| Site Description |
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 | | Torr Head, Co. Antrim, looking across the North Channel to the Mull of Kintyre. |
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| Introduction: |
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| Torr Head is a rocky coastal outcrop located, about 7km north of Cushendun and sign posted from Torr Road, the scenic coastal route between Cushendun and Ballyvoy. The site is popular with visitors and provides spectacular views of the North Channel and Scotland. |
| On the first edition of the one inch to one mile geological map of the area (Sheet 8, Ballycastle) (Geological Survey of Ireland, 1887), the rocks at Torr Head were classified simply as "gneiss and limestone and were referred to jointly as "crystalline schists". |
| McCallien (1931) produced the first detailed lithostratigraphic map of north-east Antrim and adopted the Dalradian lithostratigraphic nomenclature already in use in Scotland. His stratigraphy for north-east Antrim was predicated on a structural interpretation which viewed this area as a south-westerly continuation of regional scale recumbent fold structures already identified on the Cowal Peninsula, SW Scotland (Gunn et al., 1897). |
| The second edition of the Ballycastle Sheet, (Geological Survey of Northern Ireland, 1963, Wilson et al., 1966) was based on detailed a mapping of the Dalradian rocks and included a revised lithostratigraphy. This work also resulted in an alternative to Bailey and McCallien's (1934) interpretation of the structure of north-east Antrim. |
| A more formal approach to lithostratigraphic nomenclature for the Dalradian was adopted on the first edition of the Solid Geology Map of Northern Ireland (Geological Survey of Northern Ireland, 1977) and for the first time, a series of local lithostratigraphic names was proposed including terms such as "Altmore Schists" and "Torr Head Limestone". |
| Arthurs (1976) produced a major revision of the Dalradian geology of north-east Antrim and defined a new lithostratigraphic framework. Although this work was largely based on reconnaissance mapping, and further detailed work is required in this area, Arthurs' linework and lithostratigraphy were incorporated in the second edition of the Solid Geology Map of Northern Ireland (Geological Survey of Northern Ireland, 1997), see table below. |
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| TABLE TO BE ADDED Dalradian stratigraphy and ESCR sites in North-east Co. Antrim. |
| Description: |
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| Torr Head is notable for being the type locality of the Torr Head (Limestone) Formation. The most accessible exposures of this and other formations are on the south-eastern side of Torr Head where limestone is closely associated with metabasite ("epidiorite") intrusions and the contact with the structurally inverted stratigraphic base of the Southern Highland Group (Altmore Formations) is exposed. |
| On the southern side of the headland, from the top of the sea-cliff south-east of the abandoned coastguard station, it is possible, to make a safe, steep upwards traverse of the succession. The succession is inverted and dips at about 25 degrees towards the north-west. At the base of this section, coarse to very coarse-grained crystalline metabasite (intrusive sill) has a chilled concordant contact with pale-pink gritty psammite (Altmore Formation is equivalent to the Pitlochry Schists) formerly referred to as quartz schists (Wilson et al., 1966). |
Up section there follows approximately 10-15m of pale pink and buff- grey platy quartz psammite with occasional pebbly lenses and grit units. Graded-bedding is common in many of these units which often have gritty bases and semi-pelitic tops. Grading, together with bed morphology, (erosive based channel fills and other cross-cutting structures)
 | | Channel-bedding in psammite (Altmore Formation), Torr Head. |
, provides confirmation that these beds are inverted, i.e., they young towards the south-west. The stratigraphic base of the Altmore Formation is marked by a 1m thick laterally continuous coarse grit bed. |
Structurally overlying the Altmore Formation, but below it stratigraphically, is a 2m thick interval of dark grey, cream and fawn coloured metasediments which represent a transition between the coarse psammites of the Altmore Formation and the limestones of the Torr Head Formation. These transitional beds include a number of thin units of limestone, calcareous schist and variegated finely laminated pelite. These rocks pass upwards into 6m of grey to black parallel- bedded limestones
 | | Thin limestone beds at the top of the inverted Torr Head Formation, Torr Head. |
which are often slightly schistose and contain elongate quartz grains and laminae standing proud on weathered surfaces (Torr Head Formation is equivalent to the Lough Tay Limestone in Scotland). |
Highly distinctive, coarse-grained, black graphitic limestones occur sporadically towards the top of this section on the eastern side of Torr Head. These consist of an interlocking mosaic of coarsely crystalline lamellar calcite with minor amounts of angular quartz, chlorite and muscovite. At the top of the section, the limestone is overlain by a massive metabasite which extends at least 50m to the summit of Torr Head. Throughout the Torr Head area thick, concordant and lenticular, intrusive bodies of metabasite are intimately associated with the limestones. Field relationships are complicated by a series of north-west to south-east trending normal faults which cross Torr Head and juxtapose metabasite and limestone at several different structural levels.
