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The Glaciation of the Barmouth and Arthog area

Bernard O’Connor 2005-6


It is thought that the ‘Harlech Dome’ of North Wales has been affected by at least two major ice events, separated by a period of warmer conditions. The last “ice age” was geologically very recent – between about 16,000 and 11,000 years ago. As temperatures dropped, the winter snowfall on the high land did not melt in the summer, in particular on the north-facing slopes of Cadair Idris and the Rhinogs. The snow built up over hundreds and thousands of years to accumulate as a major ice sheet with finger-like glaciers flowing down the valleys, particularly the Mawddach and the Dysynni,  only to be hemmed in to the north and west by the frozen Irish Sea.


In Austin Miller’s paper on the river development in the Dolgellau district, he located one of the major ice dispersal areas as the cwms (corries or cirques) of Llyn-y-Gadair and Lynn Cau below Cadair Idris. The ice flow was from the NNE to the SSW and, from the lateral erosion of the valley sides, he estimated that the Mawddach glacier was at least 900 feet (315 m.) thick and over six miles (9.6 km.) in length. (Miller, A.A. (1946), Some physical features related to the river development in the Dolgelley district, Proc. Geol. Ass. Vol. 57, pp.174-203)


There is plenty of evidence of glacial erosion and deposition in the Barmouth and Arthog area. At times, only the peaks of the highest mountains were exposed above the ice sheet. Cadair Idris is a good example of a pyramidal peak (SH 711130). Its steep slopes were heavily eroded by ice-plucking at the rear of the glaciers that descended from it. Freeze-thaw action behind the cwms just below the peaks, resulted in huge scree slopes. There are several cwms below the summits of Diffwys, Craig-y-Grut and Cadair. These armchair hollows with steep back slopes were the sites of ice build-up and the start of glaciers. Many have lakes in them like Llyn Bodlyn (SH 648239), Llyn Irddyn (SH 630222), Llyn Cyn (SH 657118). Llyn-y-Gadair (SH 708135) Llyn Gafr (SH 710141) and Llyn Cau (SH 710124). Aretes, knife-like edges separating two or more cwms, have left the ridges favoured by walkers for their dramatic views for example Llawlech (SH634217), Craig-y-Llyn (SH662117) and the numerous ones approaching the summit of Cadair Idris.


‘Roches Moutonees’ can be seen in the valleys and on the hills. These are large, smoothed, rounded and exposed rocks left as the glacier scraped away and plucked off rock from their surface. On the south bank of the estuary are the islands of Ynysgffylog (SH 639138), Fegla Fawr (SH 629147), Fegla Fach (SH 638154) and Coed-y-garth (SH 661168), heavily glaciated rocky outcrops close to the snout of the glacier. Those ‘roches moutonees’ on the hillsides can resemble sheep from a distance – hence their name. Around Cadair Idris there are examples of striations, scratch marks on the rock caused by the rocks being scraped across them as the glacier flowed over them. Dotted over the slopes on both sides of the river are numerous rocks and boulders deposited when the ice melted. These are glacial erratics, dropped sometimes hundreds or thousands of kilometres from where they originated.


The glaciers that once filled the valleys of the Ysgethin, Mawddach and Dysynni have left good examples of U-shaped troughs. Once the Irish Sea ice melted, the ice travelled roughly ENE to WSW into the Irish Sea.  When the ice eventually retreated, the flat valley floor was covered in deposited rocks, boulders, clay (ground volcanic rock) sands and gravels. The pressure release on the valley sides caused rock falls which built up scree slopes at the foot of the valley sides. Whilst many millions of years of subsequent wind and river erosion have lowered the slopes, it is still possible to see hanging valleys. These were left up to 900 feet above the glacial trough, for example the upper courses of the Afon Dwnant, Afon-Cwm-Llechen and Afon-Cwm-mynach on the northern side of the Mawddach and Afon Arthog and Afon Gwynant on the south side.


Al these rivers have a sudden change of gradient where they drop down steeply to the Mawddach. The word Mawddach means flat seashore, a good description of the estuary at low tide. Steep-sided ravines, waterfalls and rapids can be seen in all these valleys. Where they enter the Mawddach they have deposited their load of rocks, boulders and pebbles that have built up as alluvial fans.


Austin identified thirteen glacial overflow channels in the Mawddach estuary. These were cut when the tributary streams feeding the main river had to cut new channels when they encountered the glacier filling the valley.


They follow the line at the foot of the back slope and thus have the effect of severing the shelf from its attachment to the hillside and isolating, as a knoll or ridge, what had previously been a bevelled spur. It is noteworthy too, that the larger ones all lead westwards, intaking from high up the sides of the tributary valleys, whose meltwater discharge they have carried round the corner by severing the spur. Many of them come to an abrupt end and hang, as much as 20 feet, above the valley of a very different type. This probably marks the height of the contemporary ice surface at the point of escape.


