Motus Mentis

One of the cleverer ways that archaeologists date the artifacts they find is a technique known as dendrochronology, which relies on the patterns of growth rings in the trunks of trees.

If you were to cut down a 100-year-old tree and examine its trunk, you’d see a ring for each year the tree had been alive, making it trivial to determine the tree’s exact age. Just as important, though, is the pattern of spacing between the rings themselves. In warm, wet years, the tree would grow rapidly, and the resulting rings for those years would be thicker than for the cooler, drier growing seasons. Because these yearly patterns of weather usually affect broad geographical regions, the same ring pattern will be found in any of the trees taken from the same area — a distinctive historical fingerprint.

So let’s say we now examine a tree that was also cut down at the age of 100 — but 80 years ago, when our first tree was still in its youth. The outer layers of rings, then, will correspond to the inner layers of the more recent tree, but it will be possible to map the pattern of one tree’s rings onto the other, after which we now have a record of ring patterns that goes back 180 years. In this way, by finding older and older trees of overlapping age, scientists have been able to construct detailed chronological mappings that go back as far as 10,000 years. This means that when a wooden artifact is found — a roof timber in an ancient dwelling, say, or a plank from a tabletop — archaeologists can often date it back to the exact year the tree was cut.

These rings record, in the very tissue of the tree itself, the big events and global patterns of their external world: patterns and events that affected not just themselves as individuals, but all the millions of trees around them as well, in vast areas spanning hundreds or even thousands of miles. It occurs to me that we too are marked in the same way by the salient events of our world. For example, people my age carry a mark, deep inside and covered by thick deposits of later growth, that records the shocking assassination of John F. Kennedy. Slightly further from the center we find the Moon landing of July 1969, and moving toward the surface we will pass the Nixon resignation, the horrors in Cambodia, the eruption of Mount St. Helens, the Challenger disaster, the fall of the Soviet Union, and so on. Our children are marked by some of the same events — the advent of the Internet, the attack of 9/11, but for them these rings will be closer to the heartwood than in my case, and they will in turn come to record events that I won’t live to see.