The BGS Lexicon of Named Rock Units — Result Details

Marchlyn Formation

Computer Code: MAF Preferred Map Code: MaF
Status Code: Full
Age range: Merioneth Series (EM) — Merioneth Series (EM)
Lithological Description: Grey siltstones and sandstones with rare ironstones near the base. Top marked by coarse-grained quartzose pebbly sandstone.
Definition of Lower Boundary: Conformable and gradational from mainly coarse-grained sandstones of the underlying Bronllwyd Grit Formation.
Definition of Upper Boundary: Unconformable upper contact with overlying late Cambrian,Tremadoc or basal Arenig strata, (Dolgellau, Dol-cyn-afon or Allt Llwyd formations). The base of the overlying Arenig is characterized locally by sandstones.
Thickness: 240 to 500m
Geographical Limits: Northern Snowdonia and Arfon.
Parent Unit: Mawddach Group (MWG)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Type Area  Around Marchlyn Mawr, above Llanberis, North Wales. 
Howells, M F and Smith, M. 1997. Geology of the country around Snowdon. Memoir of the British Geological Survey, Sheet 119 (England and Wales). (London: The Stationery Office). 104pp. 
Howells M F, Reedman A J and Leveridge B E. 1985. Geology of the country around Bangor: Explanation for 1:50 000 geological sheet 106 (England and Wales). British Geological Survey Sheet Explanation. [London: HMSO.] 34pp. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E119 E106