 | | Massive metabasite, Torr Head. |
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| Importance: |
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| This is a site of both national and international importance. It contains the stratotype (type section) for the Torr Head Formation which is the key marker horizon within the Dalradian of north-east Antrim. The site contains the best known exposure in Northern Ireland of the Middle-Upper Dalradian boundary and is likely to remain the focus of interest for future generations of geologists from Northern Ireland and abroad. The exposures at Torr Head contributed significantly to the early understanding of the lithostratigraphy of north-east Antrim and were crucial in establishing the initial stratigraphic correlations between Scotland and Northern Ireland. |
| Interpretation: |
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| Approximately 600 Ma, at the end of the Middle Dalradian (Argyll Group), the continental crust along the margin of Laurentia came under the influence of a developing extensional tectonic regime and the comparatively stable shallow-water depositional environments, which had characterised much of Lower and Middle Dalradian times, gave way to less stable conditions. Crustal extension and thinning was accompanied by a major upsurge in basic volcanic activity and the eruption of basaltic lavas into comparatively shallow near shore marine sedimentary environments (Tayvallich Subgroup). Throughout the Dalradian belt in Northern Britain, this stratigraphic interval is characterised by the intimate association of limestone and basaltic magmatism such as is seen at Torr Head. |
| Rifting along the continental margin continued into Upper Dalradian (Southern Highland Group) times by which time a series of ensialic basins had developed. These basins were infilled with siliciclastic sediments derived mainly from the nearby continental landmass. At Torr Head, these events manifest themselves by the sudden influx of coarse arenites and grits seen at the base of the Altmore Formation. |
| During the Caledonian Orogeny, the Dalradian rocks in north-east Antrim were deformed, folded and subjected to low grade regional metamorphism. The mineralogy of the limestone, which contains black graphitic calcite, white calcite, quartz and phlogopite, is consistent with metamorphism in the quartz-albite-epidote-biotite sub-facies of the Greenschist Facies. |
| Structurally, Torr Head and the Torr Head Formation lie on the downwards facing, limb of the Altmore Anticline, the north-east to south-west trending recumbent anticline that is interpreted as the south-westerly continuation into Northern Ireland of the Cowal Anticline (Bailey and McCallien, 1934; Wilson et al., 1966). |
| Conclusions: |
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| The Torr Head Formation is a key marker horizon for those studying the Dalradian geology of north-east Antrim or making correlations between Northern Ireland and Scotland. The Torr Head Formation, together with its correlatives elsewhere in Northern Britain, marks the transition from the Argyll Group (Middle Dalradian) to the Southern Highland Group (Upper Dalradian). Torr Head is one of just a handful of Precambrian sites in Northern Ireland which has demonstrable international importance. |
| Notes: |
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For site specific information and references on other Dalradian sites in North-east Antrim see the following sites: |
| Key Site 155 - Benvan Key Site 156 - Escort Port Key Site 158 - Altmore Burn Key Site 159 - Leckpatrick Point Key Site 160 - Carnaneigh Key Site 161 - Loughan Bay Key Site 162 - Loughareema Channel Key Site 163 - Knocknacarry Bridge Key Site 164 - Cushendun Bay and Rock Port |
| For site specific information and references on other Dalradian sites in the Sperrin Mountains, Londonderry and Tyrone see the following sites: |
| For Central and Southern Sperrin Mountains: |
| Key Site 130 - Butterlope Glen Key Site 131 - Craig Key Site 132 - Mullaghcarbatagh Key Site 133 - Henry's Bridge, Glensass Burn Key Site 134 - Oughtboy Burn Key Site 135 - Garvagh Burn Key Site 136 - Barnes Gap Key Site 137 - Barnes Burn Key Site 138 - Golan Burn Key Site 139 - Glendarragh Burn, Glenlark Key Site 140 - Mountfield Old Quarry Key Site 141 - McNally's Burn, Broughderg |
| For North Sperrin Mountains and Londonderry: |
| Key Site 142 - Letterbrat Quarry Key Site 143 - Banagher Glen Quarry Key Site 144 - Strabane Quarry Key Site 145 - Glenmornan River, Artigarvan Key Site 146 - Balix Hill Key Site 147 - Drain Quarry Key Site 148 - Kildoag Quarry Key Site 149 - Loughermore Mountain Key Site 150 - Burntollet and Tamnymore Wood Key Site 151 - Dog Leap, Limavady Key Site 152 - Kittybane Quarry Key Site 153 - Prehen Quarry Key Site 154 - Creevagh Hill Old Quarry |
| For general information on the Dalradian rocks of Northern Ireland see the following site: |
| Key Site 124 - The Dalradian Rocks of Northern Ireland |
| For general and site specific information on the Pre-Dalradian rocks of Northern Ireland see the following site: |
| Key Site 123 - The Pre-Dalradian Rocks of Northern Ireland |
| For general information and an extensive reference list on the Precambrian of Northern Ireland see the following site: |
| Key Site 122 - An Introduction to the Precambrian of Northern Ireland |
| This site is also important for Structural Geology. See Key Site 648 - Torr Head. |
| References |
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| Arthurs, J.W. 1976a: The geology and metalliferous mineral potential of the Sperrin Mountains area Geological Survey of Northern Ireland Special Report |
| Bailey, E.B. and McCallien, W.J. 1934: The metamorphic rocks of North-east Antrim Transactions of the Royal Society of Edinburgh: Earth Sciences, vol. 58, pp.163-177 |
| Gibbons, W. and Harris, A.L. (eds) 1994: A revised correlation of Precambrian rocks in the British Isles Geological Society of London Special Report, numb. 22 |
| Gunn, W., Clough, C.T. and Hill, J.B. 1897: The Geology of Cowal Memoir of the Geological Survey of Scotland |
| McCallien, W.J. 1931: A contribution to the correlation of the Dalradian rocks of Scotland Geological Magazine, vol. 68, pp.153-175 |
| Wilson, H. E. and Robbie, J. A. 1966: Geology of the country around Ballycastle. Memoirs of the Geological Survey Northern Ireland, |
| Map(s): | GSI (1887). 1:63,360 Geological Sheet 8 (Ballycastle), Dublin, OS for GSI / GSNI (1963). 1:63,360 Ballycastle, Northern Ireland Sheet 8, Drift, Southampton, OS for GSNI / GSNI (1977). 1:250,000 Northern Ireland, Solid Geology (1st edition), Southampton, OS for IGS / GSNI (1997). 1:250,000 Northern Ireland, Solid Geology (2nd edition), BGS, Keyworth, Nottingham |