Portrush Rocks

The “Portrush Rocks” discovered on the Lansdowne foreshore in the late 18th Century were of such importance as to make the site one of the most important cultural sites in international geology.

Lansdowne Foreshore Rocks
Lansdowne Foreshore Rocks

These rocks proved instrumental in the scientific argument between Vulcanists and Neptunists, over whether minerals in the ocean crystallised together to form the Earth’s rocks (Neptunism) or if rocks were created either by volcanic activity or the result of compacted sediments (Vulcanism). These two theories were named after the Roman god of the sea, Neptune, and the Roman god of volcanoes and fire, Vulcan, and were extensively debated in the 18th and 19th centuries.

In 1786 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 Neptunist observers as basalts.

Since the Vulcanists 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.

People - John Playfair FRSE FRS
John Playfair FRSE FRS

However, John Playfair, an eminent Scottish geologist saw specimens soon after the death of his mentor James Hutton, a leading proponent of the Vulcanist movement in 1797, and visited the Portrush site in 1802. He immediately (and rightly) recognised the rock as a hornfels.

Playfair showed that the Portrush fossils were originally formed in sedimentary mudstone rock which was later ‘baked’ (or metamorphosed) when an underlying massive flow (or sill) of volcanic rock known as dolerite came into contact with it.  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 Vulcanists 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.