The area to be dealt with in this paper lies south-west of the Corris-Aberllefenni district, recently described by Prof. W. J. Pugh. It includes about 35 square miles, extending from the coast, in the vicinity of Towyn, to Taren y Gresail in the east. The northern boundary is the Dysynni Valley, and a line due east from the Birds' Rock (Craig yr Aderyn) to the head of Glyn Iago; while the southern limit runs through Ffridd Cefn-isaf, Cwm Dyffryn, and Mynydd y Llyn, and thence north-east towards Corris. The area includes the mountain-country lying south and south-west of Cader Idris. Its most striking feature is the long, straight, steep-sided valley which runs south-westwards from Tal-y-llyn to Towyn, namely, the Tal-y-llyn or Bala Fault-Valley. The Dysynni river, which drains the greater part of the area, occupies this valley from Tal-y-llyn to Abergynolwyn, but at this point it is diverted through a narrow gorge into the broad alluvial valley known as Dyffryn Dysynni.