A POST-ROMAN LOCAL KING or powerful warlord, rather than an eighth-century Anglo-Saxon ruler of Mercia, now seems likely to have been the mastermind behind the construction of Wat's Dyke, the frontier earthwork that snakes through the northern section of the Welsh borderlands.
This new assessment of the historical context of the ancient earthwork follows radiocarbon dating of remains from an archaeological investigation of a site at Oswestry, Shropshire.
It means that the deep ditched boundary rampart on the Welsh border was constructed up to 300 years earlier than its previously accepted date. Hitherto the general consensus has been that it was an eighth-century near-contemporary of its more famous parallel boundary, Offa's Dyke.
According to one local historian, this latest finding has `put the cat amongst the archaeological pigeons'. The new evidence has emerged from the line of the ancient earthwork on an Oswestry industrial estate, Maes-y-Clawdd, south of the town.
A small hearth or fire site was uncovered on the old ground surface beneath the raised bank. Some heavily worn sherds of Romano-British pottery were also retrieved, though these may have been residual deposits within the soil on which the fire had burnt.
Dating analysis at Queen's University, Belfast, of charcoal and burnt clay samples centres puts the dyke's construction at around AD 446. This is an important finding with broader implications, because datable evidence from the dykes is almost non-existent. Moreover, the surviving records for the period are scanty and often garbled.
A Shropshire County Council Archaeological Service report on the excavations by H.R. Hannaford concludes that the most likely date indicates that the people who constructed Wat's Dyke were not the Mercians of Offa (r. 757-96) or subjects of his predecessor, Aethelbald (r. 716-757).
The new information places the construction of the dyke within the shadowy period that began with the formal withdrawal of the Roman administration (AD 410) and ended with the absorption of the area into Mercia.
The report concludes: `The dyke should therefore be regarded as being contemporary with that other great fifth-century linear earthwork, the Wiltshire Wansdyke, rather than Offa's Dyke, and should be considered as an achievement of the post-Roman kingdom of the northern Cornovii, rather than the work of seventh- or eighth-century Mercia.'
The latest date for the construction of Wat's Dyke indicated from the site, adds the report, would also coincide with the earliest date for the annexation by the Mercian king Penda (r. 630-655). However, the radiocarbon date suggested that a fifth-century construction for the dyke `is more likely'.
The relationship between the two borderland earthworks has long been a puzzle. In some areas they stand no more than 500 yards from each other, while in others they lie up to three miles apart. However, in both cases the deep western mountain-facing ditches indicate their underlying purpose.
Wat's Dyke, following a more easterly alignment across mostly low-lying agricultural land, is considered to be the better built and better sited of the two. But both represented military or political demarcation lines of some sort, though a measure of cross-border compromise or agreement must have been at work at the time of their construction.
With breaks for natural defensive features such as ravines and river gorges, Wat's Dyke runs northwards for almost forty miles, from the River Morda at Maesbrook, south of Oswestry. It ends at Basingwerk, on the Dee estuary in Flintshire, thus protecting the western estuary approaches to the former Roman legion city of Deva (Chester).
It does not run southwards beyond the Severn, so it was clearly built in response to military and political conditions specific to the northern Marches. This is where the suggested connection with the post-Roman `kingdom' of the Cornovii tribe comes in. …