4D Archaeology

Article excerpt

Introduction

By far the commonest absolute date estimates come from radiocarbon ages converted to dates by comparing them with the ages of tree rings of known date. There are still many problems with the technique. The quoted errors attached to most of the dates obtained between 1950 and around 1982 have to be increased by factors between 1.4 and 4 (Baillie 1990; Ashmore et al. 2000). There are plateaux in the calibration curve which mean that some ages correspond to an unacceptably wide range of calendar dates. Many archaeological sites contain pieces of charcoal much older than the main period of activity on them. Many charcoal dates obtained before about 1999 were from bulk samples and some demonstrably reflect mixing of charcoal of very different age, providing a meaningless date somewhere in between (Ashmore 1999a). There is now fairly abundant evidence that dates from poorly preserved bone, whether buried or cremated, can be centuries out. The marine effect, which has been assumed to make all Scottish shell dates 405 years too old, may fluctuate (Harkness 1983; Cook & Dugmore pers. comm.). The bones of people who ate food from marine sources show the marine effect and calculation of the required change to an age measured by a laboratory depends on a measurement of the strength of the marine effect at the time the person lived (Barrett et al. 2000). Some dates from residues on pots seem to represent accurately the time they formed; others for unknown reasons do not.

There are only about 1670 radiocarbon ages from archaeological contexts and artefacts in Scotland between about 8500 cal BC and 0 and only about 1040 from between 0 and AD 1000, around which time historical dates become useful (TABLE 1). Some of those obtained as recently as the past few years are problematical in one way or another. The inadequate number of good dates becomes even more obvious when regionality is taken into account. Scotland has a diverse topography and climate and the capacity of its soils to maintain agriculture varies widely regionally as well as locally. Its cultures undoubtedly varied also (Barclay 2000b). Therefore a date for a type of structure or artefact in one area may not be a very useful guide to the date of a similar structure or artefact elsewhere.

On the other hand, there have been some encouraging basic developments over the past few years. The excellent OxCal programme has made manipulation of groups of dates fairly easy (Bronk Ramsey 2000). The 1998 calibration curve is of high quality back to nearly 12,000 years ago and useful back to nearly 14,500 years ago (Stuiver et al. 1998). The bio-apatite fraction from highly cremated bone appears to provide consistently credible ages for a range of Irish and Scottish samples (Lanting & Brindlay 1998). SURRC has started a programme of dating matched samples of marine and terrestrial material to see whether the marine effect did fluctuate, and if so, by how much (Gordon Cook pers. comm.) Historic Scotland now pays only for single entity archaeological dates (Ashmore 1999a). It prefers to obtain large numbers of dates with more than one from individual contexts, vastly improving understanding of what the dates really mean. Re-dating of classic sites has helped to resolve some long-standing arguments.

Some dating issues

In what follows I have space to illustrate only a few of the Scottish dating issues illuminated by recent dates. I shall avoid some of the exciting developments discussed elsewhere in this Special section.

One prominent issue of the past few years has been whether a settled way of life evolved before farming reached Scotland (Finlayson 1999). The numbers of dated hunter-gatherer sites per century south of Inverness does increase sharply around 4500, reaching a peak and then dropping off dramatically at about 4200 cal BC (Ashmore forthcoming). But the evidence from the only extensive and relatively well dated regional study, Scotland's First Settlers, in Skye and the adjacent islands and mainland, does not so far show the same pattern (Cressey et al. …