Neil J. Loader, Coralie Mills, Darren Davies and Danny McCarroll
The discovery of bridge remains in the River Teviot near Ancrum in 2018, by Ancrum and District Heritage Society (ADHS), has been described as ‘one of the most exciting and significant archaeological discoveries in Scotland in recent years’ (Historic Environment Scotland, 2020). The site is believed to have been an important, and at times possibly the only crossing point of the Teviot during the medieval period. As such, Ancrum Bridge, would have assumed strategic, ecclesiastical and political importance providing a direct link between the regionally important town of Jedburgh four miles south of Ancrum and the royal castles and abbeys situated to the north (ADHS, 2023). The bridge also lies close to the site of Mantle Walls at Ancrum, a medieval residence of the Bishops of Glasgow and the subject of community excavations.
The surviving bridge structure includes two submerged stone pier bases with multiple timbers preserved within them (Figure 1). The presence of timber preserved in-situ provided an opportunity for dating the structure.
During 2019 and 2020, seven oak (Quercus spp.) timber samples were collected for dendrochronological dating (Figure 2) and were analysed through the South East Scotland Oak Dendrochronology (SESOD) project (Mills 2023a). Three of the seven samples, AOB03, AOB05 and AOB06, retained bark edge, potentially allowing for an exact date for the year of felling to be determined. Some of the ring-width series could be cross-matched, demonstrating that the samples were broadly coeval (Mills 2020, 2023b; Wessex Archaeology 2020), but it was not possible to date these timbers securely by ring-width dendrochronology, owing to the relatively short sequences and the absence of local reference chronologies for native medieval oak in south-east Scotland (Mills 2023a).
In the absence of a dendrochronological date, radiocarbon dating was used to establish chronology for the bridge timbers. Radiocarbon wiggle-match dating of timber AOB03 returned a date for the last ring formed by the tree of 1340-1360 calAD (95.4% probability). This date raised a conundrum. Ancrum Bridge is located within 13 miles of the border with England, an area repeatedly and violently contested over the last c.800 years. The date indicated by the radiocarbon dating places the construction of the bridge during the reign of David II and the Second Scottish Wars of Independence (AD 1332-1357), a period of significant political turmoil in the region and around the time of the arrival of the Black Death in AD 1350. Arguably this would have been an unlikely and uncertain time to embark upon a significant construction project. The political allegiances and controls over the region changed frequently during this time such that even with the tightly modelled age range provided by radiocarbon dating it was not possible to determine if the bridge was Scottish, English, military or ecclesiastical in origin. These questions could only be addressed with further refinement to the dating.
At the same time as these initial investigations at Ancrum Bridge, a new precision dating technique was being developed (Loader et al 2019). Stable isotope dendrochronology shares many common elements with ring-width dendrochronology, but instead of measuring ring-width, stable isotope dendrochronology measures changes in the chemical composition of the wood, in this case oxygen isotopes, and compares these against an isotopic reference chronology. Trees do not need to be physiologically stressed to record a strong dating signal in their wood chemistry, meaning that the method works well on fast-grown invariant tree-ring series as can sometimes occur in Scotland. The moist temperate climate does not strongly limit oak growth and some samples, especially those with shorter sequence lengths, can prove difficult to date using ring-width alone. Furthermore, the common isotopic signal is more coherent over a larger geographic area than for tree-ring width meaning that dating may be possible without the need for local reference chronologies. This is important for tree-ring dating in regions such as south-east Scotland, where few native oak structures survive due to historic impacts on woodlands and on the built heritage making development of local reference chronologies for native oak challenging (Mills 2023a; Mills forthcoming).
The annual rings from three of the Ancrum Bridge timbers (AOB03, AOB04 and AOB06) were selected for isotope dating. The α-cellulose from the latewood of each annual ring was isolated and dried prior to high temperature pyrolysis to carbon monoxide gas at 1400°C and isotope ratio mass spectrometry (Figure 3a,b). Results are reported as per mille deviations relative to the VSMOW standard (Coplen 1995).
Using isotope dendrochronology, it was possible to position the three samples relative to one another and to date each timber through comparison against the central England isotopic reference chronology developed by Loader et al (2019) and a new independent isotopic record derived from native Scottish oak from Neidpath Castle (43.5 km west of Ancrum Bridge) (Mills 2023c). Samples AOB03 and AOB06, with bark edge preserved, were felled in the winter of AD 1428/29. AOB04 returns a date for the last measured ring and a heartwood/sapwood boundary date of AD 1397 meaning that it was very likely felled between AD 1407-1443. This date range is consistent with the felling date for AOB03 and AOB06 (Figure 4).
Stable isotope dendrochronology has conclusively shown that timbers with intact bark edge used to construct the sub-structure of the medieval bridge at Ancrum were felled during winter AD 1428/29. Given that wood was generally worked ‘in the green’ (unseasoned) it is highly likely that the bridge would have been constructed within 12-18 months after felling (Miles 2006). Significantly, this result shows that the bridge is approximately 80 years younger than the date indicated by wiggle match radiocarbon dating (95.4% range).
Whilst it may appear that the two dating methods are in conflict, it is important to emphasise here that the radiocarbon dating is not ‘wrong’. Radiocarbon dating reports a date range with a probability that expresses the likelihood that the actual date falls within a statistically-determined range. In this case the actual date simply falls outside of the 95.4% range quoted (effectively a 1 in 20 chance). It is important for those less familiar with scientific dating methods who use radiocarbon dating to consider this when interpreting dates, particularly for periods such as the Medieval where the radiocarbon calibration curve is complex. In the case of Ancrum Bridge, if the probability range is extended higher than 95.4% then the true date would be captured by the calibrated radiocarbon date range.
As a result of the isotopic dendrochronological evidence, we now know that rather than being constructed during a period of instability and conflict during the reign of David II, the timbers used to construct the medieval bridge at Ancrum were felled during the less turbulent reign of King James I. From this new insight it is now possible to redirect archaeological and archival research to focus on this later period and to allow a better understanding of the role that the bridge played in the cultural and political history of the region (see Mills forthcoming).
Acknowledgements
The text for this case study is drawn from Davies et al 2024 Journal of Archaeological Sciences: Reports (available here). This work was supported by a UKRI Frontiers grant EP/X025098/1, with additional support from UKRI grants EP/X525637/1 and AH/X00354X/1, Marsden Fund 22-UOA-184 and SSHRC (Canada) 895-2019-1015. We thank the QUERCUS project team for laboratory support and Christopher Bronk Ramsey for helpful discussions on wiggle-match radiocarbon dating. We are extremely grateful to Ancrum & District Heritage Society (ADHS) for their generous assistance throughout this process. The SESOD dendrochronology project was principally funded by Historic Environment Scotland and Forestry Land Scotland. HES funded the underwater survey and additional sampling of the bridge remains undertaken by Wessex Archaeology Coastal & Marine and also supported the radiocarbon wiggle-match dating. Sincere thanks also to the Society of Antiquaries of Scotland and the CARD radiocarbon fund for support to ADHS for the scientific dating work.
References
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