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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.
Using local pine trees, Douglass built a 450 year record of the tree ring variability.
First used, and likely invented by archaeologist Sir William Flinders-Petrie in 1899, seriation (or sequence dating) is based on the idea that artifacts change over time.
For example, JJA Worsaae used this law to prove the Three Age System.
For more information on stratigraphy and how it is used in archaeology, see the Stratigraphy glossary entry.
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. 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.
Not only that, it varies regionally, such that all trees within a specific species and region will show the same relative growth during wet years and dry years.