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Using local pine trees, Douglass built a 450 year record of the tree ring variability.
Clark Wissler, an anthropologist researching Native American groups in the Southwest, recognized the potential for such dating, and brought Douglass subfossil wood from puebloan ruins.
Stratigraphy is the oldest of the relative dating 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.
The method is still a standard for cemetery studies.
Absolute dating, the ability to attach a specific chronological date to an object or collection of objects, was a breakthrough for archaeologists.
The main drawback to dendrochronology is its reliance on the existence of relatively long-lived vegetation with annual growth rings.
Seriation is thought to be the first application of statistics in archaeology. The most famous seriation study was probably Deetz and Dethlefsen's study Death's Head, Cherub, Urn and Willow, on changing styles on gravestones in New England cemeteries.
It was now possible to assign a calendar date to archaeological sites in the American southwest for over 1000 years.
Determining calendar rates using dendrochronology is a matter of matching known patterns of light and dark rings to those recorded by Douglass and his successors.
Many of the first efforts of archaeology grew out of historical documents--for example, Schliemann looked for Homer's Troy, and Layard went after the Biblical Ninevah--and within the context of a particular site, an object clearly associated with the site and stamped with a date or other identifying clue was perfectly useful. Outside of the context of a single site or society, a coin's date is useless.
And, outside of certain periods in our past, there simply were no chronologically dated objects, or the necessary depth and detail of history that would assist in chronologically dating civilizations.In other words, artifacts found in the upper layers of a site will have been deposited more recently than those found in the lower layers.