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TievebulliaghAntrim
Tievebulliagh, Co.Antrim: view from north.
Summary Full report
Site Type: Inland exposure
Site Status: ASSI
District: Moyle District Council
Grid Reference: D31944268
Rocks
Rock Age: Tertiary (Eocene, Palaeocene)
Rock Name: Antrim Lava Group, Interbasaltic Formation
Rock Type: Basalt, Dolerite, Porcellanite
Interest
Minerals: Corundum, Cristoballite, Hematite, Magnetite, Mullite, Pseudobrookite, Saphirrine, Tridymite
Other interest: plug, No Data

Summary of site:

Tievebulliagh is the prominent hill with east facing cliffs 4 km west of Cushendall, glimpsed in places from the Antrim Coast Road. It is the site of a steeply inclined volcanic plug, a feeder for a long-vanished volcano or possibly a line of eruptive fissures, cutting through the Lower Basalts. It is internationally famous because it is the site of a Neolithic axe factory exploiting an extremely rare rock type created by uncommon geological circumstances.

The general geology of the site is explained in record 14 (also Tievebulliagh) and is summarised here. A little over 60 million years ago massive volcanic activity flooded the Ulster White Limestone karst landscape with flow after flow of highly mobile lava. This activity last for around half a million years when the volcanoes became quiescent for a period of several hundred thousand years. During this time, in a strongly seasonal climate with a pronounced dry season, the surface of the basalts rotted to create a soil called a laterite. It is typically red in colour, 10 to 15 m thick and enriched in iron and alumina by alternate leaching and evaporation. At the end of this long period of dormancy, volcanic activity recommenced with new molten magmas surging up along lines of crustal weakness, breaching the Lower Basalts and burying the lateritic soil under new lava flows. This was the time when the vent above Tievebulliagh was active and the unique rock, so coveted by Neolithic toolmakers, was formed. At some point in the eruptive process the pulsing of the magma in the vent detached a large mass of the Interbasaltic Bed from the wall. It became suspended in the incandescent magma but the flow was not powerful enough to force it to the surface so it subsided down the 45 degree slope where it lodged at considerable depth. In temperatures exceeding 1,000 degrees centigrade, surrounded by basaltic magma, the chemistry of the laterite was altered to create the rock now described as porcellanite. Some of the changes can be appreciated from its colours, white patches are mullite, a rare aluminium silicate only formed at high temperature, yellow areas are corundum, an extremely hard aluminium oxide, streaks of brown contain pseudobrookite, an iron titanium oxide and the dominant black is due to the iron oxide minerals hematite and magnetite. Microscopic examination reveals the rare sapphirine, a magnesium aluminium silicate and two high temperature forms of quartz, cristabolite and tridymite, the latter confirming the very high temperatures involved. The block of porcellanite measures about 8 m by 1.7 m in total but the lower 2.5 m is now obscured by scree. It is completely embedded in dolerite (the solidified magma). The Neolithic toolmakers, employing primitive tools, roughed out the axes at the site and usually finished them elsewhere. The porcellanite is tough, highly resistant to fracture (unlike the plentiful local flint) and was an ideal medium for axe heads at a time of forest clearance. It also takes a keen edge and high polish to create some of the most beautiful of all stone artefacts. They were highly prized and widely traded throughout Britain in Neolithic times. The site is an Area of Special Scientific Interest (ASSI).


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