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County Antrim, Northern Ireland

""Unsung Heroes

Ramore Head is located in the busy seaside town of Portrush, Co. Antrim. Now famous for its amusement arcades, ice creams and other seaside treats, in the 18th and 19th centuries it was famous for an altogether different type of ‘Portrush Rock’. 

The ‘Portrush Rock’ was first discovered by Reverend William Richardson, and described in 1799. It was announced as a ‘fossiliferous’ basalt and thus sparked a serious scientific debate that lasted for a number of decades. 

Sill at Portrush

During the 18th century, there were two schools of thought on the origin of igneous rocks. One school, known as Neptunists, thought that these rocks were chemical precipitates from seawater. The other, known as Vulcanists recognised that igneous rocks owed their origin to the crystallisation, or solidification, of molten material.
The interpretation of the rocks at Portrush as a fossiliferous basalt seemed to support the Neptunists theory. After all, here was basalt bearing fossils, something clearly impossible if its origins were igneous. 

In 1802, John Playfair correctly identified the supposed basalts as a ‘stratified stone which had acquired a high degree of induration’. This view was supported by William Conybeare and the Reverend William Buckland in 1816. These three people being amongst the most distinguished geologists of their day. 

Ammonites at PortrushToday, it is accepted that the rocks at Portrush started out as mudstone of Jurassic age (190 million years old. These have since been intruded in to by an igneous rock known as dolerite, about 60 million years ago during the Palaeogene period (known as the Portrush Sill). This has resulted in a thermally altered rock known as hornfels, but the abundant Jurassic fossils, particularly ammonites can still be seen. 

The site is designated as both an Area of Special Scientific Interest and as a National Nature Reserve.

Text: Kirstin Lemon, Geological Survey of Northern Ireland

100 Great Geosites

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Images (top to bottom):
  • Portrush © Anne Burgess (source Creative Commons Attribution-ShareAlike 2.0 Generic License
  • Sill at Portrush © Kirstin Lemon, Geological Survey of Northern Ireland
  • Ammonites © Kirstin Lemon, Geological Survey of Northern Ireland

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October 13 - 21
Theme: 'Earth Science in our lives'