The Killough-Ardglass Dyke Swarm

Abstract

I. Introduction The dykes described in this paper are those occurring on the coast of County Down, Northern Ireland, between Killard Point in the north and Inner Dundrum Bay in the south-west (Pl. XX); the length of coast-line is approximately 20 miles. The country rock of the district is entirely of hard, compact Lower Palaeozoic grits and slates, highly folded and faulted and considerably affected by glacial action, which tend to give a very bold and rugged shoreline. Exceptions occur in only one or two places; Ballyhornan Bay is occupied by a wide stretch of sand with few rock exposures; Dundrum Bay includes a very long stretch of sand with few outcrops of solid rock. The fact that a number of dykes are found in the few outcrops that are exposed in these covered stretches suggests that the solid rock concealed by the sand may contain many more. South of Dundrum Bay, the Palaeozoic grits and slates again rise above tide-level and the rocky coast-line is traversed by the numerous dykes of the Mourne swarm, which have already been described by the present authors (1935). In the same paper the geographical relations of the dyke swarms of the Tertiary Province of Northern Ireland have been discussed, and consequently need not be referred to here. The first survey of the dykes of this swarm was completed by Traill and Egan (1871), who gave a brief account of some of the dykes. No further attention appears to have been given to the