How Do Archeologists Date Sites and Artifacts?

Archeologists use relative and absolute dating methods. They use these methods to determine the age of sites, artifacts, and the behaviors they represent.

• Relative dating arranges past events in a sequence in relation to one another (think: earlier than, later than, more recent than, and so forth).
• Absolute dating attempts to pinpoint a discrete, known interval in time such as a day, year, century, or millennia. Very few artifacts recovered from an archeological site can be absolutely dated.

Method	Type	Description	Example
Cross-dating	Relative	Compares evidence found in one place to other places to determine relative age. The evidence might be within a site (even within stratum) or at different sites.	Dendrogeomorphical Analysis of Earthwork Stability at Poverty Point
Dendrochronology	Absolute	Study of tree ring dating. Seasonal conditions affect annual tree growth, causing all trees of the same species within a given geographical region to have the same tree-ring pattern. Cross sections of cut or dead trees from a single region are compared and the tree-ring patterns are matched.	Tree Ring Dating at Aztec Ruins National Monument
Geologic Dating	Relative	Assigns dates using geological stages related to documented climate and geology events.	Keonehelelei - The Story of the Footprints Area
Historical records	Absolute	Uses of precise dates on (or described in) government records, diaries, newspapers, or other historical records.	Historical Archeology at Yellowstone National Park
Horizon	Relative	A pattern characterized by widespread distribution of a complex of cultural traits that lasts a relatively short time.	Hidatsa Flintknappers, Potters, and Smallpox
Mean ceramic dating	Absolute	Establishes the average age of a ceramic assemblage due to known manufacturing dates and the frequency of ceramic types in the assemblage.	Ceramic Analysis at Fort Vancouver National Historic Site
Obsidian hydration	Absolute	Measures the microscopic amount of water absorbed on freshly broken surfaces. The longer the surface is exposed, the thicker the hydration band will be	Wildfire and Archeology in the Jemez Mountains
Radiocarbon dating	Absolute	Measures the decay of carbon. Carbon decays at a steady, predictable rate. It releases carbon-14 atoms with a half-life of 5,700 years. Scientists measure the carbon-14 to estimate age of organic objects, such as charcoal, shell, wood, bone, or hair.	Compliance Inspires Science on Santa Cruz Island
Seriation	Relative	Orders evidence of human behavior into a series.	Examining Artifacts on the Landscape
Stratigraphy	Relative	Layers of earth and other materials built over time. The Law of Superposition holds that, when undisturbed, deeper layers are older than the ones above them.	Stratified Prehistoric Archeological Sites in Chesapeake & Ohio NHP
Terminus post quem (TPQ)	Relative	The date after which a stratum, feature, or artifact must have been deposited. When several artifacts are recovered from a single stratum, the TPQ date corresponds with the first possible date that the latest-occurring artifact could have made its way into the ground.	Using 3D Replicas to Study Spanish Coins from La Galga and Juno Shipwrecks
Thermoluminescence	Absolute	Used for rocks, minerals, ceramics and burned features, TL is based on the fact that almost all natural minerals emit light when heated. Energy absorbed from ionizing radiation frees electrons to move through the crystal lattice and some are trapped at imperfections. In the lab, samples are heated releasing the trapped electrons and producing light.	Wildfire and Archeology in the Jemez Mountains