An approximate date for the construction of a prehistoric building can be determined by comparing the pattern of annual growth rings of trese used in the construction with the known pattern of tree growth in the area. Trees add new growth every year, and the thickness of this new growth in the trunk of a tree is determined by the amount of rainfall in the area that year. Since rainfall varies from year to year, a distinctive pattern of new growth rings develops over a period of years. This distinctive growth pattern is the basis for the dating method, fixing in time the calendar year in which any particular tree was cut.
The University of Arizona Tree Ring Laboratory is now on-line.