sentences:  Rocks of Cambrian age occur most extensively in an inlier in Merionethshire where the up-arched rocks of the Harlech Dome form the Rhinogs.The Harlech Grits Group comprises sandstones, mudstones and greywackes forms the eroded core of the dome and within this sequence it is specifically the greywackes of the 'Rhinog Formation' which provides the higher hills.Cambrian rocks are also to be found in north Pembrokeshire, Anglesey and Llŷn.