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Portrush - JurassicAntrim
Summary Full report
Site Type: Coastal section
Site Status: ASSI
District: Coleraine Borough Council
Grid Reference: C857410
Rock Age: Tertiary, Jurassic (Lower Jurassic, Palaeogene)
Rock Name: Portrush Sill, Waterloo Mudstone Formation
Rock Type: Dolerite, Hornfels, Shale
Fossil Groups: Ammonite, Bivalve
Other interest: joints, Intrusion, Marine sediments

Summary of site:

In the last quarter of the eighteenth century, the dominant figure explaining the origin of the rocks of the Earth was Abraham Gottlob Werner, a compelling lecturer in the Freiberg Mining Academy. He believed that all rocks were formed by crystallization or precipitation from a primeval ocean and, for the first time ever, he classified all known rock types according to his own system. Werner’s views were revolutionary and he was so influential that students travelled from all parts of Europe to sit at his feet. He and his followers formed the ‘Neptunists’, so called because they believed all rocks originated in the oceans (the realm of the Roman god of the sea, Neptune).

In the late 1760s, an Edinburgh natural philosopher, James Hutton, came to the conclusion that some rocks once existed in a molten state - impossible in a primeval ocean - and that supporting evidence could be seen in the rocks around Edinburgh. Although he was not a teacher, his ideas gradually gained ground and he was supported by an influential group of Scottish intellectuals and other observers who became known as the ‘Plutonists’ (from the Roman god of the underworld, Pluto). These two philosophies were bitterly defended by their adherents but in 1786, when it seemed that the Plutonists were in the ascendant, rocks were found in Portrush on the east side of Ramore Head that appeared to support the Neptunist cause. Dark grey to black rocks containing clearly preserved fossil ammonites were found along a 250m stretch of the shore. They were interpreted by early observers such as Whitehurst, Kirwan and Richardson (Neptunists to a man) as basalts. Since the Plutonists considered basalts to be solidified lava flows, how, the Neptunists argued, could ammonites live in molten rock? No animal could survive such heat so, they asserted, the rocks must have been formed in the primeval ocean - just as Werner had taught. If this were true, it undermined the very fundamentals of Hutton’s and the Plutonists’ explanations. Hutton died in 1797 without seeing the famous ‘Portrush Rock’, as it became known, but his faithful amanuensis, John Playfair, saw specimens soon after and visited the site in 1802. He immediately (and rightly) recognised the rock as a hornfels, an early Jurassic clay baked by its close proximity to a massive sill of molten dolerite immediately beneath. The result was a rock superficially resembling basalt. This view was later upheld by influential observers and has prevailed ever since. So it was that the east coast of Ramore Head was the last battleground of the Neptunists and Plutonists from which the latter emerged triumphant. In an important sense it can be claimed that geology as a science was born from that moment of victory. Beyond its historic significance the site is also important because it exposes one of the thickest dolerite sills (a tabular injection of molten rock) in Northern Ireland, and shows how the effects of heat (thermal metamorphism) altered the surrounding rocks to hornfels. There is also a fine-grained chill zone where the molten dolerite cooled quickly on contact with cooler rock. This is one of the most important cultural sites in international geology.

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