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Port Calliagh-BallycastleAntrim
Port Calliagh, W of Ballycastle, Co.Antrim, showing the type section of the Port Calliagh Chalk Member.
Summary Full report
Site Type: Coastal section
Site Status: PASSI
District: Moyle District Council
Grid Reference: D109420, D120416
Rocks
Rock Age: Cretaceous (Campanian, Maastrichtian)
Rock Name: Ballycastle Chalk Member, Ballymagarry Chalk Member, Port Calliagh Chalk Member, Tanderagee Chalk Member, Ulster White Limestone Formation
Rock Type: Flint, Limestone
Interest
Fossil Groups: Belemnite, Brachiopod
Other interest: No data, Marine sediments

Summary of site:

This fine cliff coastline between Port Calliagh in the west and Ballycastle in the east exhibits the youngest Cretaceous rocks in Northern Ireland. The rocks are white limestones and consist of the last two members of the Ulster White Limestone Formation, the Port Calliagh Chalk and the Ballycastle Chalk. These two members are named and their characteristics are defined from this stretch of coast. Key outcrops of this kind are called type localities (or stratotypes) and are essential to our understanding of the closing stages of deposition of the warm, shallow Cretaceous sea beds of around 75 million years ago.

The Port Calliagh Chalk Member follows immediately after the Tanderagee Chalk and forms a unit 12.4 m thick with 6 separate beds, recognisable from their consistent thickness and diagnostic flint bands. The first four beds are each around a metre thick but the last two at 3.65 and 4.27 are easily differentiated. Green-stained chalk pebbles can sometimes be found about 1.3 m below the final bedding plane. The Ballycastle Chalk Member is 13.7 m thick and best seen in the cliff immediately west of Ballycastle pier. It consists of 7 beds separated by clearly defined bedding planes. Thicknesses are consistent in each bed and range from around 2.5 m for the basal pair to 1.5 further up the sequence. Again the distribution and type of flint is diagnostic, further aided by a crowded bed of brachiopods in the second bed and rotted flints in the final three. The members are not restricted to this area and are also present in the Rathlin Basin, on the Londonderry Shelf and in the Lagan Valley though the Ballycastle/Port Calliagh cliffs are by far the most complete and accessible outcrops. The rocks are broadly dated from their fossils, particularly the belemnites (bullet shaped skeletal rods of extinct squid-like animals and their distant relatives) they contain. In this case the long slender genus Belemnella predominates over the much thicker Belemnitella indicating the time stage called the Maastrichtian. This site is important because it exhibits the youngest Cretaceous rocks in Ireland, the Port Calliagh and Ballycastle Chalk Members and includes the stratotypes of both. It is an essential component of the geological history and heritage of Northern Ireland and should be designated for protection.


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