Ramore Head and The Skerries

Also within

Antrim County Causeway Coast Scenic Area

Geology & landscape

Geological Feature: Igneous Intrusion · Geological Period: Jurassic · Rock Type: Basalt, Gabbro

Landscape processes: Volcanic / Igneous

Birds & fish recorded

Birds

Description

Ramore Head and The Skerries Tertiary igneous, Jurassic stratigraphy An occurrence of the igneous rock dolerite of Tertiary age, about 60 million years old M.y., has been intruded as a sill into shales of Jurassic, Lower Lias age, some 190 M.y. This has produced a fine grained, dark rock known as hornfels, which contains abundant fossils, particularly ammonites. Supposedly discovered by Reverend William Richardson, this site was first described in the literature by the Neptunist Kirwan in 1799. With Kirwan's announcement of a find of fossiliferous 'basalt', the 'Portrush Rock' became a critical site in the debate over the origin of igneous rocks. Igneous rocks are now recognised as owing their origin to the crystallisation, or solidification, of molten material. Through the 18th century especially, geological consensus was that these rocks were in fact chemical precipitates from seawater. People holding this view were termed Neptunists while those supporting an igneous origin were known as Vulcanists. By 1771 Nicho1as Demarest had recognised the association between basalts and volcanic structures in France and seeing engravings of the Giant's Causeway, had proposed that the Co Antrim basalts were also igneous in origin. Kirwan's interpretation was therefore of great importance to the Neptunists, as here was a basalt bearing fossils, something clearly impossible if its origins were igneous, but evidently supporting formation in a marine environment. 'Portrush Rock' became known across the entire scientific world of the time. In 1802 John P1ayfair correctly identified the 'supposed basalts' as 'stratified stone which had acquired a high degree of induration', namely thermally altered to hornfe1s. This view was supported by William Conybeare and the Reverend William Buckland in 1816 following a visit to the site. These three persons were amongst the most distinguished geologists of their day. A final salvo of papers by the Reverend William Richardson between 1803-08, dealing in part with the 'Portrush Rock', was the last of fensive of the Neptunists. The increasing weight of evidence from careful field studies throughout Europe, and in particular observations of active volcanoes, inevitably saw the Vulcanists as victors in a major, and often bitter, scientific battle. The Portrush sill, which forms Ramore Head and The Skerries, is of interest in its own right. The chilled contact between the underlying dolerite and the overlying hornfels is exposed in a number of places, although there are numerous examples of the Lias being partially or fully incorporated into the sill material. The sill is unusually complex compared to others in Northern Ireland. It shows a range of features not usually seen within a single intrusion. Such features include clear textural and mineralogical divisions within the sill producing distinctive upper, middle and lower layers. Multiple and variable late-stage veining of the sill has also occurred and includes veining by altered sedimentary material. Possible flow channel structures have also been identified. The nature of the sill is explained in part by fractional crystallisation whereby repeated development of early phase 'solid' minerals leaves a residual magma depleted in certain elements. The upper and lower dolerite layers appear to have cooled and solidified relatively quickly leaving a mobile middle layer depleted in olivine, which is amongst the first minerals of crystallise, and tending towards gabbro, a coarse grained basic igneous rock. Blocks of dolerite in the latter level resulted by units breaking off from the already solid upper mass and settling into the still liquid middle division. The Skerries support a diverse range of breeding seabirds with the most notable being kittiwake Rissa tridactyla, black guillemot Cepphus grylle and eider duck Somateria mollissima. https://www.daera-ni.gov.uk/publications/ramore-head-skerries-assi

Species mentioned