Archaeologists use lots of different techniques to determine the age of a particular artifact, site, or part of a site. The two categories of chronometric techniques that archaeologists use are called relative and absolute dating. Relative dating determines the relative age of artifacts or site, as compared to others, but does not produce precise dates. Absolute dating, methods that produce specific chronological dates for objects and occupations, was not available to archaeology until well into the 20th century.
Stratigraphy and the Law of Superposition
Stratigraphy is the oldest of the methods that archaeologists use to date things. Stratigraphy is based on the law of superposition--like a layer cake, the lowest layers must have been formed first.That is, artifacts found in the upper layers of a site will have been deposited more recently than those found in the lower layers. Cross-dating of sites, when one compares geologic strata at one site with another location, and extrapolates relative ages in that manner is still used today, primarily when sites are far too old for absolute dates to have much meaning.
The scholar most associated with the rules of stratigraphy (or law of superposition) is probably the geologist Charles Lyell. The basis for stratigraphy is quite intuitive, but its applications were no less than earth-shattering to archaeological theory. For example, Worsaae used this law to prove the Three Age system.
Seriation
Seriation, on the other hand, was a stroke of genius. First used, and probably invented by the archaeologist Sir William Flinders-Petrie in 1899, seriation (or sequence dating) is based on the idea that artifacts change over time. Like fins on the back end of a Cadillac, artifact styles and characteristics change over time, coming into fashion, then fading in popularity.Generally, seriation is manipulated graphically. The standard graphical result of seriation is a series of "battleship curves," which are horizontal bars representing percentages plotted on a vertical axis. Plotting several curves can allow the archaeologist to develop a relative chronology for an entire site or group of sites.
For detailed information about how seriation works, see Seriation: A Step by Step Description. Seriation is thought to be the first application of statistics in archaeology. It certainly wasn't the last.
The most famous seriation study was probably Deetz and Dethlefson's study on changing styles on gravestones in New England cemeteries. The method is still a standard for cemetery studies.
