Craigahulliar

Also within

Antrim County

Habitats

Fast flowing river and stream

Geology & landscape

Mining / Industrial: Quarry · Rock Type: Basalt, Breccia, Mudstone, Siltstone

Landscape processes: Volcanic / Igneous

Description

Craigahulliar Tertiary igneous The features of geological interest in Craigahulliar are associated with the volcanic activity experienced by this region during the Tertiary period, some 60 million years ago. The Tertiary succession exposed in Craigahulliar quarry consists of the lowest member of the Interbasaltic Formation, the red laterites of the Port na Spaniagh Member, representing the weathered surface of the Lower Basalts. This surface had been weathered and eroded to form a north-south trending river valley with a cover of vegetation, before being inundated by renewed volcanic activity producing the flows of the Causeway Tholeiite Member. In a number of localities, these eruptions moved into lakes or shallow rivers and pillow-lava and hyaloclastite deposits formed as a result of the interaction between hot lava and water, including an excellent example of a flow foot breccia. This shows a 2-3m thick deposit of rounded pillow-like masses of chilled basalt in a matrix of palagonite. Palagonite is the clay alteration product derived from the volcanic glass which formed when the hot lava chilled against the water. Associated with the lava flows are thin deposits of mudstone or siltstone formed as river or lake deposits which contain plant remains. The flow top underlying this sediment shows a remarkable series of small domes and blisters. This occurrence is extremely rare within the Antrim Lava Group as a whole, and this exposure is the first found within the Causeway lavas. Quarry exposures show good examples of the colonnade and entablature sub-divisions which are typical of the columnar structure found in these basalts. These also show very clear horizontal markings known as 'chisel marks'. The precise origin of these marks is still under discussion but it seems certain that they represent stages in the gradual growth of the colonnade columns as the interior of the lava cooled. In places the laterite is overlain with thin pockets of blackish lignite containing abundant leaf and bark fragments and imprints. A range of micro-and macro-fossil remains have been recognised from this horizon including cedar, spruce, pine, hazel and alder. A large volume of this lignite was worked commercially by opencast methods during the 1940s while early attempts utilised the adit method of mining. The first flow of the Causeway lavas erupted over this surface and, in places, cavities have formed, probably the result of gases released from the soil and vegetation. These take two main forms being either elongated cylinders, known as pipe vesicles, or more rounded or almond shaped cavities. The former are often lined with crystalline quartz while the latter are often completely filled with the mineral chalcedony; this is very unusual within the Antrim basalts. A range of other minerals have also been recorded from the basalts including the zeolites chabazite and phillipsite. https://www.daera-ni.gov.uk/publications/craigahulliar-assi

Plant communities