Summary

The Lower Palaeozoic succession consists mostly of slates but contains a large volcanic series, apparently of Bala age. This is composed essentially of rhyolitic pyroclasts but contains some basic lavas in parts. It reaches a thickness of more than 8000 feet of virtually uninterrupted volcanic rocks in the north-east, but fades out rapidly towards the west.

The basic structure of the area is a fold of synclinal type. In the north-east this is sharply overturned towards the north-west, but it changes south-westwards to a shallow, symmetrical synclinorium. This fold has been cut through by numerous thrust-planes with displacements averaging two to three miles and by many high-angle faults, some of considerable dimensions.

Evidence on the relative ages of intrusions and faults is abundant and allows the complex Caledonian history to be largely deciphered. The initial formation of the synclinorium was succeeded by numerous phases of faulting and a variety of igneous intrusions. Metasomatic events and periods of mineralization are also found in this history.