In contrast, dendrochronology attempts to provide absolute dates for the rings present in individual timbers. This is achieved by measuring very precisely the widths of each successive ring within a sample and comparing the pattern of narrow and wide rings with reference chronologies built up by previous work. The technique can be successful and reliable only when a number of conditions are met. Firstly, there have to be contemporary chronologies of the relevant species, or genus, of timber from sufficiently nearby that some degree of cross-correlation is possible. For Britain and Ireland there is now a composite tree-ring chronology for oaks stretching back just over 7000 years. There are some periods and areas that are under-represented in this composite. However for the last 1500 years almost any building or archaeological structure should, all other things being suitable, manage to be reliably dated by the method. However, the timbers have to have a long enough sequence of tree-rings that they match in only one position to other chronologies. For oaks from Britain a widely accepted minimum for each sample is 50 annual rings. However, it is actually almost always necessary that at least some samples from a building contain significantly more than this minimum figure.

Analysis of many thousands of timbers across Britain has also revealed that there is a consistent number of samples for which no reliable date can ever be obtained, even when many more than the minimum number of rings are present. Usually, for any sample group, between a quarter and a half of all samples cannot be reliably dated, although at some sites virtually every timber dates and at a few sites none can be dated. There are several interesting observations that can be made about undatable material. It seems fast growing trees are more likely to be undatable and trees that have suffered extreme growth reductions at certain points always give problems. Since many of these influences are potentially anthropogenic in origin it follows that at times of extensive woodland exploitation more timbers are potentially unsuitable for analysis than when under-exploited woodlands are being used.

For more information please contact the secretary of the Pembridge Amenity Trust info@pembridgeamenitytrust.co.uk