|White Rocks - Cretaceous||Antrim|
|Cliff section near Sliddery Cove, White Rocks, east of Portrush, Co.Antrim, showing Glenarm Chalk on top of Ballintoy Chalk; base of Glenarm Chalk marked by weathered-out bedding plane near base of section, below which is Altachuile Breccia.|
|Site Type: ||Coastal section|
|Site Status: ||PASSI|
|District: ||Coleraine Borough Council|
|Grid Reference: ||C888408, C903413|
|Rock Age: ||Cretaceous (Campanian, Maastrichtian)|
|Rock Name: ||Ballintoy Chalk Member, Ballymagarry Chalk Member, Garron Chalk Member, Glenarm Chalk Member, Port Calliagh Chalk Member, Portrush Chalk Member, Tanderagee Chalk Member, Ulster White Limestone Formation|
|Rock Type: ||Flint, Limestone|
|Fossil Groups: ||Belemnite, Brachiopod, Crinoid, Echinoderm|
|Other interest: ||No data, Marine sediments|
Summary of site:
The Cretaceous rocks on this superb cliff coastline were formed in the second half of the Campanian and at the start of the succeeding Maastrichtian stage, a period of around 10 million years. This was only 10 million years before the end of the Age of Dinosaurs which closed around 65 million years ago at the end of the Cretaceous.
At that time the Portrush area, indeed most of Ireland, formed the bed of a warm, shallow sea blanketed in a white, chalky ooze composed of the minute plates of a group of planktonic marine algae called coccoliths. These oozes lithified into the rocks now named the Ulster White Limestone Formation, which are subdivided into 14 members. The White Rocks area exposes 6, possibly 7, of these divisions (the Ballintoy, Glenarm, Garron, Portrush, Ballymargarry, Tanderagee, and Port Calliagh Chalk Members). Three of these members are of particular importance in this area, the Portrush Chalk, the Ballymargarry Chalk and the Tanderagee Chalk, all exposed in the faces of Long Gilbert Quarry. All geological formation names are based on particular exposed sections of rock, ideally showing the full thickness of the section, its main features but particularly defining its base; such sections are described as type sections or stratotypes. The stratotypes of these three members are in the quarry.
The lowest, the Portrush Chalk Member, over 14 m thick, is exposed on the eastern quarry face and is easy to recognise because it is sandwiched between two units containing massive flints. It has been subdivided into four beds determined by their characteristic flint bands and the presence or absence of certain fossils.
The second, the Ballymagarry Chalk Member, is 11 m thick and divided into 3 beds, the first with 9 levels of nodular flints, the second, unmistakeable because it contains the largest flint nodules anywhere in Northern Ireland as well as a few paramoudras (enigmatic, large cylindrical to flask-shaped, hollow flints standing erect in the beds). The third bed lies immediately beneath the prominent Long Gilbert Flint Band. The member is best viewed on the south face of the central quarry.
The third, the Tanderagee Chalk Member, has 5 divisions and is about 14 m thick. Again a combination of diagnostic flint bands and fossils is employed in the recognition of the divisions with long belemnites featuring for the first time.
The Portrush Chalk has many wavy bedding planes suggesting considerable disturbance of the sea at that time: bedding planes are smoother for the later members.
Two major time stages, the Campanian and Maastrichtian, are present in these rocks, recognised from their different faunas. Here the line of division between them is placed at the base of the Tanderagee Chalk Member at the Long Gilbert Flint Band. The fauna below the flint band is dominated by small sea urchins of the genus Galerites while that above sees the first appearance of long, thin belemnites quickly followed by skeletal fragments of starfish and crinoids.
The importance of the stratotypes gives the sequence from Sliddery Cove up through Long Gilbert Quarry national significance. The area is ideal for demonstrating the three stratotypes and the wide range of coastal cliff features.