The geology of the Chwilog area of Southeastern Lleyn (Caernarvonshire)

Abstract

Due partly to folding but mainly to faulting, the Caradocian volcanic rocks outcrop in three districts within this area, and this repetition of outcrop reveals the variation shown by this dominantly rhyolitic volcanic series. A thick lower group of submarine bedded tuffs, thinning and fading out northwestwards, is followed unconformably by subaerial andesite and rhyolitic rocks in the north, but these volcanic rocks are absent or represented by bedded tuffs in the southern districts. The overlying sediments are generally poorly exposed but have yielded Upper Longvillian fossils from a number of localities. Caledonian intrusions are neither so numerous nor so varied as in northern Lleyn, but include granodioriteporphyries and felsites. Explosion breccias are present at the margins of some of the felsites. Nine chemical analyses indicate the variety of composition in the volcanic series while two analyses of Caledonian felsites are also supplied.