Cwm Idwal is one of the most complete mountain landscapes in Britain. A glacially-carved corrie tucked into the northern face of the Glyderau above Llyn Ogwen, it contains geology that helped Darwin understand evolution, rare Arctic-alpine plants that survived the Ice Age, legends attached to a drowned prince, and a walk that combines accessibility with extraordinary mountain drama. If you had to choose a single location to explain what makes Eryri special, Cwm Idwal would be a very strong candidate.
The cwm — the Welsh word for a bowl-shaped mountain hollow, equivalent to the geological term corrie or cirque — was carved by a glacier that occupied it during the last Ice Age. The ice plucked rock from the headwall and sides, deepening the bowl, depositing the excavated material at the mouth as a moraine dam that now holds Llyn Idwal in place. The dramatic cliffs above — the Idwal Slabs, Suicide Wall, and the great gash of the Devil's Kitchen — are the work of 10,000 years of ice, water and freeze-thaw action on ancient volcanic rock.
Key Facts
| Location | Ogwen Valley, Eryri National Park |
| Status | National Nature Reserve (first in Wales, designated 1954) |
| Main lake | Llyn Idwal |
| Geology | Ordovician volcanic rocks, glacial landforms |
| Walk distance | Approximately 4km circuit of the lake |
| Difficulty | Easy — well-maintained path, suitable for families |
| Accessibility | First section of path accessible to most visitors |
Charles Darwin and Cwm Idwal
In 1831, before his famous voyage on the Beagle, Charles Darwin visited Cwm Idwal with his mentor Adam Sedgwick. Somewhat embarrassingly, Darwin later admitted that both he and Sedgwick completely failed to notice the glacial features that would have been obvious to a trained geologist. He returned in 1842, by which point his understanding of geological processes had transformed, and he correctly identified the cwm as glacially formed — one of the earliest such interpretations in Britain. The Idwal Slabs, where the smooth glacially-polished rock is most evident, demonstrate the power of moving ice to reshape even the hardest volcanic rock.
The Rare Plants
Cwm Idwal is one of the most important sites for Arctic-alpine plants in Wales. The north-facing aspect, poor soils, and late-lying snow create conditions similar to high Arctic tundra. Species that survived the Ice Age in ice-free refuges, or that colonised as the glaciers retreated, persist here: Snowdon lily (Lloydia serotina), the only place in Britain where it grows; purple saxifrage; mountain sorrel; and a remarkable array of mosses and liverworts. The National Nature Reserve status protects these plants from disturbance.
The Legend of Idwal
Llyn Idwal takes its name from the young prince Idwal Foel, son of Owain of Gwynedd, who according to legend was drowned in the lake by his foster father Nefydd Hardd. It is said that no bird will fly over the lake — a legend that local birdwatchers will readily dispute, since ravens, peregrine falcons and ring ouzels are regularly seen above it. The story is one of many that give Eryri's landscapes their weight of association and meaning.
Independently written — Snowdon.com