Harlech Grits Group

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Harlech Grits Group
Stratigraphic range: early Cambrian
Type Group
Sub-units Gamlan Formation, Barmouth Formation, Hafotty Formation, Rhinog Formation, Llanbedr Formation, Dolwen Formation
Underlies Mawddach Group
Overlies Bryn-teg Volcanic Formation
Thickness about 2000m
Lithology
Primary sandstones
Other mudstones, siltstones, greywacke
Location
Region northwest Wales
Country Wales
Type section
Named for Harlech (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[edit]

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[edit]

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[edit]

  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)