The Dark Ages really are dark ages and one of the anomalies we have with these dykes is there's nothing written about them. But what we have got is two linear earthworks:
Wat's Dyke, which runs about 40 miles from Basingwerk, Holywell, Flintshire, down to the River Severn at Maesbury, Shropshire, and then the other one that is very similar to it but much better known is Offa's Dyke, which runs from Prestatyn down to Chepstow, about 180 miles.
Both of them are similar in construction, they're earthworks that are a bank and a ditch, the ditch being on the Welsh side, the bank on the Mercian side. Mercia was the kingdom which is now the Midlands area. The dykes are assumed that they were boundaries delineating the edge of the Mercian territory.
One of the principle quesions is why these bank and ditch structures were put there. A boundary to a kingdom can be defined in many ways. You can put stones up as the Bronze Age people did for instance, lines of boundary stones.
They could have built hedges, they could have done all sorts of things, but to built a ditch 2-3m deep and a bank that's probably 4m high and about 20m wide is a fair excavation just to delineate a boundary.
So the first question that arises from it is what was the purpose of the dyke? Was it a defensive boundary rather like Hadrian's Wall? There's no evidence in the excavations so far that the dyke is a defensive structure.
There's no evidence of palisades or battlements or watchtowers where soldiers could be stationed. So as a defence, it wouldn't be impenetrable by any means.
The second question that arises is why have we got two dykes in very close proximity to each other.
Offa's Dyke and Wat's Dyke at Caergwrle are only about two miles apart. If you go down to Ruabon they're only about three quarters of a mile apart. If they are boundary demarcations why two within a very short distance of each other?
Another anomoly is that the early thinking was that Wat's Dyke pre-dated Offa's Dyke and in a way that would be fairly logical because Wat's Dyke runs down through Wrexham and Wrexham was a Saxon settlement so the logical sequence would be that Wat's Dyke was built first then along comes Offa, the successor to Aethelred, and decided that he wanted to shift the boundary further into Wales.
But recent excavations on the site of Wat's Dyke and dating of material from the dyke is putting it after Offa, only by a period of about 75 years or so which throws the whole thing into turmoil because if you had Offa's Dyke and they were retreating and built Wat's Dyke 75 or so years later, it seems very odd that they were so close together.
You'd have thought they would have actually defended that territory because Mercia was a very strong kingdom. These kingdoms waxed and waned but Mercia remained strong into the 800s then started to decline and it seems very odd at that particular time that they should retreat from Offa's Dyke - it's a real enigma. 
