// Workers AI · dad joke modeWhat did the Mawddach Group say to the rock? You're a stone-cold member.

Mawddach Group
Stratigraphic range: mid to late Cambrian
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 2,400 m (7,900 ft)
Lithology
Primary	mudstones
Other	sandstones, siltstones
Location
Region	northwest Wales
Country	United Kingdom
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.

Outcrops

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

Lithology and stratigraphy

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.