The Cadair Idris district lies at the southern edge of the Snowdonia National Park and, from the high ridges of the Cadair Idris massif to the tidal reaches of the Mawddach and Dyfi estuaries, it is an area of spectacular beauty. For many geologists it is a special place as it lies close to the heart of Sedgwick's Cambrian System. The survey of the Cadair Idris district has followed those of the Harlech, Snowdon and Bangor districts to the north and has broadened the correlation of the Cambrian and Ordovician sequences in north-west Wales.