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Earth Science Conservation Review
| Giant's Causeway - Palaeogene | Antrim |
| Site number: | 61 | File Number: | C94 |
| ESCC: | EC | ||
| Locality Type: | Coastal section | Status: | ASSI |
| Grid Reference: | C947448 Centroid | ||
| Google maps: | 55.24155,-6.51118 | ||
| County: | Antrim | Council area: | Causeway Coast & Glens Borough Council |
| Period: | Tertiary | ||
| Stages: | Eocene, Palaeocene | ||
| Lithostrat: | Lower Basalt Formation, Port Na Spaniagh Member, Tholeiitic Middle Basalt Member | ||
| Site Description | |
|---|---|
| Highlights: | |
The multi-tiered columnar basalt of the first flow of the Causeway Tholeiite Member is exposed at the Giant's Causeway. The thick laterites of the Port na Spaniagh Member and several flows of the Lower Basalts are exposed beneath three flows of the Causeway Tholeiites in spectacular cliff sections to the east of the Causeway. | |
| Introduction: | |
The Giant's Causeway locality is contained within the Giant's Causeway National Nature Reserve (NNR), extending from Portnaboe in the west to Benbane Head in the east, a distance along the coast of some 4 kms. This NNR is contained within the recently designated Causeway Coast Area of Outstanding Natural Beauty (AONB) which stretches from Portrush in the west to near Ballycastle in the east. The principal geological features are the well developed and regular columnar jointing exhibited at the Giant's Causeway, and the sequence of Lower Basalt flows, Interbasaltic laterites and multi-tiered columnar causeway basalts exposed in the spectacular cliff sections along the coast. This locality has been an important geological feature since the 17th century and the early literature is reviewed in Wilson and Manning (1978). Contributions to the volcanology and geochemistry of the area have been made by Tomkeieff (1940), Patterson (1955), Patterson and Swaine (1955) and Lyle and Preston (1993). | |
| Description: | |
A period of volcanic quiescence at the end of the first cycle of volcanism in Antrim (Lyle, 1980) is marked by the development of thick laterite beds, represented here by the Port na Spaniagh Member of the Interbasaltic Formation. This quiescent period was long enough to allow the formation of extensive river valleys by erosion and the development of a vegetation cover; pockets of lignite are found in places on the interbasaltic surface. This dormant phase was interrupted in north Antrim by the eruption of the Causeway Tholeiite lavas within a block bounded by the Tow Valley Fault to the SE of the Causeway coast and the Foyle Fault trending down Lough Foyle to the NW. The first flow of this eruptive phase buried the lateritized Lower Basalt surface, and where river valleys had existed on this landscape the lava was ponded to greater depths than normal. One such valley was at the Giant's Causeway where the first flow is almost 100 m thick due to ponding against the valley sides. The cooling of this thick body of chemically and mineralogically very homogeneous lava led to the formation of a complex set of cooling joints forming spectacular and often very regular columns. Tomkeieff (1940) described these columnar joints in classical architecture terms, using colonnade for the lower, wide and very regular columns, and entablature for the upper zone of narrower, more irregular and often curved columns. The variations in the colonnade/entablature pattern are seen in the exposures along the coastal path in the bogs and cliffs to the east of the Causeway. The first flow encountered standing water in places on the Lower Basalt surface, indicated by the presence of hyaloclastite flow-foot breccias in a number of localities, notably to the east of Port na Spaniagh at Port na Callan. The role of displaced river and lake drainage in the formation of multi-tiered columnar lava flows will be discussed in a later section. Details of the Geology I - The Lower Basalts Flows of the Lower Basalts occur beneath the Interbasaltic laterites in the coastal section to the east of the Causeway, but are generally inaccessible. The path from the visitors centre towards the Causeway passes through Lower Basalts at Windy Gap where they show good examples of spheroidal weathering, and typical Lower Basalt flow junctions are exposed in the crags of the Great Stookan just to the north of Windy Gap on the coast. II - Port na Spaniagh laterite of the Interbasaltic Formation This is readily viewed on the path down from the visitors centre to the Causeway. Here it forms the western slope of the infilled valley on the Lower Basalt surface. The eastern slope of this valley occurs on the eastern side of Port Noffer where the coastal path is roughly at the level of the laterite horizon. Here the relict structures of the weathered Lower Basalt flows can be