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Dendrochronology
Dendrochronology - also known more informally as Dendro or Tree Ring dating - is one of the most accurate methods
of absolute dating in archaeology. It is also possibly the easiest for the lay person to understand since it
depends on seasonal variations in the past producing recognisable patterns of tree growth which can be measured
in wood found in archaeological contexts.
Each growing season, trees produce a new layer of wood under the bark; this varies in width slightly depending on
the climatic conditions that year and this proportional width will be shared by all trees of the same species
within that climatic zone. Scientists can count and measure annual ring widths to a very accurate degree, these
measurements are analysed statistically to produce reference sequences. Any piece of wood from the present day
backwards will usually overlap its tree ring pattern with an older piece - e.g. a tree felled in autumn 1945 may
contain rings in its early (inner) pattern which will match to the outer (later) pattern of a tree felled in spring
1870. That tree may match wood from 1790 and so on. Many years of painstaking research have now compiled detailed
sequences for many parts of the world, in Britain covering the last 7,300 years.
Patterns of seasonal variation are mostly unique and recognisable, and can date wooden artefacts, furniture,
structural timbers in historic buildings, bridges and ships, or finds of wood from excavations. In well-preserved
samples, the date of felling can be refined to within a season of a particular year. Note however that the date of
felling is not the same as the date that the timber was used. It may have been stored for many years to 'season'
before being used in buildings, etc and then reused a second time before it finally ended up in archaeological
deposits.
The drawbacks to dendrochronology are straightforward: it requires a substantial piece of wood with no less than
fifty years' worth of rings, so that the pattern can be established beyond doubt. Some regions and periods (eg. the
British Iron Age) have poor sequences and are therefore more problematic for this technique. With the exception of
waterlogged sites, wood from excavations is very often not preserved well enough to use for dendro dating, although
it may still be used for radiocarbon. The role of dendro sequences in calibrating radiocarbon dates is the other
major contribution of this technique (see Radiocarbon).
Learn more:
- http://www.shef.ac.uk/uni/academic/A-C/ap/dendro/
Return to Dating Methods.
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