Summary of site:
The importance of Portnaloub lies in its good exposure of an uncommon association, for Northern Ireland, of sedimentary rocks called oil shales and the high concentration of fish remains, some fairly complete and preserved in 3 dimensions that they contain.
The massive tumbled dolerite and sandstone blocks forming the scree skirts around Fair Head obscure almost all the underlying rocks but at Portnaloub about 7.5 m of a grey to black shale are exposed within the tidal zone. The lowest part of the succession is best seen in offshore rocks. 2 m of the shale immediately above the low water mark contain bands and nodules of ironstone. The nodules are centred on fossilized fish droppings but, less commonly, around plant or three dimensional fish remains. Above the first 2 m the shales pale and grade into 4 m of siltstones and sandy shales containing sparse plant remains. Phosphatized fish droppings, scales and spines become more concentrated towards the top. These beds also contain fragments of the external skeletons of unrecognisable crustacea and abundant fossils of a single ostracod species. A further 1.5 m of grey shales with ironstones complete the sequence. The much sapped entrance to the Portnaloub Mine lies just above high water mark here, originally sunk to work the Portnaloub Coal, one of the lower seams in the Ballycastle Coalfield, not exposed at the present time.
These deposits were formed in the late Brigantian, towards the end of the Lower Carboniferous period, a little over 330 million years ago, in an area dominated by freshwater lakes surrounded by dense vegetation. The high concentration of plant debris falling or blowing into still, fresh water led to stratification of the deeper water in one of the lakes. Bacteria were unable to break down all the organic matter but in the attempt absorbed all the available oxygen creating an anoxic layer of water immediately above the lake bed. Unwary fish swimming into this water were asphyxiated and their remains lay undisturbed in the oxygen starved water for long periods, slowly becoming phosphatized resulting in preservation in the round. The lake gradually silted up (the grey silty shales) and eventually the area became a more typical coal swamp.
Good fish remains are uncommon in Northern Ireland so this fauna assumes importance beyond its numbers. Remains of acanthodians, primitive, mostly bony fishes have been recognised along with palaeoniscid fishes which have skeletons completely constructed of bone. The genera Rhadinichthys, Elonichthys and Megalichthys all belonging to this latter group, have been identified from here and are held in the Ulster Museum.
These rocks bear a strong resemblance to the Carboniferous oil shales of Scotland and may have been continuous with them at the time of these events.
This locality contains rocks and fossils unique in Northern Ireland and has considerable unrealized research potential justifying its designated status.