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.
Tree ring growth reflects the rainfall conditions that prevailed during the years of the tree's life.
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.
When archaeologists have access to the historical records of civilizations that had calendars and counted and recorded the passage of years, the actual age of the archaeological material may be ascertained—provided there is some basis for correlating our modern calendar with the ancient calendar.
With the decipherment of the Egyptian hieroglyphics, Egyptologists had access to such an absolute timescale, and the age, in calender years, of the Egyptian dynasties could be established.
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.
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.