There was in Mercia in recent times a certain king, who was dreaded by all the neighbouring kings and states. His name was Offa, and it was he who had the great dyke made from sea to sea between Britain [i.e. Wales] and Mercia.Vita Alfredi §14
There is no reason to doubt Asser’s word that Offa was responsible for the ditch-and-bank earthwork that bears his name – though there is, at the moment anyway (September 2022), insufficient scientific dating evidence to corroborate (or disprove) the statement. However, after some 1200 years of the activities of nature and, particularly, of people, it is only from Treuddyn (Flintshire) in the north to Rushock Hill (Herefordshire) in the south that a continuous earthwork is evident. (There is a gap of about five miles, north of Buttington, where the Severn apparently acts in lieu of the Dyke.) Indeed, in their Offa’s Dyke: History and Guide (2003), David Hill and Margaret Worthington, based on thirty years’ study of the Dyke, proposed that Asser’s assertion that it stretched “from sea to sea” should be considered to be “a literary device” (p.105), and that the sixty-four mile stretch from Treuddyn to Rushock Hill is in fact the full extent of Offa’s Dyke. They believed that other “intermittent and puzzling short lengths of bank and ditch” (p.107) between Rushock Hill and the Wye, to the west of Hereford, were nothing to do with Offa’s Dyke: “Although there are various short lengths of upstanding earthworks, when each was investigated from the air, with geophysical survey, with fieldwalking or with the spade, no continuous earthwork could be discovered and no connection could be made with Offa’s Dyke.” (p.143). Recent work, involving the study of lidar data, however, has indicated that the Dyke really did continue from Rushock Hill to the Wye.[*]
Hill and Worthington dismissed the notion that “supposed earthworks” running north-south along the lower Wye valley in Gloucestershire were remnants of Offa’s Dyke,[*] but survey and reconnaissance has demonstrated that there was an earthwork, exhibiting build-characteristics similar to the earthworks of Offa’s Dyke, starting at Sedbury Cliffs on the Severn estuary, shadowing the Wye northwards and then turning eastwards.[*] Moreover, there is a reference dating from 1321 to part of this earthwork (to the west of St Briavels) as Offedich.[*] There remains, however, a large gap between this apparent southern section of Offa’s Dyke and the point where the central section seemingly picks-up west of Hereford.
It has traditionally been imagined that the Dyke must have continued northwards from Treuddyn to join the coast around Prestatyn. Various lengths of earthwork in that general direction have been called Offa’s Dyke, but when investigated have proven not to be. Some of these candidates have turned out to be prehistoric features, some post-medieval, and some belong to another massive linear earthwork called Wat’s Dyke [Map]. Recent reconnaissance, however, has raised the possibility that there is a route to be traced from Treuddyn, reaching the coast to the east of Prestatyn.[*]
WAT’S DYKE (the reason for the name is not certainly known) is some 40 miles long. It begins, in the north, at Basingwerk, on the Dee estuary, and then runs in a southerly direction, to the east of, and virtually parallel with, Offa’s Dyke, ending below Oswestry. “It is very similar to Offa’s Dyke, but better made”, says David Hill in British Archaeology (Issue 56, December 2000). It has generally been supposed that Wat’s Dyke was an earlier Mercian earthwork than Offa’s Dyke – Æthelbald (716–757) being popularly proposed as the work’s instigator – however, following an excavation in 2006, at Gobowen (Shropshire), OSL dating of samples from the ditch has suggested that it was actually constructed in the early-9th century. If that is really the case, then Cenwulf, who became king following the very brief reign of Offa’s son, Ecgfrith, and who died at Basingwerk in 821, must be a strong contender for being the project’s ‘mastermind’.
Why these dykes were built and how they were operated are subjects of ongoing speculation.