[2] The basic meaning of the word cadair (Middle Welsh/Early Modern Welsh kadeir or cadeir) is 'seat, chair' (borrowed from the Greek cathedra, καθέδρα, 'chair').

The Pony Path (Welsh: Llwybr Pilin Pwn), which begins in the north from either Dolgellau or the Mawddach estuary, is the easiest but the longest of the main trails.

They comprise the mudstones and siltstones of the Ceiswyn Formation which form much of the southern part of the mountain, together with the varied rocks of the underlying Aran Volcanic Group.

The imposing cliffs which characterise the north side of the mountain are formed from a microgranite intruded into the Ty'r Gawen mudstones.

[14] The crater-like shape of Cwm Cau has given rise to the occasional mistaken claim that Cadair Idris is an extinct volcano (though as noted above, some of its bedrock is volcanic in origin).

This theory was discounted as early as 1872, when Charles Kingsley commented in his book Town Geology:[15] I have been told, for instance, that that wonderful little blue Glas Llyn,[16] 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.

There are several tear-drop shaped knolls above the edge of Llyn Cau, each of which is a roche moutonnée formed by the abrasive action of the moving ice.

Much of the area around Cadair Idris was designated a National Nature Reserve in 1957, and is home to Arctic–alpine plants such as purple saxifrage and dwarf willow.

This story is an account of a girl from Llanfihangel-y-Pennant at the foot of the mountain who walked 25 miles to Bala in order to buy a bible, which were scarce at the time.

Jones' determined journey to get hold of a copy of the book in 1800 was a major factor behind the foundation of the British and Foreign Bible Society in 1804.

[21] "Tongues of fire on Idris flaring" is mentioned in the opening verse of the John Guard version of the song "Men of Harlech".