The BGS Lexicon of Named Rock Units — Result Details
St Tudwal's Formation
| Computer Code: | STUD | Preferred Map Code: | ST |
|---|---|---|---|
| Status Code: | Full | ||
| 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 [Obsolete Name and Code: Use STUD]
(-4350)
St Tudwal's Sandstone (-549) St Tudwals Group [Obsolete Name And Code: Use STUD] (STGP) |
||
| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Partial Type Section | Near Machroes. Young et al., 2002. | ||
| Partial Type Section | Coastal cliff exposure at Trwyn Liech-y-doll (lower part of formation). Young et al., 2002. | ||
| Partial Type Section | Pared Mawr, near Pwllheli. Coast cliff exposure on the north side of Porth Ceiriad. Young et al., 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 | |||