Geosynclinal sediments are predominant in the whole area of outcrop; graywacke and acidic and spilitic volcanic rocks are found in Anglesey and Lleyn, and graywacke and shale in County Wicklow and in Ingleton. In the English Midlands the main sedimentary types are of flysch and molasse character--sandstone, shale, and coarse conglomerate. These rocks are everywhere extremely folded, and in certain areas are highly metamorphosed. Low-grade metamorphism has occurred in County Wexford, North Wales, south Pembrokeshire, and the Welsh Borders, and high-grade metamorphism has formed gneiss and migmatite in County Wexford, North Wales, south Pembrokeshire, and the Malverns. These rocks are intruded by cross-cutting soda-granite bodies in County Wexford, Anglesey, and Lleyn; dior te and granite in south Pembrokeshire may be of this generation. In the Malverns the gneiss was intruded by syenite-rich pegmatitic material after a major plutonic igneous episode.

The whole sequence, representing largely an orogenic event within a geosynclinal belt, culminates in an acidic volcanic sequence in North Wales--and possibly also in South Wales--which is not much older than Cambrian.

In the English Midlands and Welsh Borders an acidic and intermediate volcanic sequence (Uriconian), sub-aerial and submarine, was deposited before the period of flysch and molasse deposition (Longmyndian) and its subsequent tight folding. At Charnwood Forest a phase of volcanic activity took place before a period of molasse-like deposition succeeded by gentle folding.

The whole sequence may be the expression of one orogenic period of sedimentation, folding, and plutonic and volcanic activity, because available radiogenic dates indicate folding and major metamorphism to have been within the period from 700 to 600 m.y. ago.