Relative dating considers how old artifacts and sites are, in comparison to other artifacts and sites. Stratigraphy and style are both used for relative dating.
Although relative dating can tell us what is older or younger, it doesn't tell us exactly how old something is. For many years archaeologists had no way of determining the real age of sites, and had no good idea of the time depth involved.
The main principle behind stratigraphy is that of superposition. This says that older things are found below younger things. When archaeologists excavate sites, we find layers of soil, each marking a period of use of the site. Artifacts in the upper layers were laid down after those in lower levels. Stratigraphy is the record of these different layers or strata. By excavating sites and separating the artifacts from each layer, it is possible to see changes through time.
Styles and Diagnostic Artifacts
The style of many artifacts changes through time, even though the function remains the same. We can see this today as styles of cars or clothing change regularly. If you have a photograph of a person, and know when their style of clothing was popular, you can tell when the photograph was taken.
For archaeologists, the changing styles of pottery and projectile points provide the best known sequences. Different styles have been found in different layers of sites, so based on stratigraphy, we can tell the order in which the styles were popular. Once we know where one style belongs in time, any time we find an artifact of that style it dates the site where it is found. Several sequences of pottery from Wisconsin are described in the section on ceramic analysis.
Archaeologists have two main ways to tell the age of sites and artifacts. Absolute dating provides a specific calendar year for the occupation of a site. Relative dating (discussed elsewhere) tells how old something is in relation to other objects, but cannot provide a year or specific date of use.
Several new methods of absolute dating have been developed since the 1950's that allow us to calculate the calendar ages of artifacts. The most important for American archaeology is radiocarbon dating. Radiocarbon dating can be used back to about 50,000 years.
For sites older than that, in Europe or Africa for example, methods such as potassium-argon dating are available, that measure the amount of various radioactive elements in volcanic or other deposits.
Radiocarbon dating is critical to archaeologists. It works on the principle that there are two different isotopes or forms of carbon. Carbon 14 is produced in the atmosphere and is absorbed by all living things. When a plant or animal dies, the carbon 14 begins to break down at a known rate. This half life is 5370 years. Radio-carbon laboratories can measure the amount of carbon 14 remaining in organic materials and calculate how long it has been since death.
Archaeologists can date charred plant remains, animal bones and shells. We cannot directly date stone tools and pottery, because they are not organic. But we can date the organic materials found associated with the stone tools or pottery, and thus get dates for the use of each different type of pottery and point.
Radiocarbon labs and carbon samples
Until recently, radiocarbon laboratories needed about a hand full of charcoal or bone to measure the remaining carbon-14 and provide a date. Recent advances allow dating of very small samples through Accelerator Mass Spectrometry or AMS dating. This technique counts the actual carbon-14 atoms remaining in an organic sample. Now something the size of a single kernel of corn can be dated.
There are only about a dozen labs in the country that run radiocarbon samples. Carbon 14 and AMS dates cost several hundred dollars each, but are essential for understanding culture change through time.