The remains of Offa's Dyke near Clun, in Shropshire.
Asser, the Welsh biographer, and contemporary, of Alfred the Great, states:
“There was in Mercia in recent times a certain valiant 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 Wales and Mercia.”
‘Vita Alfredi’ §14
There is no reason to doubt Asser's word that Offa was indeed responsible for the ditch-and-bank earthwork that bears his name. The claim that it stretched “from sea to sea”, however, seems to be a case of poetic license.
In the north, the Dyke ends in the vicinity of the village of Treuddyn, Flintshire – the exact terminus has not been located. Archaeological investigations have demonstrated that it did not continue to the coast around Prestatyn, as was once thought.*
The Dyke runs south from Treuddyn to Rushock Hill, Herefordshire. Its presence has not yet been detected for a length of about five miles along the Severn, north of Buttington.
To the south of Rushock Hill, occasional short lengths of earthwork continue the line a distance of about twelve miles, to the Wye. There then follows a very large gap – some thirty-seven miles – until a final section of earthwork shadows the lower ten miles or so of the Wye, in Gloucestershire, concluding at Sedbury Cliffs on the Severn estuary. Cyril Fox, who investigated the Dyke in the late-1920s, reckoned it must have been the case that, in its southern stretch: “the frontier consisted of dense forest and a large river along which an artificial line was necessary in certain places only”.*
In their ‘Offa's Dyke: History and Guide’ (2003), David Hill and Margaret Worthington, based on thirty years' study of the Dyke, controversially proposed that the sixty-four mile stretch from Treuddyn to Rushock Hill is in fact the full extent of Offa's Dyke – the more southerly sections of earthwork are nothing to do with Offa's Dyke – and that it was: “a defensible earthwork rather than a defended one” (no evidence of forts, towers or gateways have been found), built as: “a significant defence for Mercia against pressure from a virile Powys at the opening of the reign of Offa.” (Chapter 4).  More specifically: “we see that the strength of Powys under Eliseg at the beginning of Offa's reign in Mercia gives a reason for why the Dyke was built.” (Chapter 5).  Hill and Worthington envisaged teams of Mercian troops patrolling the Dyke (which could have been topped by a wall or palisade – no evidence has been found), to provide early warning of Powysian transgression.
However, Ian Bapty, who was at the time Offa's Dyke Archaeological Management Officer for Clwyd-Powys Archaeological Trust, wrote a scathing review of Hill and Worthington's book, talking of: “the overstretched presentation of ideas and interpretations as proven facts”.  For instance: “Offa's Dyke ‘proper’ is presented by Hill and Worthington as a uniformly designed structure always having a western ditch, and is unerringly shown as such on the accompanying maps. Yet there are significant sections of the dyke in Radnorshire and south Shropshire where the surviving earthwork clearly appears to have an eastern ditch. Whatever explanatory significance may or may not be attached to such patterns, it is mystifying that this sort of basic detail is entirely overlooked. This is especially so when the existing Offa's Dyke explanations Hill and Worthington dismiss – such as Fox's negotiated frontier hypothesis – stemmed from observation of precisely this kind of localised variation in the form of the earthwork.”*  And: “Take the case of what, for Hill and Worthington, is the ‘rogue’ section of Offa's Dyke in Gloucestershire ... 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 well documented (VCH Gloucestershire Vol 5, pg 249). More fundamentally, their damning characterisation of this length as no more than slight and intermittent “supposed earthworks” with “little similarity to the form or siting of Offa's Dyke in the undisputed central area” [Chapter 6] bears scant relation to the often massive, mostly continuous and generally impressive earthwork which anyone who visits Offa's Dyke in Gloucestershire will actually find on the ground.”
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, the northern section of 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 defensive earthwork (the ditch is on the western side) 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’.
Asser ‘Vita Alfredi’ translated by Albert S. Cook
Cyril (later, Sir Cyril) Fox carried out fieldwork along Offa's Dyke in the summers of 1925 to 1930. His findings and opinions were published in 1955, under the title ‘Offa's Dyke: a Field Survey of the Western Frontier Works of Mercia in the Seventh and Eighth Centuries AD”. Sir Cyril thought that Offa's frontier did stretch from sea to sea (from Prestatyn in the north, to the Severn estuary at Sedbury Cliffs).
Founded in 1899 and originally dedicated to Queen Victoria, the VCH (Victoria County History) is an encyclopaedic record of the traditional counties of England. Based at the Institute of Historical Research in the University of London since 1933.
Hill and Worthington write (Chapter 5): “We now come to the difficult question of what it was for. Consider what we have left – a bank of varying sizes and a ditch that can be quite large but is often silted or ploughed over. Much of the evidence for purpose would come from the upper layers of the bank, but this has been eroded away by agricultural use and by the wear and tear of the centuries. However, we do have some pointers:
The Dyke dominates and overlooks the land to the west, Wales.
The ditch and bank always have their steeper face against Wales.
For many years it was believed that the purpose of the Dyke was to provide an agreed frontier between two different peoples, the Welsh and the Mercians. This is almost certainly disproved by the very shape of the surviving monument that stands so clearly against the Welsh. Where we have known agreed frontiers from various areas and periods of time ... they are symmetrical and seem to be built by both groups with a ditch on both sides and the earth thrown up in the middle.”
Optically Stimulated Luminescence dating: a technique in which the length of time that has elapsed since a mineral was exposed to daylight is determined.
The line between Treuddyn and Prestatyn is twenty-two miles long. Some fourteen, dyke-free, miles distant from Treuddyn, extending along the next five miles or so of that line, three distinct sections of linear earthwork are to be found. The middle section, about a mile long and containing the largest and best preserved of the earthworks, is of very different construction to Offa's Dyke – comprising two ditches, one either side of a relatively low bank – and is now known as the Whitford Dyke. It was probably an estate boundary marker. The two adjacent sections may be part of the same scheme, but this has yet to be established with any degree of certainty. Whatever the case, it has, since the 1980s, become generally accepted that these earthworks – that were called Offa's Dyke by the Royal Commission on the Ancient and Historical Monuments of Wales in 1912 – are not part of Offa's Dyke.