Summary of site:
The exposed and dramatic promontory of Torr Head displays a revealing section of Precambrian rocks, bridging the key boundary between the Middle and Upper Dalradian periods and providing vital links with rocks of the same age in Scotland and further west in Tyrone and Donegal.
The section on the shore between Portadoon (about 2km north of the head) and Leckpatrick (about 1km south) show four formations: two from the Argyll Group (Middle Dalradian) and two from the Southern Highland Group (Upper Dalradian). All four have been regionally metamorphosed by low temperatures but powerful pressures caused considerable deformation.
The oldest division is the Owencam Formation, consisting primarily of quartz rich schists with quartz-feldspar grits and a thin limestone, with evidence of early injection of molten rock (now metadolerite) into fractures.
The Torr Head Limestone Formation, which succeeds the Owencam, is by far the most interesting in the area. Exposures can be seen in easterly draining streams, about 0.7km south-west of the head and 0.5km north-west of Farranmacallan, incised into the lower slopes of Carnanmore. Here the grey to black limestone has weathered, leaving grains of quartz projecting proud of the general rock surface. An outcrop on the east side of the head comprises coarse, interlocking calcite crystals, again black in colour, with angular quartz, chlorite and muscovite. The head itself appears to consist of thick, metamorphosed dolerite intrusions (injections of basic molten rock), closely associated with the limestone.
The top of the limestone marks the end of the Middle Dalradian and is succeeded by the Altmore Formation, consisting of altered shales and muds with thin sandstones and quartz schists. The top of the formation is not seen either along the coast or inland because the south-west trending Altmore Fault cuts out all the upper section.
South of the fault to Leckpatrick, a short section of the topmost Runabay Head Formation is exposed, here consisting of quartz and mud-rich schists but incorporating the Leckpatrick Green Beds. Further down the coast by 4km, the Carnaneigh Green Bed Member marks the base of the Runabay Formation.
These four formations, with many quartz rocks and limestones, show much evidence of sedimentary deposition into a shallow basinal sea flanking a landmass. The frequent metadolerites and greenschists in the Owencam, Torr Head Limestone and Runabay formations suggest persistent volcanism in the area during Middle and Late Dalradian times.
First examination of these rocks at Torr Head suggests that the Runabay Formation is the base of the succession with the Altmore Formation on top followed by the Torr Head and Owencam formations, exactly the reverse of what has just been described. But close examination of well-preserved sedimentary structures within some beds makes it abundantly clear that the entire outcrop of the area is upside down. This inversion has been caused by the highly complex structural history of the area, with two main phases of deformation and several minor episodes within each. In the first phase, compression generated a huge fold with a north-east/south-west trending axis. Further lateral pressure intensified the folding so that the crest was pushed over, overturning the rocks to the south-east and thrusting the rest of the fold over the top. Erosion has now stripped away the upper part, leaving the inverted limb of the fold seen at Torr Head as clear testimony to the enormous forces operating in the first phase of the Caledonian period of mountain building.
This small patch of ancient basement rocks revealed in the north east corner of Co. Antrim has proved to be a crucial link. The Torr Head Limestone correlates with the Dungiven Limestone of the Sperrins and Culdaff Limestone of Donegal to the west, and the Tayvallich and Loch Tay limestones of the Scottish Dalradian to the east. The main structural features also demonstrate the continuity of trends right across the north of Ireland and into Scotland. It also provides sections through four formations that bridge the Middle and Upper Dalradian, providing a continuous record, albeit blurred by regional metamorphism. This concentration of interest, and its dramatic setting, marks out Torr Head as a vital component of the geological history and heritage of Northern Ireland.