The BGS Lexicon of Named Rock Units — Result Details

Fron Newydd Member

Computer Code: FRN Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Caradoc Series (O4) — Caradoc Series (O4)
Lithological Description: Black pyritic mudstones with abundant phosphate nodules. Ooliths common in nodules - less common in matrix. Localised developments of pure oolitic ironstone.
Definition of Lower Boundary: Conformably overlies the Llyn y Gafr Volcanic Formation in the east of the outcrop where the boundary is sharp but poorly exposed. Unconformable contact to west, greatest at Llwyngwril, where it overlies the Allt Llwyd Formation.
Definition of Upper Boundary: Conformable transition from black nodular mudstones to paler mudstones of the Ty'r Gawen Mudstone Formation lacking in nodules. Contact not seen but makes a distinct feature or hollow.
Thickness: Mostly about 25m in thickness but locally ranges up to 180m thick at western end of outcrop. Some rapid lateral variations in thickness.
Geographical Limits: Continuous outcrop from the coast as far as Cross Foxes (at least). Changes facies however.
Parent Unit: Ty'r Gawen Mudstone Formation (TYGN)
Previous Name(s): Oolitic And Nodular Beds (-2192)
Oolitic Ironstone (-2193)
Alternative Name(s): none recorded or not applicable
Type Section  Two small quarries (100m eastnortheast of Llwyn Cottage and adjacent to Peniarth Lodge respectively) and a series of small hillside exposures between the two. 
Jones, B, 1933. The geology of the Fairbourne-Llwyngwril district, Merioneth. Quarterly Journal of the Geological Society, Vol.89, 145-171. 
Cox, A H. 1925. The geology of the Cadair Idris Range, (Merioneth). Quarterly Journal of the Geological Society of London, Vol.81, p.539-592. 
Ridgeway, J. 1975. The stratigraphy of Ordovician volcanic rocks around the south and east of the Harlech Dome in Merionethshire, North Wales. Geological Journal, Vol.10, 87-106. 
Pratt, W T, 1990. Geological notes and local details for 1:10 000 Sheet SH50NE, Llanegryn. Technical Report of the British Geological Survey, WA/90/37. 
1:50K maps on which the lithostratigraphical unit is found, and map code used: