Earth Science Conservation Review

Summary Full report
Site number:518  
Locality Type:Coastal section Status: ASSI
Grid Reference: D180438 Centroid
County: AntrimCouncil area:Causeway Coast & Glens Borough Council
Lithostrat:Fair Head Sill
Site Description
The Fair Head Sill represents the thickest and most extensive of the Tertiary sills associated with the plateau lavas of northeast Ireland and forms the dominant headland in north Antrim. This headland was referred to on Ptolemy's map of the world, produced in the second century A.D.
The Fair Head Sill forms the upper part of the Fair Head or Benmore promontory on the north coast of Antrim, some 7km northeast of Ballycastle. The sill is around 85m thick at maximum and consists of columnar jointed olivine dolerite, intruded into Carboniferous sediments in the north of the outcrop, with the base of the sill transgressing steeply through Carboniferous, Triassic and finally Cretaceous rocks to the south in the cliffs above Murlough Bay. Associated with the main Fair Head sill are a number of smaller intrusions including the Binnagapple Sill which occurs just beneath the base of the Fair Head Sill on the coast north of Fall Point (D190432). The most detailed account of the sill is given by Wilson and Robbie (1966) in the Memoir to Sheet 8 of the Geological Survey of Northern Ireland, Geology of the Country around Ballycastle. An excursion guide to the area is included in Emeleus & Preston (1969).
The base of the sill can be traced round much of the headland of Fair Head, approximately horizontal and underlain by the shale and tuffs of the Carboniferous Murlough Bay Coals. The cliff around the Grey Man's Path (D184436) is formed by the massive vertical columns and for much of the headland is over 80m high. The upper surface of the sill dips to the south and over much of the area northeast of Lough na Crannagh (D178427) dips more steeply than the base so that at its eastern side the sill thins fairly rapidly to the south. To the south of the outcrop the base of the sill is seen to transgress steeply upwards through the Carboniferous sediments until it intrudes Triassic and eventually Cretaceous rocks. The overall thickness of the sill decreases to around 15m in this area.
To the west at Farragandoo (D170430) the sill thins out sharply with the upper surface dipping and the base rising and the sill interdigitates with Carboniferous sediments.
Fig 2 from Emeleus & Preston (1969) shows the geology of the area.
A complex of minor but extensive sills occurs to the south and west of the main sill and a thick sub-parallel sill, the Binnagapple Sill, crops out below the main sill on the northeast side of the headland southeast of the Grey Man's Path. The sill about 15m thick also shows coarse vertical columnar jointing and is separated from the main sill by 5-10m of shales showing signs of thermal metamorphism.
Thermal Metamorphism:
The base of the sill which can be traced around most of the headland shows little induration of the underlying sandstones. Some bleaching is apparent for up to 1m from the contact but some of the shales are altered to hornfels up to 6m from the sill. West of Fall Point (D187430) there are large xenolithic rafts of sandstone and on the upper surface of the sill at the northeast corner of Lough Doo (D173434) there is a fairly extensive 30m long exposure of indurated Carboniferous shales. Above Murlough Bay (D190419) the Tertiary age of the sill can be verified by its thermal action on Chalk and Greensand (Emeleus & Preston, 1969).
Jointing and Structure:
Though unaffected by any major faulting the sill is traversed by several thrust belts most of which have the same northerly trend as the major joint system. The Grey Man's Path is weathered along the eastern of two crush zones which intersect at the cliff edge. The crush zones and the major northerly trending joint system are near vertical, hence forming the massive columnar structure commonly seen on the cliff face. The block scree surrounding the foot of Fair Head is considered by Wilson & Robbie (1966) to be probably of late and post-glacial age. The scree comprises fallen blocks of columnar dolerite, many several metres in width and making passage difficult.
Petrography and Geochemistry:
The petrography of the sill is described in detail by Wilson and Robbie (1966). The normal rock of the sill is an ophitic olivine-dolerite composed of fresh olivine with serpentinization on cracks, lamellar-twinned laths and abundant pleochroic titanaugite in large ophitic grains. Skeletal and irregular grains of ilmenite are ubiquitous. Zeolites occur occasionally. Veins and patches of a coarse dolerite-pegmatite are abundant in the upper part of the sill. Glomeroporphyritic olivines and feldspars have been observed in specimens from near the base of the sill and, where best developed, feldspar phenocrysts form complete rosettes round a large olivine phenocryst or a group of olivines. Partial chemical analyses are given in Wilson & Robbie (1966).
The Fair Head Sill is the most important of the Tertiary sills associated with the Antrim Lava Group in terms of lateral extent and thickness. It represents a very visible demonstration of intrusion processes with transgression from Carboniferous sediments up to Cretaceous Chalk. Within the British and Irish Tertiary Volcanic Provinces it is important in terms of the scale of the intrusion.
The Fair Head Sill is one of a number of sills which occur around the lava plateau, generally intruded into pre-Tertiary sediments. They are probably bodies of magma which failed to reach the contemporaneous Tertiary surface or they represent standing columns of magma which leaked away through conduit walls into the adjacent sedimentary basins (Preston, 1981). The Fair Head Sill illustrates the controlling process of olivine fractionation. Glomeroporphyritic olivines encased in plagioclase crystals accumulated near the base of the intrusion while relatively olivine-free pegmatites occur as veins and lenses under the indurated Carboniferous shale of the roof.
The Fair Head sill is a clear example of a large scale intrusion showing transgression and internal modification of crystallisation products, with clearly exposed boundaries showing thermal metamorphism on variable scales. It is an important demonstration of the relationship between landform and the underlying geology with the formation of the most prominent headland in north Antrim.

