These rocks are exposed, though often poorly, in a belt of country which runs east from the Cardigan Bay coast northeast of Llangranog. A narrow outcrop also stretches northeast from Aberdyfi. A more extensive outcrop reaches south from Machynlleth to the Llanidloes and Rhayader areas and indeed through the eastern part of Claerwen Reservoir where the turbiditic mudstones of the Derwenlas Formation are exposed in the hillside beneath the dam. They are seen again in the vicinity of Llyn Brianne and in the core and margins of the Cothi and Doethie anticlines around Mynydd Mallaen.