The shore sections of both Lighthouse and Mew Islands off the northeast coast of County Down provide impressive displays of Caledonian folding. On both islands the folds occur in plan on the wave-cut platform and in elevation in the vertical walls of numerous sea-gullies eroded along fault lines. The highly tectonised Ordovician rocks of the area display three phases of deformation though the folds on both islands primarily belong to the First Movement Phase.
On Mew Island and Lighthouse Island there are numerous first folds (F1) with features typical of concentric folding. On Lighthouse Island the F1 folds can be seen to be periclinal, although the modal fold based on bedding plane measurements consists of one limb dipping at 80° towards 327° and the other at 60° towards 165° while the fold axis plunges at up to 20° towards 242°. The most characteristic feature of concentric folding in the area is the facility with which differential slip occurred along the incompetent layers and the consequent development of associated minor structures. Between competent members of the series differential slip occasionally produced bedding plane slickensides with lineations orientated normal to the fold axes. At the same stage, particularly over the axes of folds, concentric shear planes developed and later were infilled to form lenticles of quartz or, less commonly, calcite. In addition, the crestal zones of anticlines are often irregularly net-veined with quartz and, very occasionally, there are longitudinal quartz-filled gashes. On Lighthouse Island, in the crest of an anticline, thin quartz veins occur in several beds of massive sandstone, while in the shale layers between, there are only occasional stringers of quartz - the competent beds having continuously yielded to pressure without the development of tension cracks.
Due to the relative movement along bedding planes separating competent and incompetent strata during the F1 fold phase, two groups of minor folds with axes parallel to the F1 fold axes developed: monoclines and sigmoidal flexures of the regional cleavage. On Lighthouse Island the relationship of these sigmoidal cleavages to the axis of an anticline is clear and the sense of differential slip on the bedding planes is that normally to be expected in a concentric anticline.
