By matching the tree rings on an archaeological sample to the master sequence of tree ring patterns, the absolute age of a sample is established.The best known dendrochronological sequences are those of the American Southwest, where wood is preserved by aridity, and Central Europe, where wood is often preserved by waterlogging.
In dendrochronology, the age of wood can be determined through the counting of the number of annual rings in its cross section.
Tree ring growth reflects the rainfall conditions that prevailed during the years of the tree's life.
The varved-clay method is applied with fair accuracy on deposits up to 12,000 years old.
Furthermore, Egyptian trade wares were used as a basis for establishing the age of the relative chronologies developed for adjoining regions, such as Palestine and Greece.
Thus, Sir Arthur Evans was able to establish an accurate absolute chronology for the ancient civilizations of Crete and Greece through the use of Egyptian trade objects that appeared in his excavations—a technique known as cross-dating.
Because rainfall patterns vary annually, any given set of tree ring patterns in a region will form a relatively distinct pattern, identifiable with a particular set of years.By comparing the pattern of tree rings in trees whose lifespans partially overlap, these patterns can be extended back in time.These techniques are based upon the measurement of radioactive processes (radiocarbon; potassium-argon, uranium-lead, thorium-lead, etc.; fission track; thermoluminescence; optically stimulated luminescence; and electron-spin resonance), chemical processes (amino-acid racemization and obsidian hydration), and the magnetic properties of igneous material, baked clay, and sedimentary deposits (paleomagnetism).Other techniques are occasionally useful, for example, historical or iconographic references to datable astronomical events such as solar eclipses (archaeoastronomy).Absolute dating can be achieved through the use of historical records and through the analysis of biological and geological patterns resulting from annual climatic variations, such as tree rings (dendrochronology) and varve analysis.After 1950, the physical sciences contributed a number of absolute dating techniques that had a revolutionary effect on archaeology and geology.