Sheffield Dendrochronology Lab - What is Dendrochronology?

Dendrochronology or tree ring dating is a method of dating which makes use of the annual nature of tree growth. Each year trees such as oak, put on a layer of new wood under the bark. The thickness of that layer - the tree-ring - depends on various factors, particularly climate. Conditions favourable to growth will result in a wide ring; unfavourable ones will produce a narrow one. Trees growing at the same time will show similar patterns of tree-rings. This can be seen best by measuring the widths of the tree-rings and plotting them as graphs. There will be some similarity between graphs from contemporary timbers.

In order to make use of the information contained in the tree-rings, long ring patterns, known as tree-ring chronologies, have to be constructed for use as reference data. These are produced by overlapping ring patterns from successively older timbers, starting with living trees, then buildings, and finally samples from archaeological sites and peat bogs. The Sheffield Laboratory now has a continuous master sequence for England going back to about 5000 BC. This is made up of numerous regional tree-ring chronologies, particularly in the medieval and post-medieval periods, for which the laboratory now has more than 200 reference chronologies from many areas.

To date a timber sample of unknown date, its rings are measured and its ring-pattern matched against the reference chronology rather like finger-printing. Each ring on the test sample can then be given a calendar date. the precision of the tree-ring date with relation to the felling of the tree will depend upon the completeness of the sampled timber. If bark edge is present the date of the outer ring will be the year the tree was felled. If the timber has been trimmed, the felling date will be less precise but it will be more accurate than any other dating method.

For further information the "ESF Handbook for Archaeologists: No2 Dendrochronology Dating" is recommended. This costs around £2 and is available by mail order from:
Oxbow Books
Park End Place
Oxford
OX1 1HN