The BGS Lexicon of Named Rock Units — Result Details
Allt Lwyd Formation
| Computer Code: | ALL | Preferred Map Code: | All |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Arenig Series (OR) — Arenig Series (OR) | ||
| Lithological Description: | Interbedded siltstone and coarse- to medium-grained, bioturbated and locally slumped sandstone, locally quartzose or volcaniclastic, subordinate conglomerate and rare mudstones; good sedimentary structures. | ||
| Definition of Lower Boundary: | Unconformable on rocks of the Mawddach Group and the Rhobell Volcanic Group. | ||
| Definition of Upper Boundary: | Succeeded chiefly by the Offrwm Volcanic Formation in southern Snowdonia and by mudstones of the Nant Ffrancon Subgroup in the north. Locally unconformable contact in the Snowdonia district. | ||
| Thickness: | 130 to 330m | ||
| Geographical Limits: | Northern, central and eastern Snowdonia. | ||
| Parent Unit: | Nant Ffrancon Subgroup (NFR) | ||
| Previous Name(s): | Carnedd Iago Formation [Obsolete Name And Code: Use ALL]
(CRI)
Graianog Sandstone Formation [Obsolete Name And Code: See Allt Lwyd Formation And Supplementary Information And ] (GRA) Cwmorthin Formation [Obsolete Name and Code: Use ALL] (-3797) |
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| Alternative Name(s): | Graianog Sandstone |
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| Stratotypes: | |||
| Reference Section | Exposures north of Tan-y-Grisiau. Howells and Smith, 1997. | ||
| Type Section | Upper Mawddach Valley. | ||
| Reference(s): | |||
| Pratt, W T, Woodhall, D G and Howells, M F. 1995. Geology of the country around Cadair Idris. Memoir of the British Geological Survey, Sheet 149. (England and Wales). (London: HMSO), 111pp | |||
| Waters, C N, Smith, K, Hopson, P M, Wilson, D, Bridge, D M, Carney, J N, Cooper, A H, Crofts, R G, Ellison, R A, Mathers, S J, Moorlock, B S P, Scrivener, R C, McMillan, A A, Ambrose, K, Barclay, W J, and Barron, A J M. 2007. Stratigraphical Chart of the United Kingdom: Southern Britain. British Geological Survey, 1 poster. | |||
| 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. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E119 E135 E149 E136 E120 | |||