The outer year should correspond to the year the tree was cut or logged, and we count backwards from there. This gives us a starting point, but we never know the outer year of ring formation. We cross-date all samples using marker years. Beaty refers to a sheet of paper bearing a complex table of dates, dashes, and codes indicating the average ring-width for each year. Marker years are years that were either uncommonly wet or uncommonly dry.
Wet years yield wider rings, while dry ones are much narrower.
Those are marker years. He gestures toward the pile of stumps towering behind him. Almost all of these stumps will have those same narrow rings. After a tree has been cross-dated, it is easy to pinpoint when a particular fire occurred. Gazing through his microscope at the alternating light and dark bands, Beaty can determine, not only the year of scarring, but also the season.