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Many of the first efforts of archaeology grew out of historical documents--for example, Schliemann looked for Homer's Troy, and Layard went after the Biblical Ninevah--and within the context of a particular site, an object clearly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.
In 1929, they found a charred log near Show Low, Arizona, that connected the two patterns.
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.
Each tree then, contains a record of rainfall for the length of its life, expressed in density, trace element content, stable isotope composition, and intra-annual growth ring width.
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.