A new geological map covering over 1000 square km of n.w. Co. Mayo is presented. Metasediments, probably Moine and Dalradian in age, have been deformed and metamorphosed during the Caledonian orogeny together with the pre-Moine Erris Complex (Laxfordian? rocks). The gneissose Erris Complex is commonly preserved except where significant Caledonian reconstitution to schists has occurred. Rocks of the Erris Complex were tectonically emplaced within the metasediments as major anticlinal cores, and as transgressive thrust wedges during the first Caledonian deformation.

A significant structural and metamorphic break occurs across the Corraun Fault which is probably the south-westward continuation of the Leannan Fault of County Donegal.

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