Examples of mass movement are not common in Snowdonia but there is a concentration along the glacially excavated Bala lineament through Talyllyn. The largest is that which originates on the steep northwest slopes of Graig Goch and which has resulted in a mass of debris up to 30m high covering the otherwise flat floor of this trough and behind which is the lake. There are further smaller examples which form Bwlch Cyfyng southwest of Abergynolwyn and on the southeastern slopes of Mynydd Gwerngraig east of Cadair Idris. A further mass of slipped material is recorded beneath the peak of Pen yr Helgi Du at the head of Cwm Eigiau.