What is the X in X-mas?
The study of growth rings in trees for the purpose of analyzing past climate conditions or determining the dates of past events. Because trees grow more slowly in periods of drought or other environmental stress than they do under more favorable conditions, the size of the rings they produce varies. Analyzing the pattern of a tree's rings provides information about the environmental changes that took place during the period in which it was growing. Matching the pattern in trees whose age is known to the pattern in wood found at an archaeological site can establish the age at which the wood was cut and thus the approximate date of the site. By comparing living trees with old lumber and finding overlapping ring patterns, scientists have established chronological records for some species that go back as far as 9,000 years.