Accurate dating is the holy grail of archaeology. It allows us to bring together information from different sources and sites to create a picture of society in the past. Archaeologists use dates of two kinds – inferred (or relative) and absolute (or scientific) dates.
Until the 1950s absolute dating was largely restricted to the Roman and later periods for which we had historical information, such as names and dates of rulers. Even then it was very patchy. Scientific advances have now transformed our ability to discover absolute dates from a variety of materials, though all present some problems. The two most widespread techniques – which are to some extent inter-dependent – are radiocarbon dating and tree-ring dating.
Tree ring dating (dendrochronology)
Throughout the last 10,000 years, wood was the main source of building material, tools and fuel. As a consequence it is one of the commonest materials on archaeological sites and occasionally it will survive as large fragments of timber and even as intact trees.
Using tree rings
In general, trees grow by adding one ring of new wood each year. The analysis of tree rings can reveal how the tree responded to the weather on a season by season basis and thereby offer a record of past climate but in archaeology the main focus is the calculation of the age of the tree when it was felled or died. This process, termed dendrochronology, works best on long-grown timber where a large span of rings survive (approx 120 years); it also works best on Oak although much work has also been done on Alder and Scots Pine. In ideal conditions it can provide an accurate date for the felling of the tree and by inference for the use of the wood within the use of the site. Several such pieces of wood from the same site then can be used to reconstruct the life span of site itself. Such sequences of dates are based on statistical analysis and are therefore inherently imprecise Inferences drawn from such analysis must therefore be treated with caution.
The analysis of tree rings from many oak timbers has produced long sequences of measurements of ring width. In many parts of Britain, Ireland and Europe. These sequences have allowed us to reconstruct the history of tree growth spanning many centuries. For instance, in Ireland there is now a history of Oak growth spanning 7000 years. In Scotland, Historic Scotland has supported the ongoing development of chronologies for Oak and Pine and though this has yet to match the Irish example that is certainly the ambition.
The pattern of tree growth over many years is closely related to local weather conditions. By comparing, for instance, timber used in the 16th
century in Stirling Castle, to sequences of ring rings from across Europe it has been possible to demonstrate that the timbers almost certainly came from tree felled in Poland in the late 1500s.