Cadair Idris is formed from a complexly fractured and folded pile of both sedimentary rocks and igneous rocks of Ordovician age. 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 steep cliffs rising above Llyn Cau to the north are formed from hard-wearing basalts and tuffs of the Pen y Gadair Volcanic Formation whilst those on the south side are acid tuffs of the Craig Cau Formation. The imposing cliffs which characterise the north side of the mountain are formed from a microgranite intruded into the Ty'r Gawen mudstones. In common with the rest of Snowdonia, the faulting and folding of this rock succession took place during the Caledonian Orogeny.