The BGS Lexicon of Named Rock Units — Result Details

Drefach Group

Computer Code: DRE Preferred Map Code: notEntered
Status Code: Full
Age range: Llanvirn Series (OE) — Caradoc Series (O4)
Lithological Description: The Drefach Group comprises mudstones, limestones and tuffs of Llanvirn to Caradoc age exposed on the southwestern margin of the Lower Palaeozoic Welsh Basin. In its type area, south of the village of Drefach, Carmarthenshire, the Drefach Group comprises a lower Felin Wen Formation (formally the Didymograptus murchisoni Beds) overlain by the Asaphus Ash Formation. This is succeeded in turn by the Hendre Shales Formation, Mydrim Limestone Formation and Mydrim Shales Formation (eg Strahan et al. 1909).
Definition of Lower Boundary: The lower boundary of the Group is taken at the base of the Felin Wen Formation. The boundary is not exposed, but the incoming of the Formation was previously described by Strahan et al. (1909), from outcrop in a road bank opposite the corn mill in the village of Felin Wen, which yielded Didymograptus murchisoni graptolites.
Definition of Upper Boundary: The upper boundary is taken at the top of the Mydrim Shales Formation. The boundary is not exposed, but in the type area is defined by a faulted contact with pale grey burrow-mottled mudstones of the Nantmel Mudstones Formation of Ashgill age.
Thickness: Greater than 600m in the type area.
Geographical Limits: The Drefach Group crops out in a narrow belt extending from Fishguard Bay in the west to Llangadof in the east.
Parent Unit: Not Applicable (-)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Type Area  Type area round the villages of Drefach and Mydrim. Outcrops of the component formations can be seen aound the villages and in scattered roadside exposures and quarries along the B4299 Mydrim road heading south towards St Clears. 
Strahan, A, Cantrill, T C, Dixon, E E L and Thomas, H H. 1909. The Geology of the South Wales coalfield. Part 10. The country around Carmarthen. Memoir of the Geological Survey of England and Wales. [London: HMSO.] 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable