The BGS Lexicon of Named Rock Units — Result Details

Trygarn Formation

Computer Code: TGN Preferred Map Code: Tgn
Status Code: Full
Age range: Llanvirn Series (OE) — Llanvirn Series (OE)
Lithological Description: Mudstones and siltstones with coarse-grained sandstone interbeds containing ferruginous ooids.
Definition of Lower Boundary: Overlies the sandstones and mudstones of the Bryncroes Formation, possibly with disconformity or non-sequence, the boundary being taken at the base of the first ooidal ironstone within the Tregroes Formation.
Definition of Upper Boundary: Conformable, gradational boundary where ferruginous sandstones of the Trygarn Formation pass upwards into undivided micaceous siltstones of the Nant Ffrancon Subgroup.
Thickness: 90m
Geographical Limits: Pwllheli area, northwest Wales.
Parent Unit: Nant Ffrancon Subgroup (NFR)
Previous Name(s): Carw Formation (-3721)
Alternative Name(s): none recorded or not applicable
Partial Type Section  Bank of small stream 300m northeast of Ty Ruttan (exposes section of basal part). Young et al, 2002. 
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). 
Beckly, A J, 1988. The Stratigraphy of the Arenig Series in the Aberdaron to Sarn Area, Western Llyn, North Wales. Geological Journal, Vol.23, 321-337. 
1:50K maps on which the lithostratigraphical unit is found, and map code used: