Summary of site:
Fair Head, or Benmore, is the striking headland 7km north east of Ballycastle, Co. Antrim, famously prominent cliff in tourist literature and Irish imagery.
The entire upper profile of the headland is a massive sill which is responsible for its main geological interest. A sill is a tabular body of once molten rock injected into horizontal rocks, usually well-bedded sedimentary strata.
The sill at Fair Head is 85m thick and composed of olivine dolerite (a basaltic rock). Its base rests on Carboniferous rocks to the north and east but, in Murlough Bay immediately around the headland to the south, it intrudes above into Triassic and Cretaceous rocks. This means that its intrusion was later than the Cretaceous and associates it with the main eruptions of the Antrim Plateau basalts.
The Fair Head Sill is inclined to the south and thins rapidly to both the south and west, where a complex of thinner sills can be seen. At the north end of Murlough Bay, a substantial sill appears below the main sill, separated from it by up to 10m of Carboniferous shales. This is the Binnagapple Sill, around 15m at its thickest, and is part of the same complex.
Despite its considerable thickness, the heat from the sill has little altered the underlying sandstone but shales have been converted to hornfels extending up to 6m from the contact in some places. A crude columnar jointing of the rock is the result of stresses caused by the cooling and solidifying of the melt and there are minor crush zones associated with later compression. Although the cliff face is remarkably stable, weathering (particularly frost action) periodically brings down columns producing blocks of massive dimensions on the scree below. Even major storms barely disturb these blocks and the bulk of the scree may be little changed since late glacial times.
The sill is made of dolerite, a coarse-grained rock equivalent to basalt in composition, showing crystals of the mineral olivine. It has a number of interesting mineral fabrics (particular associations of crystalline minerals) and illustrates the changing chemistry of the rock as minerals formed during solidification.
The importance of Fair Head is due to this combination of features and its exceptional size.