(Austin, op.cit. p.194)


They are listed anti-clockwise from the southwest as Tyddyn Sieffre Channel, Capel Horeb Channel, Tyn-y-coed Channel, Coed-y-Garth, Llyn Jericho Channel, Abergwynant Channel, Maes Angharad Channel and Fiddler’s Elbow Channel on the south side of the river and Rhuddallt Channel, Bontddu Channel, The Farchynys “Horseshoe”, Ynys Dafydd Channel and Panorama Walk Channel (Barmouth) on the north side. Details of each are provided in Austin’s paper. (Ibid.pp.194-200).   


Moraine, the material plucked or scraped off the surface, was deposited when the ice melted. Potentially, millions of tons of rocks and boulders were eroded and dumped into what is now known as Cardigan Bay. Over the millennia constant wave action carried much of them out to sea but vast quantities were deposited north of Barmouth. The 13th century poem, "Boddi Maes Gwyddno", (The Drowning of Gwyddno's Realm) tells the story of how fifteen settlements off the Meirionydd coast from Aberdyfi, Barmouth and Borth were submerged when the dykes protecting them, were breached in the sixth century. The legend is said to be one of numerous ancient folk myths about extensive flooding following rising sea levels after the last Ice Age. According to the BBC Legacies website


Cantre’r Gwaelod was said to cover much of the lowlands now beneath Cardigan Bay, and many geographical features are connected to the legend. The Sarnau, single ridges several miles long, which run at roughly right angles to the shore, are located between each of the four river mouths in the north of the Bay. Legend has it that these ridges are the remains of causeways built to give access to the present mainland at high tide, but they are probably the remains of glacial moraines – formations of gravel, clay, sand and boulders left behind as the glaciers melted away at the end of the last Ice Age.


There are few areas of coastal lowland in the area so drumlins, large oval-shaped mounds of glacial moraine, are not much in evidence. Better examples can be seen in the Wnion valley. Lateral moraine, ground up rocks and boulders, were dumped along the sides of the glacier. Much of this has been carried away by the Mawddach. It is more in evidence in the Dysynni valley. The terminal moraine of the Mawddach glacier, rocks, boulders, clay, sand and gravel dumped at the snout of the glacier has been washed away by the river. The heavier stones may well have ended up being washed up on Fairbourne beach to form the bar.  


As the Mawddach glacier cut its way through the Maentwrog Beds and the Ffestiniog Beds it left what are called truncated spurs, steep-ended ridges. There are a number about 900 feet high, indicating the height of the glacier side. The Austin noted that the spurs coming down from Y Garn both upstream and downstream of Llanelltyd are steeply truncated up to this height, as are Foel Ispri and Y Vigra.


 In exposed areas eroded rock falls have built up as scree slopes, not to be confused with the enormous deposits of waste rock left after mining operations, especially the slate quarries.


 Sea levels are estimated to have risen by as much as 100 metres following the melting of the icecaps, flooding much of the coastal lowlands. As the coastal strip is generally made up of high land, it helps explain why the settlements of the prehistoric peoples tended to be sited between 200 and 300 metres above sea level.


The Drainage of the Barmouth and Arthog area


The Mawwdach estuary is relatively recent in geological terms. Before the last period of glaciation, about 16,000 years ago, the southwestern edge of the Harlech Dome was drained by an ancient river system which some geologists think originally flowed out into Cardigan Bay further south. In Austin Miller’s work on river development in the Dolgellau district he identified the Wnion valley, NE of Dolgellau, as the original upper course of the river, draining WSW through the valley followed by the minor road past Gwernan Lake, through Ffordd Ddu and then down what is now Afon Gwril into the sea at Llwyngwril. (Figure 31 p. 189)


Subsequent upthrust and faulting changed the course of this river and caused rejuvenation, over deepening of the river channels. The western side of the SSW-facing geosyncline  was drained by a tributary river that flowed roughly NS from Bwlch Y Rhiwgr, Llawlech and Braich to Ffordd Dhu. The western side of the Harlech Dome was raised and complex faulting occurred. Whilst you may have heard of the Tal-y-Llyn  - Bala fault to the south, other faults, following roughly the same NE to SW trend, caused drainage NNW down the slope of Braich Dhu to produce the valley of Afon Arthog. The Dwynant, the upstream section of the earlier river, then fed its waters into the new vale that drained into Cardigan Bay at Barmouth.


A series of further river captures followed. At some later stage the river then turned north at Hafotty Fach, 791 feet (293 m.) to flow down the Gwynant into the glacial valley. It later captured the Upper Cwm Mynach, the Ganllwyd and the Wnion.



Arthog Bog


In William Condry’s 1981 Natural History of Wales he comments that

Sadly, Arthog Bog has degenerated through drainage. Its sphagna have greatly diminished and the great sundew has quite disappeared. Yet here, at its only site in north Wales, brown beak-sedge still holds on. Now so nearly water-less, this bog would be completely covered by a scrub of rhododendron and birch if it were not regularly fired by the farmers who use it for rough grazing. Its ditches and some old peat cuttings still hold water and in them grow two beautiful plants, both yellow-flowered – greater spearwort and, at its most northern British locality, wavy St John’s wort (Hypericum undulatum).