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In 1901, Douglass began investigating tree ring growth as an indicator of solar cycles.Douglass believed that solar flares affected climate, and hence the amount of growth a tree might gain in a given year.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.Without those, the archaeologists were in the dark as to the age of various societies. The use of tree ring data to determine chronological dates, dendrochronology, was first developed in the American southwest by astronomer Andrew Ellicott Douglass.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.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.Dendrochronology has been extended in the American southwest to 322 BC, by adding increasingly older archaeological samples to the record.
The method is still a standard for cemetery studies.
For example, since each Roman emperor had his own face stamped on coins during his realm, and dates for emperor's realms are known from historical records, the date a coin was minted may be discerned by identifying the emperor depicted.
Many of the first efforts of archaeology grew out of historical documents--for example, Schliemann looked for Homer's Troy, and Layard went after the Biblical Ninevah--and within the context of a particular site, an object clearly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.
His research culminated in proving that tree ring width varies with annual rainfall.
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.