Summary of site:
Karst is most extensively developed in the Carboniferous limestones of Northern Ireland, particularly in Co. Fermanagh where conditions are near ideal. The Cretaceous Ulster White Limestone is another pure limestone but has limited outcrop and tends to occur as cliff faces. Investigations in the 1980s and 1990s have unexpectedly revealed karst features and cave systems where conditions are suitable.
On the Red Hall Estate, north of the Hall, there is a substantial gorge feature developed below a Tertiary basalt cliff. At the foot of the cliff, the gorge is floored by Ulster White Limestone and in normal conditions water sinks into limestone fissures in the valley floor. The water resurges at the junction of the limestone with the underlying Liassic (Lower Jurassic) clays.
The limestone is not permeable to water but the acidic river water penetrates the joints and bedding planes and widens them by dissolving the calcium carbonate to a point where they can now carry the flow of the river underground in all but torrential flooding. Once this subterranean water reaches the impermeable clay at the base of the Ulster White Limestone, its further descent is arrested and it flows on to the surface at the lowest convenient point.
There are a number of associated sinks south of the gorge.
This is one of a group of karst phenomena in the Cretaceous Ulster White Limestone that together form a unique cluster of interest restricted to Northern Ireland. Research on this type of karst is in its infancy and all these sites warrant protection.