies, refer to Barca 2012.  Figure 1 presents the dates derived from pipe stem data.  The pipe stem date (using Lewis Binford’s formula) is 1746.67. 

Harris Matrix.  Creating a Harris Matrix allows archaeologists to put excavation contexts in order from the bottom of the site to the top.  We created a diagram capturing the relationships of the layers (or strata), which was digitized and made available for download on www.daacs.org.

TPQ.  This dating technique can provide a more accurate measure for the date after which a particular layer or phase was formed by calculating the beginning date of manufacture of the latest dating artifact in an assemblage or sub-assemblage.  For example, if you had a handful of coins from a particular layer, dated 1750, 1775, 1800, and 1802, the TPQ (or terminus post quem) is 1802.  In other words, the layer could not have formed until after that 1802 coin was created.  Archaeologists often use the beginning dates of manufacture for ceramic ware types to calculate TPQs for layers ordered within a Harris Matrix.

Tell Time figure2

Figure 2 presents the MCD and TPQ data for the midden’s phases using the ceramic data.  Also provided are two other TPQ measures, TPQ95 and TPQ90.  TPQ95 estimates the date after which the deposit was formed based on the 95th percentile (or 90th for TPQ90) of the beginning manufacturing dates for the sherds from that layer.  These measures can refine TPQs by removing ceramic outliers that may have been introduced through excavation error or disturbance.