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Skeagh and CraigclugganAntrim
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Summary Full report
Site Type: Crag, Crags
Site Status: PASSI
District: Larne Borough Council
Grid Reference: D302062, D294081
Rock Age: Tertiary (Eocene, Palaeocene)
Rock Name: Antrim Lava Group, Upper Basalt Formation
Rock Type: Basalt, Dolerite, Olivine Basalt
Other interest: dyke, fissures, flow-banding, phenocrysts, plug, Intrusion

Summary of site:

A plug is a cylindrical feeder to a volcano, filled with solidified lava and volcanic debris and often forms a prominent feature in the landscape. In the Antrim basalts there are several apparent plugs which have an elongate form and all share a north north west/ south south east orientation. Two of these are the plugs at Craigcluggan and Skeagh.

Craigcluggan is composed of olivine dolerite (a medium-grained rock with the composition of basalt with crystals of olivine) and penetrates the Upper Basalts, the last pulse of lava flows of the early Tertiary, dating from around 58 million years ago. East of the plug there are extensive Upper Basalts of the same composition. The Skeagh plug, directly aligned with Craigcluggan and about 2 km to the south, is essentially the same olivine dolerite and is similarly surrounded by Upper Basalts of the same type, confirmed by electron microprobe analysis. A further 3 smaller structures to the west and south west of Aughaboy are also elongate and directly aligned between the two. There is general agreement that these plugs were conduits along the line of an active fissure and represent the final phase of volcanic activity. Their near-identical composition makes it almost certain that they were fed from a common magma chamber along an extensive, near vertical fissure. This fissure, feeding extensive lava fields in a devastated volcanic landscape, is just one of many formed at the time of opening of the North Atlantic when gigantic slabs of crustal rocks moved apart, thinning the crust to the point of fracture allowing lavas to spill out. One plane of weakness was aligned with the Irish Sea and a whole range of fissures developed along this line for which the main evidence survives in Northern Ireland and the west of Scotland. This fracture “healed” after a period of activity but that to the west widened and Greenland separated and drifted west and north as an active mid-ocean ridge became established. Iceland was created along the ridge line and remains active to this day. This set of linked plugs is of international importance providing key evidence of events in the final phase of eruption of the basalts forming the Antrim Plateau, around 58 million years ago. They also inform the processes that operated in the creation of the North Atlantic. Their remoteness offers a high level of protection but designation would reinforce their status.

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