Ballycastle shingle

Coal, Sandstone, and Pebbles: Ballycastle, Antrim

in Fresh/Geology/Nature

The Ballycastle area is the only coastal location of Carboniferous coal seams in Ireland. It is also an unusual mix of three different periods of geological change.

It is difficult for an amateur like me to find out much information about the geology of Ireland. I am indebted to the research of others. Much of what I write here comes from the website Earthwise. And also from this excellent booklet, which can be downloaded as a PDF, from the Causeway Coast and Glens Heritage Trust – http://ccght.org/wp-content/uploads/2012/05/geology_booklet.pdf

Coal mining occurred between the 13th century up until 1967 producing between 10–15 000 tons annually in the 18th Century.  This area corresponds closely with the Machrihanism Coalfield in Kintyre, Scotland. The cliffs between Pans Rock and Carrickmore expose the upper part of the succession. Strata become older from west to east due to minor faults that down-throw to the west.

The eastern end of Ballycastle Beach is part of Ballycastle Coalfield ASSI. Ballycastle Coalfield is the best exposure of a coalfield sequence in Ireland and it contains
a series of Carboniferous sedimentary rocks (335-330 million years old) with contemporary lavas and younger Tertiary igneous rocks (60 M.y.). The sedimentary rocks were deposited in a shallow marine bay which gradually developed into a vegetated coastal swamp subject to periodic flooding by the sea. The vegetation was preserved as seams of coal. Fossils that have been found include goniatites (shellfish), fish remains, giant clubmosses and arthropod insects. The Tertiary dykes have
metamorphosed the carboniferous shales to produce porcellanite and a range of minerals. The site also contains evidence of early industrial activity: the coals and
iron ores were mined between the 16th and 19th centuries.The underlying geology and the spoil heaps give rise to both base rich and acidic habitats, including wet
grassland, base-rich flushes and maritime heath. Limited saltmarsh occurs on some of the beaches.

The cliffs at Pans Rock have been designated as an Area of Special Scientific Interest known as the Ballycastle Coalfield.  On one of the rocks there is also a mysterious face.

 

 

The lowest unit of the Ballycastle Group, the Eglish Sandstone Formation, rests unconformably on the Dalradian and is exposed at Murlough Bay. Grey mudstones 55m above the base in the stream at Ballyberidagh North contain the miospores Spelaeotriletes arenaceus and Grandispora spinosa which first appear in the VF Biozone (Brigantian). So we can see where Ballycastle sits on the map above is an unusual coming together of these three periods of rock formation.

The Carey River Basalt Formation is exposed in the north-facing, roadside cliff opposite Pans Rock. At Murlough Bay two, crudely columnar, flows are exposed on the shore. At least 10 m of the Glenshesk Tuff Formation exposed in rock platforms on the shore west of Fair Head include fireclay with plant remains and lycopod stumps with root systems. Black mudstone (oil shale) of the Murlough Shale Formation is exposed in rock platforms at Portnaloub.

Map above from the  Causeway Coast and Glens Heritage Trust

The Ballyvoy Sandstone Formation is exposed in cliffs between Ballycastle and Fair Head. Although few coal seams are visible their stratigraphical position is well
documented. The Carrickmore Shale Member consists of 13 m of grey mudstone with thin, banded ironstone and limestone and is exposed above the cliff-top waterfall 250 m southeast of Carrickmore. The 20 m thick Main Limestone Member consists of mudstone and thin limestone. The limestone component, which consists of two or three closely spaced beds with a combined thickness of up to 2.4 m, is an important marker horizon and contains the Brigantian brachiopod Gigantoproductus giganteus. Exposures are confined to headlands between the North Star Colliery and Carrickmore and to the rock platform west of the North Star Dyke. An ammonoid fauna including Sudeticeras adeps, of the late Brigantian P2b Biozone, occurs in calcareous sandstone 1.3 m above the top of the Main Limestone Member in cliffs 250m NNE of Corrymeela. McGildowney’s Shale Member is 20 m thick, of which 15 m is lenticular sandstone that forms Pans Rock. The Viséan-Namurian boundary is located at the base of the Main Coal.

Flooding of this desert landscape heralded the beginning of the Carboniferous Period (355 – 290 million years ago), when Ireland was covered by shallow tropical seas and deltas. Sediments from this era comprise limestone and sandstone formed within major river deltas in what is now the Ballycastle area. Abundant growth
in plants in the warm and moist conditions lead to an accumulation of decayed vegetation, forming the coal seams such as those found in Ballycastle.

You can see more about the geology of nearby Fairhead, Torr Head, and Murlough Bay

Ballycastle, Antrim, Shingle
Ballycastle, Antrim, Shingle

The rocks here are mainly sandstones and mudstones with abundant fossil plants and evidence of animal activity. The sediments that formed these rocks were deposited in a delta to shallow marine setting. Vegetation was abundant on the delta and built up into layers of peat that was later preserved as seams of coal. In an area famed for its Cretaceous chalk and volcanic past, pebbles of all shades and combinations with quartz, sandstone, granite it is no surprise to find a wonderful pebble beach.

Reference

Eastern Carboniferous outliers of Ballycastle, Cultra, Castle Espie and Carlingford, Northern Ireland:  http://earthwise.bgs.ac.uk/index.php/Eastern_Carboniferous_outliers_of_Ballycastle,_Cultra,_Castle_Espie_and_Carlingford,_Northern_Ireland

PDF Guidebook to Geology in the Causeway and Glens:  http://ccght.org/wp-content/uploads/2012/05/geology_booklet.pdf

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