How Archaeologists Tell Time

 Blue and green shell-edged pearlware (1800-1835), Blue floral painted pearlware (1795-1830), Early polychrome painted pearlware (1795-1830),  engine-turned  red stoneware (1763-1775), Jackfield (1740-1800)

Blue and green shell-edged pearlware (1800-1835), Blue floral painted pearlware (1795-1830), Early polychrome painted pearlware (1795-1830), engine-turned red stoneware (1763-1775), Jackfield (1740-1800)

 Archaeology is about telling time.  To be sure, knowledge of the spatial or formal properties of the sites we excavate is important, but if you can’t account for time, you can’t do archaeology.  Historical archaeologists infer time with a number of methods, but it really all comes down to careful artifact inspection and identification.  If the various materials from a given site can be correctly identified and their diagnostic attributes recorded, historical archaeologists can almost always determine occupation periods.  

For most, the venerated guidebook on correctly identifying and dating historic-period artifacts is Ivor Noel Hume’s (1969/2001) classic A Guide to Artifacts of Colonial America.  Republished many times, Hume’s Guide is probably the one artifact reference book every practicing historical archaeologist has on his or her shelf.  In it, he describes an important dating technique that, unfortunately, continues to be under-utilized by historical archaeologists.  The concept of the terminus post quem (TPQ) refers to the date after which an artifact, feature, or soil layer must have been deposited.  The most recently dated artifact within a soil layer, therefore, is that layer’s TPQ, with all layers and artifacts above being of a more recent date. Beyond its use in inferring chronology, the TPQ also has practical applications in the field. Anyone who has dug a shovel test or a 1x1 knows all too well that soil layer transitions can be deceiving, which in turn can often confuse the true location of artifacts.  When peeling away the bottom of a soil layer that is mottled with the one below, for example, how is one to know accurately which artifacts go with which layer?  This is not an infrequent or even innocuous concern while excavating, but by thinking about the TPQ concept, the issue is easily resolved.  Hume’s dictum is that “when in doubt, put transitional artifacts in the soil layer above.”  The reasoning is simple: earlier artifacts mixed into later contexts will have no effect on that context’s TPQ date, but later artifacts mixed into earlier contexts will.  The latter situation can seriously compromise the chronology of a site.

The desire to get more archaeologists to implement the TPQ concept has resulted in a recent publication that is no doubt destined to be as useful as Hume’s Guide.  George Miller’s “Telling Time for Archaeologists” provides a comprehensive list of TPQ dates for a wide range of artifacts.  The list includes pretty much any artifact type one might expect to find on eighteenth and nineteenth century American sites, including such things as rim-fired cartridges (post 1866), Victorian Majolica (post 1851), or the steel springs used in wooden clothes pins (post 1887).  Bringing together TPQ dates for such a wide range of items represents a considerable achievement for Miller and his colleagues.  Even casual perusals of printed and online dating sources reveal the often-widespread use of inaccurate dates, a pattern that is reinforced by publications that do not cite the source of their artifact dates.  Miller’s list includes citations for every date given, thereby providing a standardized TPQ list that is still flexible enough for additions or changes.

Incorporating the use of TPQ dates to interpret chronology has no doubt been hampered by the lack of consistency in the dates used.  Miller’s article remedies this situation by compiling and condensing TPQ dates and ranges.  The list provides a powerful dating tool to the historical archaeologist, and it will no doubt make its way on to all those shelves, hopefully next to Hume’s Guide.

Note: for poster versions of Miller’s article, check out the website of the Council for Northeast Historical Archaeology.  

This article was originally published in the Spring 2005 edition of Fort George Advice: The Newsletter of the Lake George Battlefield Park (Fort George) Alliance

References:
Hume, Ivor Noel
  2001    A Guide to Artifacts of Colonial America.  University of Pennsylvania Press, Philadelphia.

Miller, George, Patricia Samford, Ellen Schlasko, and Andrew Madsen
  2000    Telling Time for Archaeologists.  Northeast Historical Archaeology 29:1-22.