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'We are either killed or drowned': the 'great flood' of post-Roman Britain

Recent historical research has proven that there was, until about the 16th century, two lost islands west of the Cardigan Bay shore, near Aberystwyth, the remnants of a lost low-lying coastal landscape that was recorded by Ptolemy's observations two thousand years ago. This landscape would have been substantial, enough to support major towns, farms, ports and coastal wildlife. This land was immortalised in medieval legend as Cantre'r Gwaelod, ruled by the morose king Gwynddo and decieved by the wicked prince Seithenn. As this legend dates from the 12th century - and recounts local tradition - the submergence would have taken place long before then.



The drowning of Cantr'er Gwaelod, which possibly inspired the drowning of Númenor and the northernly region of Bereliand in Tolkein's Silmarillion. Image credit: Alan Lee.

One clue we have towards this great inundation is found with Gildas, a monastic scholar probably writing from a secure abode either in Durnovaria (Dorchester) or somewhere in western Devon. In his excoriation of the Britons, he mentions a mid-5th century plea to the magister militum of Gaul, Flavius Aetius, for monetary aid and military support. The primary reason for this was the Saxon revolt engulfing the east of the island, but also a secondary disaster: a flood or deluge of some sort in the west that 'drives us to the barbarians'. This had long remained a mystery, and most scholars believed it was an exaggeration of Gildas, who liked to draw on Biblical events such as the Plagues of Egypt or, in this case, the Flood. Now, it seems more likely that he is referring to an actual flooding event affecting the entire west coast of Britain, displacing hundreds of coastal villas and towns.




Submerged forest off Cardigan Bay, Wales. Image credit: Doug Evans.

The name of the wicked prince, who in the Black Book of Carmarthen destroys the 'sea walls' holding Cantre'r Gwaelod from the ocean, supports an inundation around the mid-5th century, as the coastal settlement of Portus Setantiorum (Port of the Setantii), in Lancarshire is thought to have been lost to the sea around this time. It is possible that until their end the Setantii held a prestigious position as a skilled maritime community, as their name is also found as the original name of the Irish culture hero Cú Chulainn, son of the god Lugh. The wording of the plea to Aetius - implying that a considerable number of people were drowned or forcibly displaced by the flood - strongly suggests that the coastal region inundated was densely populated, and was subject to political authority.


Iron objects recovered from the drowned land around Portus Setantiorum, in Lancashire.  Image credit: https://www.minelab.com/community/success-stories/search-for-the-lost-port-of-portus-setantiorum

While there are few pieces of oceanographic evidence to pinpoint the cause of the flood, it was probably either due to a collapse of a marine shelf south of Ireland, or - considering its northernly impact - due to a dramatic ice sheet fragmentation in the Arctic Circle. That does not explain why the flooding was permanent, although something similar did happen five hundred years earlier in southern Denmark - with a huge flood-tide causing a permanent rise in sea levels that forced the resident Cimbri and Teutones tribes out. Within the period of the Roman Empire itself, a massive tsunami-storm event drenched southwestern Spain in the 230s CE, permanently altering the coastline and making the province untaxable for decades afterwards. 

What might have happened in 440s Britain was that a colossal storm surge inundated the west Coast, levelling the natural coastal defences (sand dunes etc.) that had stopped the sea in the past. While the waters retreated, the loss of the natural defences meant that there was nothing to stop a more gradual erosion and submergence occurring over the following centuries. Although it would have been a salty wasteland, most of Cantre'r Gwaelod and the Setantii coast may have been visible in the early medieval period, disappearing and reappearing with the tides, reinforcing the legends. 

A huge deluge helps to explain the characters and events known about immediately afterward. The Vita Sancti Germani describes the bishop Germanus' visit in 446/7 to a stronghold in Powys (probably Wroxeter) occupied by a belligerent Irish warlord called Benlli. The city is considerably far inland to be target of coastal Irish raids; either Benlli's Irish exploited the political instability caused by people fleeing the deluge to seize control of Powys, or, with political leadership drowned by the flood, the surviving Britons willingly appointed Benlli, as a protector, to resettle them in Powys, which he then exploited to usurp the Powysian throne. The hillfort of Foel Fenlli in Denbighshire (within historical Powys), which dates to the mid-5th century, may have been Benlli's headquarters, which makes sense if he held a political position within an unstable Britonnic state which he then abused. 

The land of Cantre'r Gwaelod probably was part of the territory of Meirionydd, the tribal homeland of the Ordovices. This might explain why there is only one reference to an Ordovician in a post-Roman context - and the only inscription refers to a burial. A later genealogy ascribes Gwynddo ap Merion as its ruler, possibly the same Gwyddno as the Black Book story. Another later legend recalls a climactic battle of Y Maes Mawr between one Merion ab Tybion, and Benlli's son Beli, close to his father's hillfort in Powys. Perhaps what we are looking at here is a saga of bad blood between the Merions and Benlli's Irish, caused by the aftermath of the deluge.

There are circumstantial signs elsewhere of the deluge. Large parts of the Scilly Isles - known to the Romans as a single island, Silina -  were probably inundated, contributing to folktales of the lost land of Lyonesse which had its ultimate origins in an Early Bronze Age flooding event. The Voteporgis stone in central Camarthenshire, thought to be 5th century in date, commemorates someone called Voteporgis the Protector in Latin and Ogham. This has traditionally been thought to be Vortipor, the 6th century King of Dyfed, but his title Protector may mean he was tasked with the protection of a region, or a people, against something. Seitheynn, in the Black Book legend, is specifically tasked with protecting coastal defences known as the Royal Embankment, so there is a potential that Voteporgis was a royal relative who in the aftermath of the flood was assigned to protect his people against the sea. 

The stormy weather also must have affected Brittany, which developed its own submergence legend in the city of Ys, also ascribed as happening in the early or mid-5th century, although the name of Ys itself might have been a cultural import by migrants from Britain. A scattering of submergence and lost-land tales from the Hebrides might also be connected to the event, although the land that many of them describe places them in far earlier periods, in the Neolithic or earlier. 

References:

Haslett, S. K., & Willis, D. (2022). The ‘lost’ islands of Cardigan Bay, Wales, UK: insights into the post-glacial evolution of some Celtic coasts of northwest Europe. Atlantic Geoscience: Journal of the Atlantic Geoscience Society, 58, 131-146.

Gutiérrez-Rodríguez, M., Pérez-Asensio, J. N. et al (2022). A Third Century AD Extreme Wave Event Identified in a Collapse Facies of a Public Building in the Roman City of (Seville, Spain). Historical Earthquakes, Tsunamis and Archaeology in the Iberian Peninsula, 267-311.




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