Figure 4.16 and table 4.4 show that the average calculated archaeointensity values for four of the contexts cluster in two groupings corresponding to the two different assigned ages. This confirms the differences in relative ages between the 2nd phase of construction and the 3rd phase. The age assigned to the different features determined from archaeological evidence is early 2nd century AD or 3rd century AD. For the samples from the early 2nd century AD, the age probability of the samples is uniformly distributed over 50 years whilst the samples from the 3rd century AD have an age probability uniformly distributed over 100 years. As a consequence of the lack of variability in archaeointensities over this time period, comparison of these results with those published on the GEOMAGIA database (Korhonen et al., 2008, Donadini et al., 2006) for France does not more closely constrain the age of the features.
As full vector field information was measured for the praefurnium, it was possible to conduct archaeomagnetic dating for this context using Philippe Lanos’ ChronoModel 1.1 software. As seen in figure 4.17, the declination, inclination and intensity values measured for the praefurnium correspond to four time periods:88 AD ±80 at 16% probability. 399 AD ± 96 at 41% probability, 1058 ± 73 at 28% and 1523 ± 27 at 10% probability. Using the archaeological evidence, the second of these dates is preferred. It is difficult to get a precise date for this time period because there was a relative hiatus in the intensity in France between 100AD to 600AD (as can be seen in the secular variation curve plotted in figure 4.16) whilst over the same time period declination increased very slowly. This means there could potentially be relatively little difference between the field recorded when the praefurnium was first used and that recorded by its last usage. It is plotted in figure 4.16 at 399 AD ± 96 although it is noted that the intensity data was used in the Chronomodel software to date the praefurnium.
30 35 40 45 50 55 60 65 70 75 80 -100 0 100 200 300 400 500 600
In
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T)
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The Praefurnium The Hypocaust CAN3 CAN4 CAN5Figure 4.16
Intensity results from this study (coloured diamonds) plotted with all the French archaeointensity data in the Geomagia database (black circles) (Donadini et al., 2006, Korhonen et al., 2008) relocated to Site de la Molère. Horizontal error bars represent the beginning and end of the date assigned to the structure whilst the intensity value for each structure (displayed as a diamond) is plotted in the centre of the archaeological age
estimate. Also plotted is a secular variation curve (red line) for St-Jean-Poutge calculated by Philippe Lanos with the blue lines indicating the error envelope for this curve. As can be seen, all the intensity values determined here plot within this error envelope. Whilst the intensity calculated for CAN3 and CAN 5 is consistent with the modelled intensity over the majority of the time period plotted, the intensity values determined for the hypocaust and CAN4 are lower than the average intensity measured over between 0 and 500 BC. The intensity is, however, consistent with field strengths experienced in a certain time periods: between 0 and 150 AD, around the middle of the 3rd century AD or after the middle of the 4th century. Combined with the archaeological evidence, the most probable age for the hypocaust and CAN4 is 0 – 150 AD. The archaeological age for the hypocaust and CAN4 is early 2nd Century AD. The vertical error bars are 1 standard deviation plus or minus the mean of the average intensity calculated for the level. Note that as CAN3 and CAN5 gave identical results they plot on top of each other.
Figure 4.17. ChronoModel 1.1 prediction of the age of the praefurium based on the measured declination, inclination and intensity values. The predicated calendar dates are:48 AD ; 128 AD at 16%, 303 ; 494 AD at 41%, 985 AD; 1130 AD at 28% and 1496 AD ; 1549 AD at 10%. Based on archaeological evidence, the preferred archaeomagnetic date for the praefurnium is 399 AD ±96.
4.7. Conclusions
The results of this study cautiously confirm the archaeological interpretation that the features were constructed during two different construction events, separated in time. This is a difficult time period to study due to the lack of variability in the intensity of the field and so this study would have been strengthen by the addition of directional data. Where directional data was available, for the praefurnium, four potential calendar dates were proposed by the ChronoModel 1.1 dating software. Using archaeological information, the final heating of the praefurnium most likely occurred at 399 AD ± 96 (see figure 4.17). The site is thought to have been abandoned in the 5th century. The field vector data from the praefurnium does not contradict this conclusion.
It is acknowledged that whilst the intensity measured from features of the same age appear to group together (CAN4 and the Hypocaust for example) all five average intensity values are within the error envelopes of each other. The values determined for the early 2nd Century AD (56 ±7 and 58 ±8) appear to potentially confirm that the field was slightly lower at 100 AD before returning to former levels by 200 AD (see figure 4.16). If there was a decrease in field strength during this time period then the intensity plotted here reflects a period of increasing field strength, back to pre-100 AD values. Significantly more data is needed from 100 AD before this interpretation can be confirmed.
This study highlights the value to the archaeologist of collaborating with archaeomagnetists as archaeomagnetism can provide independent verification of dates/ interpretations and can determine if a sample has been subject to heating on more than one occasion. The value for the archaeomagnetist of working on archaeological sites is that it can either provide us with data for the construction of secular variation curves or it provides us with data with which to test geomagnetic field models e.g. Korte and Constable (2011). The difference in success rate between burnt clay samples and burnt sedimentary material is striking. This study serves to reinforce the superiority of clay as a recorder of