Offa’s Dyke is the largest, and least understood, field monument in Britain. Essentially it is a linear earthwork, consisting of a ditch and rampart, which roughly follows the modern Welsh/English border. Offa’s Dyke seems similar to, but larger than, other British linear dyke monuments. Generally it is constructed with the ditch on the Welsh-facing side, and it appears to have been carefully aligned to present an open view into Wales from along its length. But there are puzzling exceptional lengths where the ditch is east facing. As originally constructed, it must have been about 27 metres wide and 8 metres from the ditch bottom to the bank top. If it was constructed as a single development the builders must have had very considerable resources of labour and logistics.
Dating this type of linear earthwork is no simple matter and many may effectively be undatable. If excavation discovered artefacts on the original ground surface then this would at least give a date that preceded construction. Similarly finds at the bottom of the ditch should post-date the ditch-digging (although ubiquitous Roman pottery has an irritating habit of falling from the sides of any dug feature). Some form of datable component of the structure, such as timber lacing, might suitable for C14 or dendrochronology but such situations are rare. A landscape feature that is definitely datable (like a Roman Road) might ‘interact’ with the monument. Finally the name may be datable to some degree. For example ‘Wansdyke’ (Woden’s Dyke) must have been applied to the feature of that name by pagan Saxons. It is not likely to have been applied to a dyke later than 800 AD since after that date the Saxons were Christian. For Christians ‘Devil’s Ditch’ is a more likely name. Either way the name Offa’s Dyke would seem to rule out a late medieval or post-medieval origin although attempts to date chalk hill figures show that distinguishing the prehistoric from the post-medieval is not always as easy as would first appear.
The historical link with Offa depends on a single reference in Asser. There does seem to have been a boundary at this site in the Anglo-Saxon era, between the kingdoms of Mercia and Powys, for the dyke to indicate. Suggestions that it was constructed in alternative historical periods have been made but if Offa’s Dyke, as we now have it, incorporates earlier lengths of dyke then the dating problem is even more difficult. But in the Iron Age, for example, what political boundary would it be following? The Dyke does not look Roman to me nor does it follow a boundary of the Roman period but construction by the emperor Septimius Severus has been suggested recently on very slender evidence. Alternatively could it have been constructed by the Dark Age British against an Anglo-Saxon aggressor?
Offa was king of Mercia and reigned AD 757-796 AD. He is mentioned in the Anglo-Saxon Chronicle although in a rather perfunctory way considering his importance. Other evidence of his power are the legends on the coins he minted, a series of charters, and a letter of Alcuin describing him as an ‘equal’ to the Frankish king Charlemagne. His kingdom covered the area between the Trent/Mersey rivers in the North to the Thames Valley in the South, and from the Welsh border in the West to the Fens in the East. At the height of his power he also controlled Kent, East Anglia and Lindsay, and had alliances with Northumbria and Wessex which were sealed by the marriage of two of his daughters to their kings, Aethelred and Beorhtic.
Sir Cyril Fox (1955) believed the dyke was a long continuous monument with gaps left where local topography made movement impossible. Hill & Worthington (2003) considered that there were only the 64 miles of more or less unbroken earthwork in the central Marches (Shropshire, Powys, Wrexham and southern Flintshire). Yet there seem to be significant sections of the dyke in Radnorshire and south Shropshire where the surviving earthwork has an eastern ditch. According to Hill and Worthington, the naming of this section as ‘Offa’s Dyke’ is a recent phenomenon, yet an early 14th century reference to ‘Offediche’ in the area is documented.
The first question is how long is the monument? One could say that there is a short dyke and a long dyke hypothesis. Was the monument ever in fact intended to be continuous? The second question is what function did the dyke serve? An agreed boundary or a defensive structure seem most likely. Although some believe that the function of all dykes was primarily defensive, it is difficult to see what they could have defended against, except chariots and possibly cavalry. They would also prevent cattle rustling I suppose. There are no forts, turrets or other defensive structures on Offa’s Dyke although some sections might have once been topped by a palisade or wall. Surely it wouldn’t have impeded the progress of well organised infantry for very long? Offa’s Dyke is far longer that Hadrian’s Wall (116 km) and certainly testifies to the power of its builders. Being of Anglo-Saxon origin would make this dyke rather exceptional in Britain.
One solution to the conundrum might be to say that the post-Roman British reused the Iron Age technique of defensive ditch construction just as they reused IA hill forts. They probably didn’t learn the principle of linear defence from the Romans themselves since Roman military practice was not to conduct warfare from behind defensive structures. It is possible in turn that even the idea of Hadrian’s Wall was influenced by IA British examples, although the turf and stone construction techniques was Roman. In due course pre-existing short defensive ditches would then be incorporated by Offa into his dyke. The survival of a structure like this may indicate that, whatever its original function, for centuries it served a long term practical purpose. Suggestions have included a trading route for iron ore or salt, but I would have hoped for something more glamorous. I am an incurable romantic.