The Erris Ridge is a narrow structural high that separates the Erris and Rockall Troughs. It has a composite internal character and its orientation is thought to be controlled by a pre-Caledonian structural fabric which was frequently reactivated during Caledonian to Cenozoic time. In the north it is fault-bounded and probably contains rocks of Palaeozoic and older age. It formed a positive topographic feature which was progressively onlapped during Permo-Triassic to Cretaceous times. Fault reactivation occurred along its western boundary in Tertiary times and it was submerged by approximately latest Miocene times. Further south the Ridge is more complex. The Palaeozoic and older core is flanked to the SE by interpreted Permo-Triassic to Lower Jurassic strata which constitute a fault-controlled, uplifted, remnant of a half-graben basin. This composite structure was probably inverted during Late Jurassic times and remained as a topographic feature until Cretaceous times. It was progressively onlapped during Cretaceous and Tertiary times, with a minor phase of fault reactivation during the Early Tertiary. The Ridge acted as a rigid block which facilitated the focusing of several phases of crustal extension in the Erris Trough and in the Rockall Trough.