Dendrochronology relative dating method
Most famous was the attempt to estimate the duration of Pleistocene interglacial intervals through depths of soil development.
In the American Midwest, thicknesses of gumbotil and carbonate-leached zones were measured in the glacial deposits (tills) laid down during each of the four glacial stages.
Ages follow from the above equation provided there is accurate knowledge of a sample’s temperature history.
Even without such knowledge, hydration rims are useful for relative dating within a region of uniform climate.
Furthermore, artifacts reused repeatedly do not give ages corresponding to the culture layer in which they were found but instead to an earlier time, when they were fashioned.
Finally, there is the problem that layers may flake off beyond 40 micrometres (0.004 centimetre, or 0.002 inch) of thickness— Sediment in former or present water bodies, salt dissolved in the ocean, and fluorine in bones are three kinds of natural accumulations and possible time indicators.
Sometimes human observation can be maintained long enough to measure present rates of change, but it is not at all certain on a priori grounds whether such rates are representative of the past.
Consequently, the key to absolute dating of obsidian is to evaluate for different temperatures.Although no hydration layer appears on artifacts of the more common flint and chalcedony, obsidian is sufficiently widespread that the method has broad application.