Cadair Idris (or Cader Idris) is a prominent mountain in Snowdonia, north Wales. It lies at the southern end of Snowdonia National Park and reaches 893 m at its summit, named Penygadair (Welsh: "top of the chair").
It is composed largely of Ordovician igneous rocks, with classic glacial erosion features such as cwms, moraines, striated rocks and roches moutonées.
Cadair Idris is imbued with numerous legends; some nearby lakes are supposed to be bottomless, and anyone who sleeps on its slopes will supposedly awaken either a madman or a poet. The name itself - "the chair of Idris" in Welsh - derives from a giant called Idris in Welsh mythology, and the resemblance of one of the mountain's cwms, Cwm Cau, to an enormous armchair. The translation is commonly rendered as "the chair of Arthur", implying an Arthurian connection.
A number of named paths lead to the summit, such as the Pony Path, or the Fox's Path, the latter leading directly up the northern face of the mountain — a three-mile-long cliff and scree face. To the north lie the town of Dolgellau and the Mawddach estuary, while to the south is the glaciated Talyllyn lake. Two miles further west is the eastern end of the Talyllyn Railway, a narrow gauge preserved railway.
The crater-like shape of Cwm Cau has given rise to the occasional mistaken claim that Cadair Idris is a recently extinct volcano. This was debunked as long ago as 1872, when Charles Kingsley commented in his book Town Geology:
"I have been told, for instance, that that wonderful little blue Glas Llyn, under the highest cliff of Snowdon, is the old crater of the mountain; and I have heard people insist that a similar lake, of almost equal grandeur, in the south side of Cader Idris, is a crater likewise. But the fact is not so".