The Drefach Group is an Ordovician lithostratigraphic group (a sequence of

rock strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

) in west Wales. The name is derived from the village of Drefach near

Meidrim Meidrim is a community some west of Carmarthen and north of St Clears in Carmarthenshire, Wales. Meidrim (formerly also spelled 'Mydrim') and its twin village of Drefach are situated either side of the Afon Dewi Fawr at the point where this sma ...

(a.k.a. Mydrim) in Carmarthenshire. The Group comprises the Mydrim Shales Formation, the Mydrim Limestone Formation, the Hendre Shales Formation, Asaphus Ash Formation and at its base, the underlying Felin-wen Formation.

Outcrops

The rocks are intermittently exposed in a belt of country running east from Fishguard Bay via Crymych then south towards

thence east via the north of Carmarthen along the Vale of Towy to Llandeilo.British Geological Survey 1:50,000 scale geological map (England & Wales) sheets 210, 211, 212, 228, 229

Lithology and stratigraphy

The Group comprises in places more than a 400m thickness of

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

s (Mydrim Shales Formation), turbidite mudstones (Hendre Shales Formation), limestone (Mydrim Limestone Formation), tuffs and

tuffite Tuffite is a tuff containing both pyroclastic and detrital materials, but predominantly pyroclasts. According to IUGS definition tuffite contains 75 to 25% volcanic (epiclastic) material. There are several classifications that define tuffite. ...

s (Asaphus Formation) and more turbidite mudstones (Felin-wen Formation) laid down in the marine Welsh Basin during the Llanvirnian and Caradocian epochs of the Ordovician period.

References

{{Reflist Ordovician System of Europe Lower Ordovician Series Geology of Wales