Summary of site:
The Benbulben Shale Formation is part of the Visean epoch of the Carboniferous period. It accumulated around 340 million years ago as the continent it fringed crept imperceptibly northwards across the equator.
Although many streams cut through the formation, most provide discontinuous sections and access is difficult. This stream section in Rahallan townland is continuous and accessible for 250 m upstream from the road. The rocks are chiefly blue-grey mudstones, shaley in part with occasional limestones up to 10 cm thick. The beds contain a great wealth of fossils including crinoids (stone lilies, relatives of starfish), bryozoa (moss animals, minute colonial animals) brachiopods (lamp shells) and one bed near the road is richly strewn with the giant solitary corals Siphonophyllia benburbensis. They are often contorted, show daily growth rings on their outer walls and pulsed thickenings along their length thought to be due to the influence of the tidal (lunar) cycles. One remarkable specimen at this locality measures almost 90 cm long indicating the longevity of some individuals. They are not aligned suggesting that there was very little current activity on this sea bed.
The fauna as a whole suggests an Asbian age for the rocks, a time towards the end of the Visean.
The typical association of Benbulben Shale rock types and the prolific fossil fauna deserve recognition and conservation.