Summary of site:
The Ballyshannon Limestone is a substantial and important formation in the Carboniferous history of Ireland and a prominent landscape feature, particularly in Co. Fermanagh. Its considerable thickness (almost 350m in places) presents a problem that would be eased if it could be subdivided into formal rock units. No such divisions have so far been named but three informal ‘members’ are recognised on the ground and Inisway Quarry is important as by far the best outcrop of the key rock type characterising the upper ‘member’. It has the further advantage of containing a fauna of foraminifera (microscopic protozoa with rapidly evolving shell forms) that give a clear date of deposition.
Roughly 6m of the upper ‘member’ are exposed in the quarry, divided into thick beds. They consist of dark grey limestones, made up of fragments of fossil shells in contact with each other in a matrix of shaley limestone. Colonies of branching, small-diameter corals of the genus Syringopora are a feature of these beds. There are also spiral textures in the rocks, reducing in diameter upwards, that are traces of the activity of some unknown animal: these trace fossils have been named Zoophycos. The rich foraminiferan fauna gives a late Arundian age, about 343 million years ago.
Although these limestones are part of the upper ‘member’, at Inisway Quarry they are 50m below the actual top of the Ballyshannon Limestone, which can be seen 200m away to the south west, on the other side of the road.
The Ballyshannon Limestone resulted from the first major flooding of the land by the early Carboniferous sea, as the entire coastal area warped into a considerable natural basin. A progressively deepening shelf extended from the shore before falling away into deeper water. The Inisway deposits were formed on the outer fringes of this shelf, in waters still close enough to the northerly land to be receiving silts and muds.
During the Arundian, this area was on the southern fringe of a supercontinent called Laurentia, creeping imperceptibly northwards across the equator.
This is one of three neighbouring quarries that display the informal ‘members’ and the junction with the Bundoran Shale is also nearby. The firm dating from the microfauna is an additional scientific asset.
The threats to the site are those common to all abandoned quarries - dumping and overgrowth.