The stratigraphy, faulting, mineralization and metamorphism of the Upper Cambrian and Ordovician rocks are described. Newly erected formation names are the Afon Gam Formation (Upper Cambrian), the Carnedd Iago, Serw, Llyn Conwy and Rhiw Bach Formations (Ordovician). Two important unconformities, corresponding to the pre-Arenig and pregraeilis erosions widely recognized in North Wales, are described. They are controlled by major n-s gravity faulting. Many of the faults
were active as the stratified rocks were being laid down, and intrusions were injected at the same time. This gave very variable deposits and many disconformities in the Lower and Middle Ordovician. An evolutionary relation-ship between the four principal intrusive rock types is proposed to explain the restriction of certain types to particular stratigraphic horizons. Possibly the first British record of hollandite is recorded.