In the late 1920s, astronomer A.E. Douglass of Arizona discovered that trees' annual growth rings could be used to construct a window on the weather of the past. Trees add a layer of wood to their trunks every year - a wide ring during wet years and a thin one during dry years. By matching ring patterns in living trees to the patterns in old timbers, the record could be extended further back into history.
Tree-ring scientists (dendrochronologists) take samples from old, living trees in a geographic area and study the sample for patterns of tree-ring growth. By counting back from the outermost (most recent) ring, they can assign a specific year to each ring.
Scientists compare that tree-ring sequence to timber samples of unknown age from the same area. These might be old fence posts or logs used in dwellings. They look for overlaps in the patterns. By "cross-dating" old trees with still-older trees, scientists can "see" further and further back in time.
Peter Kuniholm directs the Aegean Dendrochronology project at Cornell University in Ithaca, N.Y. Dr. Kuniholm is trying to build a master tree-ring chronology for the Aegean and Near East from the present to 7,000 BC.
Kuniholm spends every summer in search of timbers to date. Using a tiny drill, he takes core samples (with permission) from old churches, houses, and archaeological sites.
"We do long miles, have hard days, and eat improbable food in even more improbable places," Kuniholm says. Now he is working on a chronology of the early and middle Bronze Age (2660 BC to about 1752 BC).
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