Dendrochronology: how it works

by Lisa Cardy

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Sea Henge, a 4,000-year-old mysterious circle of oak stumps found off the north Norfolk coast, has�caused quite a stir when it was uncovered three years ago. But how exactly do they date a piece�of history such of this

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��Press Association

Sea Henge

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Dendrochronology or tree-ring dating has been available as a technique for almost a hundred years. The method works because each year trees put on a layer of new wood under the bark. The thickness of that layer - the tree-ring ' is dependent on various factors, particularly climate, and distinct patterns are formed. During a warm year a wide ring will grow and during a cold year narrower rings are formed.

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By studying lots of trees long-ring patterns, known as tree-ring chronologies, have been constructed for use as reference data. These are produced by overlapping ring patterns from successively older timbers, starting with living trees. A kind of a linear jigsaw puzzle is created, and as long as the ancient piece of tree you're trying to date fits somewhere along the scale you can say when that tree was felled.

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For example, if we lined up the rings of a tree which, unknown to us, started life in 1500 AD and died in1700AD, with a tree which was known to be felled in 2000 AD and had 400 tree rings, meaning it started life in 1600 AD, the tree ring patterns of the two will overlap for the last hundred of the dead tree and first one hundred of the recently felled tree.

We could then date the dead tree and use it to extend the chronology back to 1500 AD and so on.

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Matching tree ring patterns, known as cross dating, allows archaeologists to give precise calendar dates for ancient structures and sometimes small samples found at sites.

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So, when Seahenge was discovered scientists just had to match the timbers with established chronologies and so were able to give it a construction date.

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Now you know how scientists dated Seahenge why not find out more about what's happening to it, click here to read a previous article on the subject in The AnswerBank's history section.