The BGS Lexicon of Named Rock Units — Result Details

St Tudwal's Formation

Computer Code: STUD Preferred Map Code: ST
Status Code: Pending Upgrade
Age range: Arenig Series (OR) — Arenig Series (OR)
Lithological Description: Coarse- to medium-grained well-bedded and bioturbated muddy sandstone.
Definition of Lower Boundary: Transgressive boundary and unconformable contact with underlying mudstones and siltstones of the Mawddach Group or Harlech Grits Group.
Definition of Upper Boundary: Is overlain by the mudstones of the Llanengan Formation. The sharp contact with the latter is a transgressive boundary, possibly a non-sequence, with a horizon of phosphatic conglomerate at the top of the St Tudwals Formation. (Machroes Member).
Thickness: 60 to 130m
Geographical Limits: Pwllheli area, northwest Wales.
Parent Unit: Nant Ffrancon Subgroup (NFR)
Previous Name(s): St Tudwal's Group (-4350)
St Tudwals Group [Obsolete Name And Code: Use STUD] (STGP)
St Tudwal's Sandstone (-549)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Coastal cliff exposure at Trwyn Liech-y-doll (lower part of formation). Young, T P, Gibbons, W and McCarroll, D. 2002. 
Partial Type Section  Pared Mawr, near Pwllheli. Coast cliff exposure on the north side of Porth Ceiriad. Young, T P, Gibbons, W and McCarroll, D. 2002. 
Partial Type Section  Near Machroes. Young, T P, Gibbons, W and McCarroll, D. 2002. 
Reference(s):
Rushton, A W A and Howells, M F. 1998. Stratigraphical framework for the Ordovician of Snowdonia and the Lleyn Peninsula. British Geological Survey Research Report, RR/99/08. 
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). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable