A recent reassessment of a medieval map, combined with geological, bathymetric, and geomythological analyses of Cardigan Bay and its surroundings – carried out by Simon Haslett and David Willis from the University of Oxford – has identified the possible historical presence off the coast of Wales of two islands, which have since disappeared.
These islands can be found on the Gough Map, a medieval map held in the Bodleian Library at the University of Oxford, and possibly the oldest map of Great Britain. In general, it is considered to be relatively accurate given that it was probably made sometime between the 13th and 14th centuries AD – but its outline of Wales has always appeared to be quite crude, with Cardigan Bay largely missing. Additionally, two shapeless islands, which definitely do not exist today, can be identified just off the mainland: the southern island is shown between the Ystwyth and Dyfi rivers, near Aberystwyth, while the more northerly appears to be located between the Dyfi and the Mawddach, near Barmouth.
Until recently, it was never widely considered whether these islands were inaccuracies or, instead, a reflection of a different coastline that had rapidly changed over the subsequent centuries. To investigate this further, Simon and David carried out an analysis of Cardigan Bay and its coastline. They found that conditions there are indeed conducive to the formation and subsequent destruction of islands. For one, the seabed of the bay and the nearby coast are largely composed of unconsolidated Pleistocene deposits, which were left there after the end of the last Ice Age and the retreat of the glaciers. These deposits are inherently unstable and prone to erosion, a process that began at the start of the Holocene with the rise in sea-level, and that has been occurring ever since.
Second, bathymetric analysis of the seabed revealed two raised areas of coarse clastic rock very near to the hypothesised location of these islands: Sarn Cynfelyn, between the Ystwyth and Dyfi estuaries, and Sarn y Bwch, between the Dyfi and Mawddach estuaries. Simon and David posit that these sarns could have acted as ‘anchors’ for islands to have ‘clung to’, while the deposits around them eroded away more quickly from both the advancement of the Irish Sea and the westward flow of the rivers.
But if these islands did indeed exist, they must have disappeared by the mid-16th century, as they do not appear on later maps. Could erosion have happened quickly enough to allow for such a rapid transformation of the coastline? Simon and David found that while the current rate of erosion at Cardigan Bay is not especially fast, at only 1m/year (above), nearby coastlines with similar conditions suggest that it is possible, with some areas seeing erosion happening as quickly as 7-8m/year. This process could also have been sped up through destruction by wind, waves, and storm surges from past meteorological events.
This new evidence also appears to be corroborated by Welsh mythology, with several sources speaking of a ‘lost’ lowland area – sometimes called Cantre’r Gwaelod – that once occupied Cardigan Bay, but which was swallowed by the sea. A clear example of this legend appears in a poem in the Black Book of Carmarthen from the mid-13th century: ‘Stand forth, Seithenhin,/and look upon the fury of the sea;/it has covered Maes Gwyddneu’ (above).
Taken together, all the evidence seems to point to the Welsh coastline having undergone significant alterations since the last Ice Age, and it is entirely plausible that these two ‘lost’ islands depicted on the Gough Map did once exist. It is hoped that further research, however, will be able to provide a more concise timeline for the possible erosion and inundation of this area.
The full results of Simon and David’s research were recently published in the Atlantic Geoscience journal: https://doi.org/10.4138/atlgeo.2022.005.