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Rathlin IslandAntrim
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Summary Full report
Site Type: Coastal section, Inland exposure
Site Status:
Grid Reference: D150510
Rock Age: Tertiary (Eocene, Palaeocene)
Rock Name: Antrim Lava Group, Lower Basalt Formation
Rock Type: Basalt
Other interest: columnar jointing, feeder dyke, fissures, Extrusion

Summary of site:

The geological structure of Rathlin Island is very simple and consists of a foundation seen in the cliffs and in Church Bay on the southern side and along a short central stretch of the north coast, of Ulster White Limestone of Cretaceous age, topped by lavas of the Antrim Basalts. The basalts immediately above the contact belong to the Lower Basalt Formation that violently breached the Cretaceous land surface around 62 million years ago laying down extensive lava flows. After this initial phase of eruption there followed a protracted quiescent period lasting perhaps hundreds of thousands or maybe over a million years in which time seasonal rains and a sub-tropical climate rotted the surface flows creating the red laterites of the Port na Spaniagh Member, the prominent red beds of this landscape, preserved in two patches at the eastern end of the island (one around the hill at Cantruan, the second immediately inland west of Doon Bay) and in a further two patches at the west end (east of the high ground of Ballygill North and surmounting the cliffs above the West Lighthouse). Above the Port na Spaniagh Member are the lavas of the Causeway Tholeiite Member, the main columnar lavas of the Giantís Causeway. They form an extensive area, partly faulted in, forming the high ground at the western end of the island and two smaller patches above the red beds already described on the east coast inland from Doon Bay.

There are two geological features of outstanding interest on the island, the first at Maddygalla on the south east coast 1 km north of Rue Point, the second at Doon Point and Portdonaghy, the southern and northern extremes of Arkill Bay. Despite the extensive lava flows and the numerous dyke swards of the Antrim Basalts it has been almost impossible to find examples of dykes directly feeding lava flows. At Maddygalla a complex dyke does exactly that. It varies between 3 and 6 m wide and can be followed for 230 m. It contains evidence of many phases of activity with many contacts where the lava has chilled against earlier cool wall rocks. At one point the dolerite of the dyke can be seen to breach volcanic dusts before bending over to lie on top of them. All the evidence here is consistent with an erupting fissure producing lava fountains for a time before returning to explosive activity in its final stages. The famous columnar basalts of the Giantís Causeway are developed exclusively in the Causeway Tholeiite Member, so the discovery of good columns, often with smaller curved columns forming an entablature above (terms adopted from classical architecture) in the Lower Basalt Formation on Rathlin comes as something of a surprise. All the columns are found in a single flow of olivine basalt extending around Arkill Bay for over a kilometre. The columns are best developed at Doon Point where they are strongly curved and in one place form a radiating mass. It is believed that slow, even cooling in association with the wetting of the upper surface of the flow provided ideal circumstances for column development. The Maddygalla dyke provides the most convincing evidence yet observed of mobile magma flowing through a fissure and then spreading immediately into a lava flow. The trend of this structure also closely aligns with a series of eruptive features and centres including the Fair Head sill and a series of volcanic plugs (feeders for volcanoes and fissures) including Tieveragh, Scawt Hill and Ballygalley forming a zone of activity over 60 km long. This is closely matched to crater rows that can be seen at the present time in Iceland. The columns at Doon Point and Portdonaghy have a curiosity value as the best examples of columnar basalts developed in a formation other than the Causeway Tholeiites. Nothing should be done that dulls the impact of these two sites.

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