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.
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.
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.
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.
Stratigraphy is the oldest of the relative dating methods that archaeologists use to date things.