Mawddach Group

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Mawddach Group
Stratigraphic range: mid to late Cambrian
Blue Lake near Friog - - 464907.jpg
Slates of the Dolgellau Formation exposed in the Golwen quarry, near Fairbourne
Type Group
Sub-units Dol-cyn-Afon Formation, Dolgellau Formation, Ffestiniog Flags Formation, Maentwrog Formation, Clogau Formation
Overlies Harlech Grits Group
Thickness about 2400m
Primary mudstones
Other sandstones, siltstones
Region northwest Wales
Country Wales
Type section
Named for Afon Mawddach (river)

The Mawddach Group is a middle to upper Cambrian lithostratigraphic group (a sequence of rock strata) in Gwynedd, Wales. The name is derived from the river known as the Afon Mawddach.


The rocks are exposed across the Harlech Dome and southern Snowdonia.[1]

Lithology and stratigraphy[edit]

The Group comprises around 2400m thickness of mudstones, sandstones and siltstones laid down in the marine Welsh Basin during the middle to late Cambrian period. The Group comprises (in descending order i.e. oldest last) the Dol-cyn-Afon Formation, the Dolgellau Formation, the Ffestiniog Flags Formation, the Maentwrog Formation and the Clogau Formation.[2]


  1. ^ British Geological Survey 1:50,000 scale geological map (England & Wales) sheets 118, 119, 149
  2. ^ (BGS on-line lexicon of rock units)