AbstractStrata ranging from Arvonian to Llanvirnian are described. These rocks outcrop in a number of cylindrical or periclinal folds trending NE‐SW. The lowest rocks are voicanic and pass upwards into mainly coarse marine sedimentary rocks intercalated with thin turf bands and ignimbrites. These lowest Cambrian rocks must have been deposited in vcry shallow water while the ignimbrites would have been laid down on land. The slates of the Nantlle slate belt are overlain, in part conformably, by the Cymmffyrch (Bronllwyd) Grit of upper Lower Cambrian‐Middle Cambrian age, and this is overlain by thick Upper Cambrian slates and quartzites in the east of the area. No Arenig rocks are exposed as the Ffestiniog slates are immediately overlain by Llanvirnian.Two sets of folds are seen: the first (F1) phase includes mainly cylindrical or periclinal folds with steep limbs and an associated axial planar cleavage (S1) which are the basis of the regional structure. Later (F3) small‐scale folds produced flexure of the (F1) folds and associated with the later movements is an S3crenulation cleavage. The riebeckite microgranite of Mynydd Mawr is briefly desc