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Torr HeadAntrim
Torr Head, Co. Antrim, looking across the North Channel to the Mull of Kintyre.
Summary Full report
Site Type: Coastal section, Crag, Crags
Site Status: PASSI
District:
Grid Reference: C234406
Rocks
Rock Age: Precambrian (Dalradian)
Rock Name: Altmore Formation, Torr Head Limestone
Rock Type: Limestone, Metabasite
Interest
Other interest: No data, No Data

Summary of site:

Sedimentary rocks and metamorphosed sediments are fairly readily interpreted. There is a simple rule. The oldest beds are at the base of the succession, the youngest at the top. But there are exceptions when gigantic structures are created. Across the entire north of Ireland, extending into Scotland, there is an immense fold in the Dalradian rocks, between 30 and 40 km wide, which has collapsed to the south along its entire length resulting in the inversion of the rocks on the southern “limb” of the fold. This is the case at Torr Head where the rocks forming the promontory are part of the overturned sequence.

The Precambrian, Dalradian rocks of Northern Ireland are divided into two substantial groups, the Argyll Group and the later Southern Highland Group. The rocks at Torr Head bridge the division between the two, the Torr Head Formation being the last division of the Argyll Group and the Altmore Formation the first of the Southern Highland Group. Being inverted here the Altmore rocks appear at the base of the cliff with the older, Torr Head, rocks at the top. The rocks will be described in their upside down sequence. On the southern side of the headland the entire thickness of the succession can be followed. The rocks are tilted 25 degrees from the horizontal to the north west. At the foot of the cliff there is a distinctive dark rock, a metabasite, which is a former sill intruded in a molten state into the original sediments, altered to metabasite during metamorphism. This is succeeded by up to 15 m of pale pink psammite, the Altmore Formation, altered sandstones, bearing clear evidence of inversion. Graded beds have their coarser grains at the top instead of the base (coarse grains settle quicker) and there are channels eroded into the psammite with their bases at the top. Above these beds there is a zone of altered sediment varying in colour from grey to fawn marking the transition from the older Torr Head Limestone (above) to the Altmore Formation. These rocks pass up into the Torr Head Formation, here 6 m of grey to black, crisply bedded, grey-weathering limestones with black bands of graphite (a carbon mineral) towards the top. The limestones are interrupted by another metabasite, at least 50 m thick, curtailed at the summit of Torr Head so the full thickness of limestone is not immediately evident. Around 600 million years ago, almost exactly at the South Pole, the giant continent of Gondwana was splitting as the continent of Laurentia separated from the main mass. As the crust was stretched by this rifting, basic volcanic lavas burst through into the shallow, narrow oceanic basin to mix with the lime-rich sediments that formed its bed. The metabasites and limestones of the Torr Head Formation are a reflection of this period. As rifting continued the oceanic basins expanded, volcanic activity eased and great quantities of quartz sands were swilled of the barren adjacent land masses, quickly submerging the limestones and creating the sandstones of the Altmore Formation. It was around 130 million years later that these rocks became metamorphosed during a period of plate collision. The deformation creating the enormous overfold was part of this process. On the overall scale of metamorphic alteration due to the elevation of temperatures and pressures, the Dalradian rocks are at the lower end. This is a site of national and international importance. Its structure and rocks extend across the whole of the Northern Ireland and establish the connection with similar structures and rocks of the same age in Scotland. It is also the type locality (stratotype) of the Torr Head Formation, the site of reference for limestone succession in the area of the same age, as well as proving to be an ideal marker allowing rock correlation across considerable distances. Additionally, it clearly exposes the boundary between the Middle and Upper Dalradian rocks (the junction of the Argyll Group with the Southern Highland Group) and adds significantly to our understanding of this key period in geological history. For these reasons it should be designated as an Area of Special Scientific Interest granting the status and protection the site warrants. Although the headland is much visited, few people venture down the cliff on the southern side where the main geological interest lies, so special protection is not an issue. While geologists should still be allowed to sample these rocks, hammering should be generally discouraged.


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