ESCR - Rathlin Island

Earth Science Conservation Review

Summary Full report
Rathlin Island	Antrim

Site number:	519
ESCC:	EC
Locality Type:	Coastal section, Inland exposure	Status:	ASSI
Grid Reference:	D150510 Centroid
Google maps:	55.2929,-6.18968
County:	Antrim	Council area:	Causeway Coast & Glens Borough Council
Period:	Tertiary
Stages:	Eocene, Palaeocene
Lithostrat:	Antrim Lava Group, Lower Basalt Formation

Site Description
Highlights:
Rathlin Island is largely underlain by lavas of the Lower Basalt Formation. These Lower Basalts are succeeded in places by laterites of the Interbasaltic Formation and overlying lavas of the Causeway Tholeiite Member. The localities at Doon Point and Little Causeway on the east coast are good examples of columnar lavas of Lower Basalt age, and the outcrops at Maddygalla show a so far unique example of a feeder dyke associated with fissure eruption of lavas.
Introduction:
The geological succession on Rathlin comprises Cretaceous Chalk which appears on the foreshore and in the lower cliffs for approximately 6km along the southern coast of the island. The succeeding Lower Basalts underlie the remainder of the island and are in turn succeeded by laterites and Causeway Tholeiites of the Interbasaltic Formation. The main outcrop of the Causeway Tholeiites is in the western part of the island, with much smaller occurrences on the north coast near Atlachuile Bay and at the south of the island near Doon Bay. Two small dolerite plugs of Tertiary age occur at Brockley in the west of the island. The southernmost of these plugs has altered the Interbasaltic laterite to porcellanite and these outcrops were the source of widely distributed axe heads in Neolithic times. The localities at Doon Point (D158488) show remarkable development of columnar structure in the Lower Basalts, showing many similarities to the columnar structure of the Causeway Tholeiites, and similarly at Portadonaghy (D498159) colonnade and entablature structures in a flow of the Lower Basalts led to the name "Little Causeway" being applied. The locality at Maddygalla (D157482) described by Preston (1971) is considered to be an episode of fissure eruption along the length of a major erupting fissure, which traversed the Antrim Basalt Plateau, with evidence of eruption of lava and pyroclastic material. Details of the lava succession and occurrences are given in Wilson and Robbie (1966).
Description:
(a) Maddygalla Dyke (D156482) Wilson and Robbie (1966) in their GSNI Memoir "Geology of the country around Ballycastle", record the presence at Maddygalla of exposures of decomposed tuff dipping steeply to the west and which appeared to be dyke-like in form, composed of sub-angular fragments of kaolinised vesicular basalt in a matrix of devitrified glass. The feature has been described in detail by Preston (1971) (see Fig. 1 from Preston) trending at around 045§ in its northern part and around 350§ at the southern end. In width the dyke varies from 3-6m and can be traced for 230m along its length. Cross-sections from Preston (op cit.) detail the multiple nature of the dyke with chilled margins of dolerite 0.5-1.2m wide lining both contacts and enclosing a central zone of tuff. At one place along the NW margin an internal chilled contact within the dolerite can be traced for some 12m and again within the SW margin a thin layer of tuff separates two bands of dolerite. The dolerite margins of the dyke intersect this tuff at right angles but at higher levels of exposure on the shore the SW margin bends over to lie on top of it (see Fig. 1 Preston, op cit.). At the southern end of the dyke the central tuff zone can be sub-divided into an overlying reddish and an underlying grey zone. Both dip to the south west between layers of basaltic rock which are continuous with the surrounding basalt lava (see Fig. 1 Preston, op cit.). (b) Doon Point and Portadonaghy (D158488) (D157498) Wilson and Robbie (1966) describe an extensive flow of olivine-basalt which can be followed north of Doon Bay for around 2km round Arkill Bay to Ruennascarrive. At several places this Arkill flow shows remarkable development of columnar structures, best seen at Doon Point. Here the base of the flow is at about the HWM and passes up into around 10m of crudely columnar basalt, which at the isthmus of the point, passes into a regular colonnade of often hexagonal columns. These columns are radiating and pass into a near-horizontal attitude in places. The regular columns are seen to pass laterally into crudely columnar flows on both sides of the promontory. The flow is clearly LBF in age and occurs below the laterites of the IBF exposed in the cliffs in Doon Bay. At Portadonaghy the same flow shows a similar development of colonnade, and to the north at Portnascorrive and Ruennascarrive, the colonnade is overlain by a clearly defined entablature zone of thinner columns. The locality at Portadonaghy is the 'Little Causeway' mentioned by Haughton (1851).
Importance:
The locality at Maddygalla represents the most convincing example yet described in the Antrim Lava Group of the direct association of feeder dyke and adjacent lava flow. As such relationships are scarce within the whole Tertiary Volcanic Province of Britain and Ireland the features at Maddygalla represent a wider significance than for northeast Ireland alone. The columnar basalts at Doon Point and Portadonaghy, like the example seen at Glen Ballyeman in north Antrim, are good examples of the formation of well-developed colonnade/entablature structures outside the better known examples of the Causeway Tholeiites. They are not of great significance, other than from their curiosity value, as any relatively thick body of magma cooling under static conditions is capable of forming at times quite complex columnar pointing.
Interpretation:
(a) Maddygalla Dyke Connections between dykes and the associated lava flows are important in supporting a fissure eruption origin for the Antrim lavas. Preston interprets the Maddygalla example as indicating initial explosive action along an open fissure to deposit a thin layer of angular tuff over the surrounding area with upward migration of magma and vesiculation to produce lava fountaining. Enlargement of the fissure with voluminous outpouring of lava to feed a surface flow followed with the internal chilled contacts and intercalated marginal tuff developing during intervals between eruptive phases. A return to explosive activity in the waning stages of the eruption left the fissure full of pyroclastic material. The siting of the Rathlin example is considered significant in that it lies in a zone 0.8km wide which includes all the known vents on Rathlin Island as well as magmatic features in Antrim such as the Fair Head Sill and the plugs of Tieveragh, Trosk, Scawt Hill and Ballygalley. This zone, 64km long, follows the general trend of the Antrim dyke swarm and in length and breadth is closely comparable to the crater rows of the neo-volcanic zones of Iceland. (b) Doon Point and Portadonaghy The occurrences in Lower Basalts at Doon Point and Portadonaghy are unusual in that elsewhere in the Antrim Lava Group the colonnade/entablature form of columnar lavas occurs in the Causeway Tholeiite flows of the Interbasaltic Formation. Flows around the Causeway area in north Antrim which show the characteristic colonnade/entablature structure are generally associated with hyaloclastite deposits and it has been suggested that the inundation of the surface of flows by displaced river water modified the internal cooling processes to produce the observed columnar structure. Wilson and Robbie (1966) record similar occurrences around Doon Point.
Conclusions:
The Maddygalla dyke can be considered as a clear example of explosive and magmatic activity along a fissure to produce an adjacent lava flow, possibly as part of a larger scale fissure zone affecting a wide area of northeast Ireland. Relationships between the tuff and dolerite components of the dyke are clearly demonstrated.

