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Archeology for Interpreters > 5. How Do Archeologists Figure Out How Old Things Are?

Absolute dating

Dendrochronology (tree-ring dating)

The annual growth rings of long-lived trees, such as sequoias, bristlecone pines, and European oaks, whose wood was used for beams, posts, and other purposes can be used to date sites. 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. Originally used on southwestern pueblos, tree-ring dating uses sequences of growth rings to determine the date when the tree was first cut down. The use of this dating method has expanded to other regions and time periods. Historic houses may be dated through dendrochronology of wooden beams.

Tree-ring dating is also used to calibrate radiocarbon dates. Radiocarbon years do not correspond exactly to calendar years. Since wood can be dated by both radiocarbon and dendrochronology, scientists have created a calibration curve using the absolute accuracy of tree-ring dates to indicate the true calendar age of carbon-14 dates (McIntosh 1999:131).

Fun fact

Tree Ring Dating at Mesa Verde National Park
Dendrochronological research on archeological and living wood in the park holds the potential to more accurately date building construction phases and provide insights into climate changes and human adaptation to these changes.

Try it yourself

This web site describes dendrochronology and includes an opportunity to use the technique.

More Dendrochronology
Now try this WebRangers dendrochronology exercise!

Historical records

Historical records can be used to date the past only as far back as the beginnings of writing and written records, which first appeared in Southwest Asia about 5000 BC. Writing was developed much later in other parts of the world. Historical archeology, or text-aided archeology, studies that portion of human history that begins with the appearance of written records and continues until today. The royal library of Assyrian King Assurbanipal from Nineveh, the tomb of Egyptian pharaoh Tutankamun, Chinese emperor Xuang Ti's burial chamber, and Mayan stone temples each contain forms of written documents that aid in archeologists' understanding (Orser and Fagan 1995:4). A few examples of historical documents are diaries, wills, official records, books, photographs, and newspapers.

Case study

Using an 18th-century midwife's life as a case study, this web site shows how the past is pieced together from the fragments—including historical records—which have survived.

Try it yourself

History Toolkit
This web site is designed to help the beginning historian conduct and organize historical research. Essays marked with include forms to print and use for research.