The Wenlock (Silurian) strata on the south shore of Clew bay, northwest Ireland, comprise three groups--Croagh Patrick quartzites, Cregganbaun pelites and calcareous beds, and Knockfadda psammites, altogether some 2,500-5,600 feet in thickness. They are bounded on the north by a fault and are repeated to the south by an overturned syncline (Croagh Patrick syncline). Northwest cross folds are present. The strata have been subjected to a regional metamorphism, not previously described as such, of late Silurian age. Fossil evidence, preserved in a few localities, supports the age determinations.