seen, including large rounded residual masses of partially weathered basalt known as the Giant's Eyes. III - Causeway Tholeiite Flows The Giant's Causeway locality is the colonnade of flow 1 of the Causeway Tholeiite Member and its entablature extends to near the top of the cliff at Aird Snout, where it is overlain by the lower part of flow 2. The Giant's Organ in Port Noffer is the colonnade of flow 1 and here the transition from colonnade to the overlying entablature zone can be clearly seen. Flow 2 of the Causeway Tholeiites can be seen here overlying Flow 1. Dykes cutting through Lower and Causeway Basalts can be seen in a number of localities along the foreshore here, notably the Camel Dyke in Portnaboe and a dyke in Port Reaston seen to traverse Lower Basalts, laterites and Causeway Tholeiite flow. On the western side of Port na Callan a well developed flow-foot breccia consisting of hyaloclastite material and brecciated basaltic pillows has formed under Flow 1 of the CTM as the hot lava flowed into standing water on the lateritized Lower Basalt surface. Up to 3 flows of the Causeway Tholeiites can be identified in sequence within the NNR, dipping very gently eastwards and apparently thinning rapidly to the south. | |
| Importance: | |
The area within the NNR contains important exposures for the interpretation of the geological history of the Antrim Lava Group. This applies particularly to the process of lateritization occurring at the end of the Lower Basalt eruptions, the volcanological conditions that existed for the Causeway Tholeiites and the formation of complex multi-tiered columnar basalts. In conjunction with the features described elsewhere at Craigahulliar Quarry all key exposures for the interpretation of the Tertiary volcanological history of north Antrim are documented. In addition the geochemical evidence available from the Causeway area (Lyle & Preston, 1993; Wallace et al, 1994) allows important relationships with Tertiary volcanic rocks in western Scotland to be established. Evidence available for the formation of colonnade/entablature structures within the columnar basalts allow comparisons to be made with occurrences elsewhere in the world such as the Columbia River Basalt Group, USA. The Giant's Causeway area is one of the few geological sites in Ireland which has a truely national and international reputation and this imparts an importance far beyond any other single locality or section. This status has been recognized by its designation as a World Heritage Site by UNESCO. The locality was important in the advancement of the concepts of volcanology in the late 18th and early 19th centuries. The etchings produced from the water colours of Susanna Drury made it one of the best known geological localities in the world at that time. | |
| Interpretation: | |
The coastal sections around the Giant's Causeway in the NNR record the eruption of the olivine tholeiites of the Lower Basalt Formation, the lateritization of the Lower Basalt surface at the end of the first cycle of vulcanicity (Lyle, 1980), and the re-commencement of volcanic activity with the eruption of the quartz tholeiites of the Causeway basalts. The Lower Basalts show typical flow junctions with lateritized and zeolitized tops and there are a number of good examples of spheroidal weathering. The eruptive conditions of the CTM forming thick, often ponded, flows on the Lower Basalt surface are clearly demonstrated, particularly at the Grand Causeway locality. Here the thickness of Flow 1 clearly increases towards the axis of the proposed in-filled river valley. The occurrence of colonnade and entablature structures within the Causeway flows is probably linked to the inundation of the cooling flow by water from displaced river drainage in the area during eruption (Lyle & Preston, 1993). This water passes down into the hot, still cooling flow, and disrupts and modifies the cooling process within the flow. The effect is to spread the cooling in the upper part of the flow producing the narrow, curvi-columnar zone of the entablature. These columns grow downwards from the flow surface, while the more regular columns of the colonnade grow upwards from the base. The Giant's Organ represents the colonnade/entablature junction of Flow 1. The occurrence of hyaloclastite material in the form of flow-foot breccias at Port na Spaniagh and Craigahulliar Quarry, and the presence of fluviatile or lacustrine sediments also at Craigahulliar, is evidence of the existence of abundant water in the eruptive environment of the Causeway Tholeiites. These occurrences are similar to those seen in the Columbia River Basalt Group in Oregon and Washington States in the USA, where there is well documented evidence of displacement of major river courses by lava flows with attendant hyaloclastite flow-foot breccias and multi-tiered columnar basalts. | |