Keywords
Rocks:	Basalt
Structures:	columnar jointing, feeder dyke, fissures
Relations:	Extrusion

Access
Management:
The outcrops at Maddygalla are in a relatively remote area on the east coast of Rathlin, accessible only after a scramble down a steep slope. They occur at or below the HWM and are boulder strewn in places and liable to covering by seaweed. No management actions are envisaged and pressure from geological sampling is not perceived to be a problem. At Doon Point and Portadonaghy no management decisions are envisaged. The relatively remote site and difficult access means there is unlikely to be pressure from sampling.

Access

Management:

The outcrops at Maddygalla are in a relatively remote area on the east coast of Rathlin, accessible only after a scramble down a steep slope. They occur at or below the HWM and are boulder strewn in places and liable to covering by seaweed. No management actions are envisaged and pressure from geological sampling is not perceived to be a problem. At Doon Point and Portadonaghy no management decisions are envisaged. The relatively remote site and difficult access means there is unlikely to be pressure from sampling.

References (3)
Haughton, S., 1851. Notes on the Geology of Rathlin Island Journal of the Geological Society of Dublin, vol. 5, pp.130-134.
Preston, J., 1971. The Maddygalla Dyke - Rathlin Island Irish Naturalists Journal, vol. 17 numb. 3, pp.88-92.
Wilson, H.E. & Robbie, J.A., 1966. Geology of the country around Ballycastle. Memoirs of the Geological Survey Northern Ireland.

Record type:	ESCR report
Entered by:	E M Porter

Enlander, I., Dempster, M. & Doughty, P., 2026. Rathlin Island, County Antrim, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=519. Accessed on 2026-11-08.