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White Rocks - TertiaryAntrim
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Summary Full report
Site Type: Coastal section
Site Status: PASSI
District: Coleraine Borough Council
Grid Reference: C887408, C900411
Rock Age: Tertiary, Cretaceous (Eocene, Palaeocene, Santonian)
Rock Name: Antrim Lava Group, Ulster White Limestone Formation
Rock Type: Agglomerate, Basalt, Chalk
Other interest: sill, tuff, volcanic vent, Intrusion

Summary of site:

In addition to their cliff morphology and Cretaceous stratigraphy, the White Rocks also provide important evidence of the violent onset of volcanism in the early Tertiary period around 60 million years ago.

At Stradly Rock in the west there are 5 sites where Ulster White Limestone (chalk) has been explosively disrupted and fallen back to fill the cleared vents with angular rubble of chalk, basalt and volcanic ash which became cemented soon afterwards. The Gulls Point locality to the east is less accessible but is also formed of an exploded mix of shattered chalk and flints resting against the undisturbed chalk of the vent wall rock. A small offshore island is composed of similarly shattered basalt and the first deposit on the limestone land surface is a 6 m layer of coarse, angular basaltic blocks (an agglomerate). There are other examples of explosive rock mixtures in the area at Sheelas Head, Jackstone Cove and Priestís Hole. The nature and position of these rocks provide clear evidence that the first volcanic breaches of the chalk landscape were extremely violent but limited to a few explosive vents. This pattern is repeated further along the coast to the east. They are explained as the products of a combination of gases from rapidly rising molten, basaltic magma and steam explosively generated as the searing magma was injected into near-surface water-saturated chalk. They were blasted out along north west/south east crustal tears. The huge Portrush Sill is the prime candidate as the source of basalt and heat. It is 45 m thick and underlies all of Ramore Head, extends 2 km offshore to the Skerries and the Storks rocks 2 km further east and 1.5 km north of Gulls Point. Such a massive intrusion would have been more than capable of sustaining this explosive degassing. This string of localities provides ample evidence of the opening round of violent volcanism around 60 million years ago before the sustained eruption of the plateau basalts and, together they provide important evidence for the interpretation of events. The addition of the volcanism to the landforms and Cretaceous stratigraphy, cumulatively amount to a coastline of great national importance.

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