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.
http://www.bbc.tv/legacies/myths_legends/wales/w_mid/article_4.shtml
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 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).