[2][3] The Proterozoic rocks of the Cymru terrane are typified in North Wales by the Arfon Group, Sarn Complex and the Twt Hill Granite.

The Granitoid Gneiss has Sm-Nd ages of 1350 Ma which is in the same temporal region as the Sarn Granite [1][5] In the southwest of Wales is the St Davids Granophyre the stratigraphical relationship of which was subject of heated debates towards the end of the 19th Century predominantly between Sir Archibald Geikie and Henry Hicks.

[1][8] The St David's Granophyre has historically been incorrectly assigned to be alaskite or trondhjemite due to low concentrations of potassium feldspar but this has been shown to be an alteration product from its original calc-alkaline granite which is noted to exhibit a volcanic arc signature.

[11] The Sarn Granite is leucocratic and covers an expanse of c.6 km^2 in contrast to the gabbro and diorite that exist as small and scattered exposures.

This is a small fine-grained, leucocratic intrusive body that cuts the Padarn Tuff at the western end of the Bangor-Caernarfon Ridge.

[1] There is evidence presented that subaerial and subaqueous deposition has occurred in a Welsh Basin that is likely to have been undergoing varying rates of subsidence thus promoting aerial emergence.

[10] In the Pebidian Supergroup the basal unit is largely basaltic and exhibits columnar jointing, autobrecciated lavas, scoriaceous and fine grained tuffs.

The Minfordd Formation is a sandstone dominated epiclastic and tuffaceous lithology which has a broad upward fining sequence with the presence of thick welded and non-welded ash-flow tuffs that elucidates an amount of volcanism.

The Fachwen and Bangor Formations are both dominated by acidic volcanic deposits but they also contain minor basic tuffs that preserve scoria textures which suggest a proximal source.

[1][3] This outcrops mainly on Anglesey but a sliver of it also occurs along the Berw Fault and its properties are similar to that of the Arfon Group ash-flow tuff and as such constrain the docking of the western Monian Composite terrane.