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.
Since the turn of the century, several methods to measure elapsed time have been discovered.
The first and simplest method of absolute dating is using objects with dates inscribed on them, such as coins, or objects associated with historical events or documents.
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.
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.
Two broad categories of dating or chronometric techniques that archaeologists use are called relative and absolute dating.
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.
Plotting several curves can allow the archaeologist to develop a relative chronology for an entire site or group of sites.
For detailed information about how seriation works, see Seriation: A Step by Step Description.