Cader Idris (English, Chair of Idris), mountain ridge, south-western Gwynedd, Wales. Located about 5 km south of Dolgellau and about 10 km inland from Cardigan Bay, the ridge extends east to west for about 11 km (7 mi) and takes in the peaks of Pen-y-Gader (892 m/2,927 ft)—one of the highest in the British Isles—and Mynydd Moel (854 m/2,804 ft). Cader Idris is composed of volcanic rocks extruded during the early Palaeozoic period, into which glacial erosion has carved some spectacular rugged scenery.