No Notes

Approach:The eastern side of the sill is National Trust property and is the area most accessible to the general public via the road down to the shore car park at Murlough Bay. Access to the upper surface of the sill is via rough tracks, one of which follows the cliff edge. The lower margin of the sill is accessible by a path which goes past the old mines near Fall Point, but access beyond this point is difficult because of the block scree. On the west side access is via the coastal road from Ballycastle to Marconi's Cottage and a footpath from the shore near Carrickmore to the top of the cliff at Farragandoo. The Grey Man's Path is the only path linking the upper surface of the sill with the coastal path along its base. All of these access points are dangerous to some extent and require a degree of agility and fitness.
Management:At present there are unlikely to be undue pressures from visitors, either as simply walkers or intending to sample the sill or associated metamorphic rocks. The area is popular with rock climbers. All mining activities around the base of the sill are long since defunct. The area also has a number of archaeological remains, including a crannog on Lough na Crannagh, which undoubtedly enhance the attraction of the area for visitors. Due to its relative remoteness and inaccessibility it is not considered likely that the sill is threatened in terms of its scientific interest. Features are sufficiently commonly developed and on a sufficiently large scale to effectively nullify any sample collection pressures which could arise. Visitors should perhaps be given more explicit warnings about the potential dangers of walking (i) on the upper edge of the cliff, (ii) walking on the lower path by the block scree, and (iii) ascending or descending the cliff via the Grey Man's Path or the path at Farragandhu.
Uses:Land use in the area is hill-farming, predominantly sheep with some cattle in the lower areas.
Emeleus, C. H. and Preston, J. 1969: Field excursion guide to the Tertiary volcanic rocks of Ireland. , pp.70 Queen's University, Belfast
Preston, J. 1981: Tertiary igneous activity In A Geology of Ireland Scottish Academic Press, Edinburgh
Wilson, H. E. and Robbie, J. A. 1966: Geology of the country around Ballycastle. Memoirs of the Geological Survey Northern Ireland,
Rec Type ESCR report    
Enterer: E M Porter
Updates: 28 OCT 00
Previous SiteNext Site