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.
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. Kuniholm spends every summer in search of timbers to date.