The BGS Lexicon of Named Rock Units — Result Details

Machroes Member

Computer Code: MCH Preferred Map Code: Mch
Status Code: Full
Age range: Arenig Series (OR) — Arenig Series (OR)
Lithological Description: Well-bedded sandstone with mudstone interbeds. A thin phosphatic conglomerate horizon at the top.
Definition of Lower Boundary: Gradational contact taken at the base of the first prominent sandstone lying above muddy sandstones of the Penrhyn Du Member.
Definition of Upper Boundary: Sharply overlain by dark grey mudstones with sandstone and siltstone laminae of the Llanengan Formation. The boundary is taken at the top of a districtive phosphatic conglomerate bed within the Machroes Member. The contact may be a disconformity or non-sequence.
Thickness: <8m
Geographical Limits: Pwllheli area, northwest Wales.
Parent Unit: St Tudwal's Formation (STUD)
Previous Name(s): Passage Beds (-3236)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Machroes, in the Pwllheli area. 
Reference(s):
Young, T P, Gibbons, W and McCarroll, D. 2002. Geology of the country around Pwllheli. Memoir of the British Geological Survey, Sheet 134 (England and Wales). 
Nicholas, T C. 1915. Geology of the St Tudwal's Peninsula. Quarterly Journal of the Gelogical Society of London, Vol.71, 83-143. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E134