Why Ancrum Bridge’s Age Surprised Historians

In 2018, the discovery of Ancrum Old Bridge beneath the River Teviot in Scotland was hailed as one of the most significant archaeological finds in recent Scottish history. This medieval bridge, once a vital crossing point in the Scottish Borders, offered historians a rare glimpse into the region's infrastructure and its socio-political landscape during the Middle Ages. However, what initially appeared to be a straightforward dating project soon took an unexpected turn. The application of two advanced dating techniques—radiocarbon dating and dendrochronology—produced conflicting results, forcing researchers to reassess their understanding of the bridge’s construction timeline. In this blog post, we’ll explore these two methods, the surprise findings, and the implications for interpreting historical structures.

Radiocarbon Dating: The Initial Findings

Radiocarbon dating, one of the most widely used techniques in archaeology, measures the decay of carbon-14 in organic materials to estimate age. When archaeologists began analyzing the timber samples from Ancrum Old Bridge, radiocarbon dating was applied to help establish when the bridge was built. In particular, wiggle match radiocarbon dating was used—a method that involves taking multiple carbon samples at intervals across the tree rings. This technique allows scientists to create a more precise model by correlating the samples with known patterns of carbon-14 fluctuations over time.

The results from the radiocarbon dating indicated that the timbers used in the bridge’s construction likely dated between 1340 and 1360 AD. This period aligned with a time of significant political instability in Scotland, during the reign of David II and the Second War of Scottish Independence (1332–1357). Given the volatile nature of this period—marked by raids, sieges, and battles—building a major structure like Ancrum Bridge seemed plausible, albeit puzzling. This dating placed the bridge's construction during a time when Scotland was engaged in a power struggle with England, with the Black Death looming on the horizon. For many historians, the idea of such an ambitious infrastructure project during a period of turmoil sparked questions about who would have commissioned it and why.

Dendrochronology: The Unexpected Twist

However, the story didn’t end with radiocarbon dating. Researchers knew that relying solely on one method, particularly one with inherent uncertainties, could result in a misleading timeline. To cross-check the findings, they turned to dendrochronology, a more precise dating technique that measures the growth rings of trees to determine their age.

Dendrochronology is often referred to as “tree-ring dating” and is highly accurate, as it provides an annual record of a tree’s growth. By analyzing the pattern of tree rings in timber samples and comparing them to reference chronologies (sets of known dated tree-ring sequences), dendrochronologists can pinpoint the year in which a tree was felled. In the case of Ancrum Old Bridge, the use of stable oxygen isotope dendrochronology—a technique that analyzes variations in the isotopic composition of the wood—enabled researchers to refine the timeline even further. The process is especially useful in temperate climates like Scotland's, where tree growth isn’t strongly limited by environmental stressors, and can be compared with isotopic reference chronologies from other regions.

To the surprise of the research team, dendrochronology produced a much later date than radiocarbon dating had suggested. The felling date for the timbers used in the bridge was identified as the winter of 1428/29 AD—approximately 80 years later than the radiocarbon results had indicated. This later date coincided with a period of relative stability under the reign of King James I of Scotland (1406–1437), a time more suited to large-scale construction projects. This new evidence radically changed the interpretation of Ancrum Old Bridge’s history and challenged the assumption that it was built during the politically chaotic mid-14th century.

Why the Dates Differed

The discrepancy between the radiocarbon and dendrochronological dates raised important questions about the reliability of dating methods and how results should be interpreted. So, why did these two techniques produce such different timelines for the same structure?

One key reason lies in the nature of radiocarbon dating itself. Radiocarbon dating provides a date range with a degree of uncertainty, which can be influenced by various factors. In the case of wiggle match radiocarbon dating, while it can offer a more precise estimate than standard radiocarbon dating, the result is still a probability model rather than an exact date. This means there is always a chance—about 1 in 20—that the true date falls outside of the most probable range. For Ancrum Old Bridge, the radiocarbon dating results provided a likely range of 1340 to 1360 AD but didn’t exclude the possibility that the actual date could be significantly later.

Moreover, the wiggle match radiocarbon dating curve for this period is multi-modal, meaning that the distribution of possible dates is not smooth or linear. When the calibration curve has multiple peaks, as it did in this case, the most probable date can be misleading, especially if only a narrow range of probabilities is considered. Expanding the probability range might have hinted at a later date, but this was not initially explored.

In contrast, dendrochronology, particularly with the use of oxygen isotopes, offers a higher degree of accuracy. Tree-ring analysis is tied directly to the biological growth of the tree, allowing for annual precision when there is a match with a reference chronology. The use of stable isotope dendrochronology in this case not only provided an exact felling date but also demonstrated that the wood used in Ancrum Old Bridge was fresh—likely felled and used within a year or two of construction.

Implications for Archaeology and History

The revelation that Ancrum Old Bridge was built in the early 15th century rather than the mid-14th century significantly alters its historical context. Rather than being a product of a time of war and instability, the bridge was constructed during a more peaceful era when Scotland was undergoing consolidation and rebuilding under James I. This shift has important implications for historians studying medieval Scotland. The bridge’s role as a strategic crossing point—linking important towns and royal sites in the Scottish Borders—can now be reconsidered in the context of post-war recovery and the strengthening of infrastructure.

For archaeologists, the case of Ancrum Old Bridge highlights the importance of using multiple dating methods to ensure the most accurate interpretation of historical sites. Radiocarbon dating remains a powerful tool, especially when dendrochronology is not an option, but it is crucial to understand its limitations and the uncertainties involved. The use of stable isotope dendrochronology, while more specialized, provides a level of precision that can clarify or challenge radiocarbon results, as it did in this case.

The discovery and subsequent dating of Ancrum Old Bridge serve as a reminder that the history of ancient structures is often more complex than it first appears. By combining different scientific techniques and reexamining the evidence with fresh perspectives, archaeologists and historians can uncover new stories about the past—stories that might otherwise remain hidden beneath the surface.

Citation: Davies, D., Mills, C. M., McCarroll, D., & Loader, N. J. (2024). The precise and accurate dating of medieval bridge remains at Ancrum, Scottish Borders, using stable isotope dendrochronology. Journal of Archaeological Science: Reports, 59, 104753. https://doi.org/10.1016/j.jasrep.2024.104753