The two Roman walls – those of Hadrian and Antoninus Pius – are well known monuments that have been studied for several centuries. Consequently, they are understood in great detail and a vast amount of archaeological evidence has been assembled not just about the walls but about their supporting infrastructure and general context. If the earthwork we know as Offa’s Dyke is in fact of early third-century date, we would expect it to exhibit many, if not all, the features of these two walls, especially the Antonine Wall, which is an earthwork embankment. On the other hand, if Offa’s Dyke is Saxon, then it ought to display features consistent with other Saxon earthworks, such as Wansdyke.
The Antonine Wall consists of a turf rampart at least 3 m (10 feet) high and perhaps as much as 3.7 m (12 feet) high; this was laid on a stone base usually 4.3 m (14 feet) wide. In this respect, it resembled other linear frontier works (such as the German and North African limites). To the north of the wall, at a usual distance of 6.1 m (20 feet), lay a ditch 12.2 m (40 feet) wide and 4 m (13 feet) deep in the eastern sector and averaging 8.4 m (27˝ feet) wide and as little as 1.8 m (6 feet) deep in the western sector. On top of the wall stood a wooden palisade and walkway.
Immediately south of the wall ran a road about 5.5 m (18 feet) wide; this was an innovation, as Hadrian’s Wall was served by the existing Stanegate, some distance to the south. The wall was built in segments by detachments from the three legions serving in the province, who recorded their work on highly decorative distance slabs. Numerous temporary camps housing the troops involved in the building work have been located. Finally, some nineteen forts were placed at intervals along the wall, at an average distance of about 3.25 km (although this varies considerably). The forts themselves vary in size but contain the usual range of buildings (headquarters buildings, commandants’ residences, barracks, stores, granaries, stables and so on). With one exception (Cadder), the forts faced north, towards hostile territory. Fortlets and beacon platforms have also been recognised on the wall.
What we have in the Antonine Wall is a complex and integrated system. There is a great deal of archaeological evidence for its construction in the form of temporary camps and building inscriptions, then of its garrison. We do not have to rely on the fourth-century Historia Augusta to tell us that it was built under Antoninus Pius, as the distance slabs record the name of the emperor.
How does Offa’s Dyke compare? If, as Blake and Lloyd assert, it was actually built by the emperor Septimius Severus, who was in Britain from 208 until his death in 211, it ought to show many similarities to the Antonine Wall, built almost seventy years earlier. It ought also to show some innovations based on the experience of that wall.
The earthwork construction of the Dyke varies considerably along its length and, unlike the Hardrianic and Antonine frontiers, it is not continuous throughout its length, with many original gaps, including one of 96 km (60 miles) where the River Wye appears to have formed the frontier. A total of 130 km (81 miles) were built as an earthwork, standing at least 7.3 m (24 feet) high. At Llanfynydd, at the northernmost end of Offa’s Dyke, the ditch was found to be at least 4 m (13 feet) wide and 1.5 m (4.9 feet) deep; there was no gap between the ditch and the base of the bank. In places, the combined width of the bank and ditch is 20 m (66 feet). There is no evidence of a continuous palisade on top the Dyke, although there seems to have been a stone wall in places and timber fencing in others, while some sections of Wat’s Dyke appear to have had a timber frontage to the rampart (as at Sychdyn, near Mold). Both Dykes lack the infrastructure seen at the Antonine Wall: there is no military road, no garrison stationed in forts attached to the Dyke (although Cwrt Llechrhyd, a moated site at Llanelwedd in Powys had been claimed as an Offan fort), no temporary camps to house the builders, no building inscriptions. Moreover, the Antonine Wall is full of Roman artefacts recovered during excavations: Offa’s Dyke has only scraps of abraded Roman material culture within its structure. Blake and Lloyd talk about the “Roman artefacts... found within the Dyke” as if they date its construction. The concept of the terminus post quem – the archaeological principle that states that no deposit can be older than the date of the most recent object found within it – ought to tell us that the Dyke is of Roman or later date. This is entirely possible because of residuality – the idea that objects can turn up in deposits of much more recent date.
The Dyke is also completely unrelated to the pattern of early third-century military sites in the region. This includes legionary fortresses at Chester in the north (undergoing considerable refurbishment early in the third century) and Caerleon in the south as well as auxiliary forts, such as those at Leintwardine, Caersŵs and Forden Gaer. The road system shows no sign of being aware of the Dyke. Moreover, the only dating evidence from stratigraphy proves it to be later than second-century occupation at Ffridd; how much later cannot be determined on archaeological grounds alone.
The most devastating argument against regarding Offa’s Dyke as a Roman defensive work is that of context. What possible function could it have performed? To the east of the Dyke, the Midlands of England were a reasonably prosperous civil province of the Roman Empire. To its west lay further areas under civilian rule (notably in the south) as well as areas under military control (predominantly in mid and north Wales); it was every bit as much part of the province as the area to the east. Linear defensive works elsewhere in the Empire mark the boundary between civilised, Roman life and barbaricum, the uncivilised world outside, which might, at best, be home to a few outpost forts. Third-century Wales can in no way be thought of as anything other than part of Britannia.
There is, moreover, a Saxon context for the Dyke. Apart from Wat’s Dyke, which marks a slightly different boundary in the north and continues to the Dee Estuary, there are numerous post-Roman earthworks across England. The majority of them are to be found in eastern England and most are short structures lying across major routes. One possibly relevant earthwork, though, is the Wansdyke, an earthwork boundary south of the Thames, defending the area to its south. This seems to have been built in the fifth or sixth century to defend the British kingdoms of the southwest against attack from the Thames Valley, where Saxon kingdoms had been established. Unlike the Roman linear frontiers, these dykes were not provided with garrisons, but often appear to be more in the nature of boundaries imposed by a militarily dominant power. They were not located to defend the areas behind them but to act as a line of demarcation. The tradition of earthwork frontiers was a long one in Anglo-Saxon England and it provides a good context for the construction of Offa’s Dyke in the late eighth century as one of the last and certainly the greatest of these structures. Thousands of men were needed to build the Dyke, proof that the kingdom of Mercia was highly organised and under strong central control. The ninth-century history of Mercia, with the destruction of its bureaucracy and ecclesiastical structure by the Vikings means that its place in the history of Britain has often been undervalued; the regard its people as barbarians incapable of such works (as Blake and Lloyd do) not only ignores their long tradition of dyke building but also shows woeful ignorance of the political sophistication of Mercia. Offa regarded Charlemagne as an equal (even if Charlemagne did not reciprocate the compliment); this was more than self-flattery.