a newly felled tree, the outer ring represents the present. The past can be exactly reckoned by counting inwards. So absolute dates can be put upon ring pattern signatures in recent trees whose date of felling is recorded. By looking for overlaps—signature patterns near the core of a young tree that match the pattern in the outside layers of an older tree—we can put absolute dates on ring patterns in older trees too. By daisy-chaining the overlaps backwards, it is in principle possible to put absolute dates on very old wood indeed—in principle even from the Petrified Forest of Arizona, if only
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