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Rising Sea Levels

The view from Bunaneer beach south to the north side of Beara.

A large and ancient pine stump high up on Bunaneer beach.

The stumps of the Bunaneer ancient woodland, exposed at low tide.

This is in fact not very unusual, there are quite a few places around the coastline of south and west Ireland and also southern England and south Wales where relict land surfaces have been covered by rising sea levels. On the coast of South Wales at Goldcliff there have been extraordinary finds of human footprints from a family group, and the skeleton of a wild boar, lodged beneath a large piece of tree, with a flint arrow or spear head lodged in it's shoulder. All of this covered by sediment and then inundated by rising sea levels, and now exposed by some quirk of fate, maybe storm or tidal removal of some sediment, and then exposed at low tide. There are several other sites along both the northern (Welsh) shore and the southern (English) shore of the Bristol Channel (Severn Estuary) where tree stumps, branches, leaves, and dark brown organic woodland floor can be seen at low tide.

In West Cork we have Tralong Bay, on the coast beneath Drombeg stone circle, where tree stumps, both in situ and also fallen and dismembered, as well as the organic woodland floor that grew up beneath the trees, can be seen at low tide. The degree of preservation in such places is sometimes quite staggering, with even whole leaves pressed flat into the compressed mud, and seeds, and small plant remains. There are quite a few other sites along the south coast of Cork and Waterford - see the Intertidal peat deposits between Toe Head and Red Strand, West Cork (IQUA 2017 research award) by Tony Beese

At Beach beach on the southern shore of Bantry Bay at the far end of the beach from the airstrip, is a flooded woodland floor, clearly visible at low tide. Sticks and branches can be seen, and initial examination of the sediment - although it is not thick here, just 40 cm, the top layers probably having been eroded away by the action of the waves and tides - suggests this was a damp alder woodland with some hazel and willow trees growing in and around the woodland, and plenty of ferns. Presumably this woodland grew up after the drumlin had been deposited by the ice and the ice had melted as the climate improved. So maybe this woodland dates to between 10,000 and 12,000 years ago. See here for the westcorkpalaeo.com webpage for Beach beach.

On the foreshore of Ringaroigy Island in the mouth of the Ilen estuary is a passage tomb (CO150-057----) which gets inundated at high tide.

At Ballyrisode on the southern shore of the Mizen peninsula west of Schull, again on the foreshore and covered at high tide, is the remains of what may be a burial cist, and also flagstones arranged upright in what appears to be the form of a tank for holding water, as we find with fulachta fia CO147-091----. This was reported by Finola Finlay and Robert Harris in their Roaringwater Journal. It may have been an intertidal fulacht fia, but more likely a dryland site that has been inundated by the rising sea level.

The Ballyrisode fulacht fia, exposed at low tide.

So we have woodland dating back to between 4,000 and 5,000 years ago, or more judging by the depth of organic sediment under the tree stumps at Bunaneer; a passage grave probably dating back to between 2,500 to 3,500 BC; a fulacht fia which may be Bronze Age c. 2,000 BC; all of which are now below high water mark. So the sea level has risen significantly since just 4,000 years ago.

The likely cause for this continuing sea level rise is the adjustment of sea level following the warming of the earth's climate that signified the end of the last glacial period. This isostatic adjustment has been investigated quite intensely over the years, and a study of Bantry Bay in particular, in 2015, resulted in a graph that suggests how sea level in SW Ireland has changed over the last 20,000 years.

A graph showing post glacial isostatic adjustment to relative sea level in the Bantry Bay area. This has been taken from Plets et al 2015.

The graph shows how pulses of quite rapid sea level rise occurred at times, and these largely relate to injections of significant amounts of meltwater as ice sheets collapsed - the ending of a glacial period is not a smooth and steady melting of ice and addition of water back to the oceans, but occurs at different rates. Meltwater in many cases was held back on land - like the large Lake Agassiz that was held on the North American continent, and then seems to have been released into the Atlantic in a massive outpouring. This caused a serious downturn in climate, affecting at least those lands bordering the North Atlantic, by bringing the circulating currents in the Atlantic to an end. The graph above implies that the sea level adjustment has now possibly more or less finished.

However, sea level rise has a different cause now, but maybe we can be reassured that the earth has managed to adjust to these changes in the past, and will continue to do so in the future, even though we as a species are threatened with rapidly rising sea levels today, caused by the human induced climate change that is so prominent in the news.

Meyssignac, B.,Cazenave, A. 2012. "Sea level: A review of present-day and recent-past changes and variability". Journal of Geodynamics. 58. 96–109.

Plets R.M.K. et al. 2015. Late Quaternary evolution and sea-level history of a glaciated marine embayment, Bantry Bay, SW Ireland. Marine Geology 369 (2015) 251–272

Finlay. 2019. Ballyrisode Fulacht Fia: A New Bronze Age Site on The Mizen. Skibbereen Historical Journal