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History and legends of Cwm Idwal

Four walkers are hiking up a craggy hill near Pen yr Ole Wen, Carneddau and Glyderau, Gwynedd. There are several huge boulders on the hill and there's a peak visible in the background.
Walkers near Pen yr Ole Wen | © National Trust Images/Arnhel de Serra

Wales is a land of mythological creatures and tales of past legends, but Cwm Idwal has its own stories. With its jagged, wind-battered pinnacles of rock, forged by volcanic fire and shaped by ice, it’s an atmospheric place to be.

Cwm Idwal, Eryri (Snowdonia)

Around 500 million years ago they were thrust upwards in a massive subterranean upheaval that saw the creation of all the mountains of Eryri (Snowdonia). A range that once rivalled the Himalayas has seen its peaks scoured back by wind and weather to the size we see today.

Charles Darwin investigates

For centuries the secret of Cwm Idwal's birth was just that - a secret. But when Charles Darwin arrived here in 1831 to investigate what would become his world-famous but controversial 'On the Origin of Species' publication, he saw the first clues to the formation of Cwm Idwal.

Tiny fossils

He noticed that the scattered rocks and boulders held tiny fossils of sea creatures and oceanic plant life, perfectly illustrating their previous incarnation as the rocky floor of the Iapetus Ocean.

Darwin’s second discovery

It was 10 years later that Darwin returned and realised that the landscape had been carved by gigantic glaciers. These rivers of ice had left behind a valley where the evidence is etched into every rock.

An elevated view looking down from Y Garn across a snowy, mountainous landscape with the cwm lake of Idwal Llyn Ogwen visible in the valley below in Snowdonia, Wales.
Looking down to the cwm lake of Idwal Llyn Ogwen from Y Garn | © National Trust Images/Joe Cornish

Legends of Cwm Idwal

The largest glacial moraine within the cwm is known as Bedd y Cawr (Giant's Grave). This is the supposed tomb of Idwal, a giant from a legend lost in the mists of time.

A tragic tale of envy, greed and death

The lake is named after a young man who died a tragic death. Legend has it that Idwal was the son of the 12th-century prince Owain Gwynedd. Beautiful and scholarly, Idwal did not have the makings of a warrior and was sent away to stay in safety with his uncle, Nefydd, while his father was at war.


Nefydd was a jealous man whose own son Rhun, in contrast to Idwal, was witless and dull. Torn apart by bitterness, Nefydd took the boys for a walk by the lake and pushed Idwal in, laughing at the young man as he drowned. Owain was devastated and banished Nefydd from his lands. He then named the lake after his son.

A different version of the legend

In another version of the tale, Idwal is an eighth-century prince, the son of Cadwaladr, who suffered a similar fate. He was murdered by a rival who coveted his estate.

The lake where no birds will fly

Legend has it that the birds that inhabited the lake flew away in sorrow at the terrible deed done there. To this day, the birds are believed to maintain that vow not to fly over the water in respect for the memory of the dead prince.

In 1954 Cwm Idwal was designated as a National Nature Reserve, the first in Wales.

Two people out hiking in the snow with a dog are crossing a stile between Tryfan and Glyder Fach, with Nant Ffrancon Pass in the background at Carneddau and Glyderau, Gwynedd.

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