St.John's Point ASSI

The area is of special scientific interest because of its coastal and intertidal flora, fauna and geology. Important habitats include intertidal rock, saltmarsh and coastal grassland. The area is also important for species, including Sea Wormwood Artemisia maritima.

The geological importance of the area relates to the Palaeogene age dyke swarm which outcrops along the coast. Dykes represent intruded igneous magma sheets, which are usually vertical in nature. The dyke series here has been intruded into older Silurian age sandstones and shales. The dykes are principally basaltic in composition but have a number of important petrological and chemical differences from the nearby Mourne dyke swarm, and also from the Antrim series. The outcrop present in this coastal area demonstrates detail of the dyke series that extends further inland. A lower density of dyke intrusion is found to the north and west of the site. The total series is referred to as the Killough – Ardglass dyke swarm. No absolute dates have been determined for this series but are likely to be between 55 and 60 million years old.