| Conclusions: | |
The Giant's Causeway is unique in Ireland in terms of the geological features displayed and the spectacular nature of the exposures. It is a major attraction for visitors, including many professional and amateur geologists, and provides much geological information in a form readily accessible to the public. | |
| Notes: | |
An area of some 70 hectares including the cliffs and seashore is owned in north Antrim by the National Trust. The NNR lies within that area. There are visitor facilities and car parks, a minibus service from the Visitor Centre to the Causeway and a footpath system that previously has allowed access along the entire length of the NNR via faults along the cliff top and also at a lower level on the cliffs at approximately the level of the Interbasaltic laterites. Farmland outside the boundary of the NNR to the south is mainly livestock and any signs of quarrying or mining within the NNR have largely been eradicated with time. The whole section runs from the Causeway to Hamilton's Seat via the lower path. The lower path is subject to blocking by rock falls, and it is now recognized that there are serious problems in providing access with sufficient safety to satisfy current legislation with respect to public liability. Conditions at the Causeway are frequently windy; the rocks can be very slippy and the waves very high. Caution is urged at all times. The state of the remaining key localities and sections is generally satisfactory since they are often renewed by rock falls or by remedial work along the coastal paths. The main localities listed earlier provide clear exposures and are generally accessible to viewing and for photography. This site is also important for Mineralogy and Metallogenesis. See Key Site 533 - Giant’s Causeway. | |
| Keywords | |
|---|---|
| Rocks: | Basalt, Tholeiite |
| Structures: | columnar jointing, flow unit |
| Relations: | Intrusion |
| Geomorph: | laterite |
| NonGeol: | Major tourist attraction; Area of Outstanding Natu |
| Access | |
|---|---|
| Approach: | The Giant's Causeway visitor centre is 3 km north of Bushmills. The route is well sign-posted. From the visitor centre the Causeway is about an 800 m walk. A minibus service also runs from the visitor centre to the Grand Causeway. |
| Restrictions: | The locality has been a nature reserve of one form or other for many years. A no-hammer ban is in place. There are access restrictions to some coastal areas beyond the Causeway locality, and particularly along the lower cliff path, due to the danger of rockfalls. |
| Management: | |
An area of some 70 hectares including the cliffs and seashore is owned by the National Trust, and management and conservation of the Causeway Coast is shared by Moyle District Council, the National Trust and the Department of the Environment for Northern Ireland, Countryside and Wildlife. | |
| Threats: | There appears to be a greater vegetation cover on those parts of the Causeway above HWM i.e. those parts receiving most impact from visitors feet; this may need to be removed. |
| References (7) | |||
|---|---|---|---|
| Lyle, P., 1980. A petrological and geochemical study of the Tertiary basaltic rocks of northeast Ireland. Irish Journal of Earth Sciences, vol. 2, pp.137-152. | |||
| Lyle, P. & Preston, J., 1993. The geochemistry and volcanology of the Tertiary basalts of the Giant's Causeway area, Northern Ireland. Journal of the Geological Society of London, vol. 149, pp.109-120. | |||
| Patterson, E.M., 1955. The Tertiary lava succession in the northern part of the Antrim plateau. Proceedings of the Royal Irish Academy, vol. 57B, pp.79-122. | |||
| Patterson, E.M. & Swaine, D.J., 1955. A petrochemical study of Tertiary tholeiitic basalts: the Middle Lavas of the Antrim plateau. Geochimica et Cosmochimica Acta, vol. 8, pp.173-181. | |||
| Tomkeieff, S.I., 1940. The basalt lavas of the Giant's Causeway district of Northern Ireland. Bulletin Volcanologique, vol. 2 numb. 6, pp.89-143. | |||
| Wallace, J.M., Ellam, R.M., Meighan, I.G., Lyle, P. & Rogers, N.W., 1994. Sr isotope data for the Tertiary lavas of Northern Ireland: evidence for open system petrogenesis. Journal of the Geological Society of London, vol. 151, pp.869-877. | |||
| Wilson, H.E. & Manning, P.I., 1978. Geology of the Causeway coast. Memoir for one-inch geological sheet 7. Memoirs of the Geological Survey Northern Ireland, HMSO, Belfast. | |||
| Map(s): | For site location see the 1:50,000 O.S. Sheet 4 (Coleraine). | ||
| Record type: | Environment Serv. GIS, ESCR report, J. Preston l | ||
| Entered by: | Hamilton, M.J. | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Giant's Causeway - Palaeogene, County Antrim, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=61. Accessed on 2026-11-06. |
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