5.0 DATING AND SAMPLE MATERIAL
In 1977 Duncan James (1990) was able to collect wood charcoal from an excavated working, described earlier in this report (location 5), which gave a C14 date of 2940+/-80 BP (HAR-4845). At the time this was the first date to be obtained from a mine of Bronze Age origin in Britain, although comparable dates had been obtained from the Mt. Gabriel mines in Ireland by Jackson (1968). It was not until 1988-90 that further sample material for dating purposes could be collected. During this period, members of the Great Orme Exploration Society (GOML) and Gwynedd Archaeological Trust (GAT) were involved with an investigation of recently opened areas of mine workings centred around Vivians shaft, as part of a reclamation scheme at the site. Wood charcoal collected from surface contexts around Vivians shaft (table 1) and from underground at location 21 gave further C14 dates of prehistoric activity (figure 33). Continued activity at the mine site for a tourist development by Great Orme Mines Ltd. (GOML) from 1990 onwards, lead to the discovery of extensive surface and associated underground workings, exposed as a result of machine excavation of the opencast to the north of Vivians shaft. Many new areas of early working were also located underground at this time. Detailed excavations at surface and underground by GAT and GOML produced both wood charcoal and bone with sufficient collagen content for several other dates. A total of thirteen calibrated C14 dates (table 1, figure 33, p89), including one Atomic Mass Spectrometer (AMS) date (ref. OxA-5397), have now been obtained from the site and a further three are awaited.
In addition to C14 dating other methods of age determination have been considered and studied at the site. Such techniques may become useful in providing dates to compare with radiometric methods and also for investigating locations where there is no suitable sample material (charcoal/bone) for C14 dating. The technique most relevant to the site is ‘Uranium Series’ dating of the calcite flowstones or speleothems that cover the deposited spoil on the floors of some early workings. This means of dating is most commonly used for determining the age of caves, their associated calcite deposits (stalagmite, stalactite and flowstone) and the corresponding age of archaeological materials and artefacts underlying or encased in these deposits. A programme of sampling and dating of calcite flowstone was commenced at the mine site in 1992 by Liverpool University as part of a larger PhD study on cave deposits from various sites in Britain and Ireland (Helen King, forthcoming). A total of six samples were obtained from underground contexts that were considered to be of an early origin.
Other means of dating considered at the site include paleomagnetism and laser luminescence. These methods again rely on the age determination of calcite flowstones and have proved useful at other sites, particularly caves. Unfortunately, initial experiments on the flowstone from the Great Orme workings indicate that there are insufficient iron oxides for effective palaeomagnetic dating. Laser luminescence dating methods have not yet seen a great use in this country as most of the research has been completed in eastern Europe (Shopov et al 1990) and therefore at this stage it has been difficult to make any useful contacts in this field. Future research at the site may also be able to make use of other dating methods such as Thermoluminescence (TL) and Chlorine 36. However, at present there are no immediate plans to adopt these methods.
5.2 Sample Material and Radiocarbon Dating
In the total of thirteen dates, three have been obtained on bone collagen while the remainder were on wood charcoal. Generally charcoal has a wider distribution through the workings than bone and, for that reason, constitutes the majority of the dates. However, where possible it was preferable to use bone rather than charcoal, for the simple reason that anomalies could arise due to the actual age of the timber at the time of burning. For example, if charcoal had been derived from the centre of a large tree, of a few hundred years in age, then there would be a difference in the apparent C14 age to younger branch wood from the same tree. Also, if any timber had been stored for any period of time, or derived from peat bogs as has been suggested by Briggs (1990) at Cwmystwyth, then even greater age differences could be expected (Lewis 1994). This situation is, however, unlikely on the Great Orme as the limestone topography is not particularly conducive to the formation of any peat deposits.
Charcoal fragments are typically blocky shaped with an irregular distribution through the spoil deposits of the recognised early workings. Sometimes this charcoal occurs as small pieces no more than 5mm across disseminated through the accumulated spoil. On other occasions it occurs as larger blocks up to 40mm across which are large enough to observe small radius growth rings, implying that branch wood was utilised. Wide spacings of these rings indicates fairly rapid growth, suggesting the possibility of active coppice management for a timber supply (Jenkins&Lewis 1990). Identified species to date include oak (Quercus), ash (Fraxinus), hazel (Corylus), holly (Ilex), elm (Ulmus), alder (Alnus) and blackthorn (Prunus). It is now generally accepted that charcoal found amongst the early spoil is considered to be predominantly the result of firesetting, with a small amount possibly representative of lighting as discussed in section 4.3.
