The natural bowl-shaped depression to which the name of chair has been attached, was formed by a cirque glacier during multiple ice ages when snow and ice accumulated on these slopes partly through avalanching. The glacier was up to a square kilometre in size surrounded on three sides by steep cliffs the highest being the headwall. The chair of Cadair Idris is this bowl-shaped hollow in the bedrock formed by the movement of glacial ice across the floor of the hollow, abrading it and the lip over which it then flowed.