The Late Glacial Human Reoccupation of North-Western Europe: New Approaches to Space-Time Modelling. (Research)
Blackwell, Paul G., Buck, Caitlin E., Antiquity
In recent years there has been much interest in trying to understand how and when the north western parts of Europe were reoccupied as the ice sheets retreated during the Late Glacial period. In this paper we reinvestigate the radiocarbon data available from this period using a statistical, model-based approach. The models we adopt are simple and are only the first step towards fully spatio-temporal interpretation; much of what we advocate can be tackled within existing software. Nonetheless, our models are flexible and generalisable and thus suitable for use on a wide range of archaeological projects, in many places and periods, in which dating of occupation, colonisation or settlement is of interest. In the context of Late Glacial Europe, for example, we are able to provide probabilistic answers to the question 'in what order were the regions of north-western Europe reoccupied?'. We are also able to make suggestions for refining and tailoring existing models for future research.
The radiocarbon determinations associated with human activity during the Late Glacial period were recently assembled by Housley et al. (1997), who scrutinised the nature and quality of the dating evidence and the stratigraphic and cultural contexts of the organic samples (ibid. 34-35). These authors made suggestions about how the landscape was reoccupied as the ice sheet retreated at the end of the Wurm/Weichsel glaciation and sought to address a number of questions. Among them were two major issues:
* what is the earliest date at which we have evidence for late glacial reoccupation in northwest Europe, and
* in what order were the various regions reoccupied?
In an attempt to address these issues, Housley et al. (1997) summarised the radiocarbon dates associated with each of eight regions in north-west Europe (the regions identified were Belgium, British Isles, Middle Rhine, Northern Germany, Paris Basin, Southern Germany, Thuringian Basin and Upper Rhine). They chose not to calibrate the determinations and related the dates of settlement in each region to summary histograms of all the uncalibrated radiocarbon ages. They reported the earliest non-zero cell in each histogram as the age of the onset of a 'pioneer phase' for that particular region and the cell with highest frequency as the age for a 'residential phase'. The age of the pioneer phase was seen as providing an estimate of the earliest date of reoccupation in a given region and the order in which reoccupation took place was estimated simply by chronologically ordering the regions using the earliest ages.
Blockley et al. (2000) addressed these issues in two ways. They repeated the analysis by Housley et al. (1997) with what they saw as a more appropriate allowance for error in the determinations, and showed that Housley et al.'s phases were ill-defined and uncertain in age. Blockley et al. also noted that radiocarbon determinations really need calibration before they can be seen as providing reliable estimates of the dates of events of interest. They used 'summed probability distributions' to draw together a number of calibrated radiocarbon date distributions and again concluded that 'population movement is difficult to argue and there is no clear evidence for a 'pioneer' and 'residential' phase' (ibid. 116).
The tools that Blockley et al. used to compute these 'summed probability distributions' are available in the OxCal radiocarbon calibration software, which offers the comment: 'Combining probability distributions by summing is usually difficult to justify statistically but it will generate a probability distribution which is a best estimate for the chronological distribution of the items dated.' (OxCal manual, Ramsey 2000). We agree with the first part of this statement but feel that the second part (from 'but') is unjustified.
In adopting Ramsey's terminology of a 'chronological distribution' Blockley et al. …