Observations suggest that the uneven distribution and often minor quantities of charcoal at the Great Orme could indicate that firesetting was not as prevalent as at other similar dated sites, particularly Cwmystwyth (Timberlake 1990) and Mount Gabriel (O'Brien 1990) where fragments are notably larger and more evenly dispersed through the spoil. It is is also contended (Lewis 1990 b) whether timber was the main fuel for fire-setting at the Great Orme, partly because charcoal may have been the more favourable fuel. Firesetting experiments conducted on the headland suggested that charcoal as a fuel for firesetting has certain advantages over timber for a number of reasons; it is useful in small confined workings or specific parts of a vein, provides concentrated heat with less fumes and is more convenient to use. The pronounced difference in weight between timber and charcoal may have also been an important factor dictating the use of these fuels at the Great Orme. It could be argued that because of the steep slopes that surround the headland, such a site may have favoured the more easily transported lighter charcoal rather than heavier timber. This may indeed have been the case during certain periods during the Bronze Age when local timber had been depleted as fuel for firesetting and for domestic use. However, at present there is no direct evidence to support this supposition.
Fortunately, at the Great Orme the neutral-alkaline groundwater conditions created by the calcareous host rocks have favoured the preservation of bone. These same conditions have also assisted with the impregnation of the bone surfaces by copper (0.9%) and iron (0.5%) to give the characteristic blue-green colour, and less commonly by manganese (1.6%) to give dark purple to black staining (Jenkins&Lewis 1990). The organic content of the bone has not been markedly affected by the mineral replacement, standing at about 15%, compared with fresh bone at 23%, comprising collagen at 95% and protein at 5%. Occasionally items of bone display no signs of mineral staining and are markedly softer with more indications of rotting than those that are coloured. This implies that mineral replacement of the bone surface has aided their preservation and, correspondingly their suitability for radiometric dating.
5.2.2 Results and Interpretation
BP (before present ) and calibrated BC (before Christ) radiocarbon dates from surface and underground contexts are presented in table 1 and figure 33. Calibrations were based on CALIB2 (Stuiver & Becker 1986). From these it can be seen that there is a continuous range between Nos. 1-12 for 1 sigma dates (68.2% confidence limits) from 1747 to 1015 calBC, with an obvious grouping for Nos. 8-12 (table 1, p89) from 1374 to 1015 calBC.
There appears to be a general trend, as would be expected, for the oldest dated workings to be at or near the surface (No 1, Table 1, p89 ), becoming progressively younger with increasing depth of mining activity (No 12 at location 5, & Nos. 8-9 at location/ vicinity 21, Table 1, & drawing 5, appendix B). However, this trend is not always obeyed and there are dates from surface contexts that are contemporary (No 10) or younger (No 13) than those from underground. It is possible these may be interpreted as spoil from an underground context that has been redeposited on surface, or may represent reworking of surface rock exposures and/or reprocessing of surface spoil at a later date. Overall, it appears that the surface opencast and the large underground stope (location 18) were being worked at the same time, with later activity concentrating on smaller more confined workings to the east, west and north of this area. Some caution must be placed on all the above interpretations, partly due to the lack of C14 dates from contexts outside those already sampled and also due to redeposition of both surface and underground derived spoils as part of the overall development of the mine through its period of early working.
At present only two comparable dates (Nos 1&2, Table 1, p89) from charcoal and bone from the same context are available. This lack of similar dates, could present problems of interpretation, as each material may be representative of a different mining technique with a corresponding relationship to the nature of the ore deposit. For example, firesetting is more likely to have been used on the harder ore-bearing rock formations, while bone, by virtue of its comparative weakness, would have been more suited to the removal of the softer rotted dolomite host rock. Therefore, bone and charcoal do not necessarily occur together in the same context underground, as has been observed in certain localities, although they do occur in surface and some underground spoils that are considered to be in a redeposited context.
5.3 Sample Material and Uranium Series Dating
The distribution and form of the calcite flowstones or speleothems have been described in section 4.2. Typically these deposits vary in thickness from a few millimetres to at least 0.3m. For effective Uranium series dating a sample of at least 50-75mm in length is required so that samples can be removed at a minimum from the upper, middle and lower parts of the formation. This is necessary to reduce any anomalies introduced from the effects of leaching due to ground water flow particularly in the lower section of the deposit and to a lesser extent in the upper portion. Such affects can reduce the proportions of isotopes available for sampling and can result in anomalous dates which need to be recognised and accounted for. Many of the samples from the Great Orme workings ranged between 70-100mm thick and therefore a number of sampling points were obtained, these varied between 4-17 depending on the thickness of the flowstone.
The results of the investigation (table 4) indicate that a proportion of the dated layers within each sample, fall between 4000-2500BP, which correlates with the Bronze Age period. Anomalous dates, seen to occur in the upper and lower portions of the flowstone are thought to be due to the effects of leaching as detailed above. Overall, the results indicate that Uranium series dating can be useful for determining the age of flowstone deposits in the early workings, although they do not appear to be as accurate as C14 dates on charcoal or bone. At one location it was possible to obtain sufficient charcoal from the base of a flowstone (location 7a) for a C14 date . Comparability between the two methods was not unreasonable, with charcoal giving 3150+/-50BP (OxA-5397) while the flowstone averaged between 3000-3500BP (sample 91EF, Table 4) .
