Harlech Grits Group

Harlech Grits Group
Stratigraphic range: early Cambrian
Sandstones of the Rhinog Formation, Rhinog Fawr, Snowdonia
TypeGroup
Sub-unitsGamlan Formation, Barmouth Formation, Hafotty Formation, Rhinog Formation, Llanbedr Formation, Dolwen Formation
UnderliesMawddach Group
OverliesBryn-teg Volcanic Formation
Thicknessabout 2,000 m (6,600 ft)
Lithology
Primarysandstones
Othermudstones, siltstones, greywacke
Location
Regionnorthwest Wales
Country United Kingdom
Type section
Named forHarlech (town)

The Harlech Grits Group is a lower to middle Cambrian lithostratigraphic group (a sequence of rock strata) in northwest Wales. The name is derived from the town of Harlech in Gwynedd.

Outcrops

The rocks are exposed beneath Harlech and across the Harlech Dome to its east; a broad anticlinal structure which encompasses the Rhinogydd range.[1]

Lithology and stratigraphy

The group comprises about 2000m thickness of sandstone, mudstones, siltstones and greywackes laid down in the marine Welsh Basin during the early to mid Cambrian period. The group comprises (in descending order i.e., oldest last) the Gamlan Formation, the Barmouth Formation, the Hafotty Formation, the Rhinog Formation, the Llanbedr Formation and the Dolwen Formation.[2]

References

  1. ^ British Geological Survey 1:50,000 scale geological map (England & Wales) sheets 119, 135, 149.
  2. ^ http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=HGG (BGS on-line lexicon of rock units)


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