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Summary Full report
Tievebulliagh
Rec. Number:14File Number: D12
ESCC:IM
Locality Type:Crag, Crags Status: ASSI
Grid Reference: D194268 Centroid
County: Antrim District:Moyle District Council
Period:Tertiary, Cretaceous
Stages:Eocene, Palaeocene, Santonian
Lithostrat:Antrim Lava Group, Interbasaltic Formation, Lower Basalt Formation, Ulster White Limestone Formation
Site Description

Tievebulliagh, Co. Antrim, from the north-east; the old track below the hill ends at the gravel pit in the main scree.
Highlights:
An occurrence of baked laterite and lithomarge (porcellanite) exploited by Neolithic Man for his stone artifacts. The type locality for a full understanding of the geological processes involved in its origin.
Introduction:
Tievebulliagh Hill (D194268), is a conspicuous crag of olivine dolerite at the edge of the Antrim (basalt lava) Plateau 4.5 km WSW of Cushendall. It overlooks the Tavnaghorna Burn, a small tributary stream in Glenballyemon. The dolerite intrusion is an inclined plug or sheet which intersects the Tertiary lavas of the Lower Basalt Formation. A high temperature and low pressure thermal metamorphism recrystallised the basalt lavas at the contact. Crystal settling of early formed olivine has enriched the rock along the footwall. A large mass of laterite and lithomarge from the overlying Interbasaltic Formation has slipped down the footwall, ploughing into the olivine-rich substrate and has been baked into a tough fine-grained porcellanite. This material was quarried by Neolithic Man and worked into stone axes; flakes and fragments of the rock abound in the scree.
E.E. Evans discovered the in situ source of the stone axes in the 1930s (Jope 1952). Tomkeieff gave a general description of the intrusion in 1940 but failed to find the porcellanite outcrop. Morey and Sabine (in Jope 1952) offered a petrographic review of the porcellanite axes. Agrell and Langley (1958) gave the first definitive account of the intrusion form and the origin of the metamorphic assemblages.
Knowles (1903 & 1906) first described the stone axe factory site, Jope (1952) dealt in detail with the form and distribution of the Tievebulliagh axes and Sheridan (1986) updated this last work and listed the known dating of axe finds. Mallory (1990) made trial excavations to locate the working floors.
Description:
The olivine dolerite intrusion appears as an inclined disc-shaped mass with a N-S strike and a dip of some 45 deg. to the E which adheres to the basalt scarp face. The outcrop is some 90 m across and about 18 m thick. It is characterised by a well developed joint pattern at right angles to the footwall; because the dolerite/basalt contact cannot be traced laterally along the scarp in either direction, the intrusion is regarded as an inclined plug rather than a sheet. A geophysical survey with a proton magnetometer (Preston, unpublished work) failed to find any contacts against the underlying Cretaceous White Limestone; in consequence the exact nature of the volcanic conduit is somewhat uncertain.
The dolerite adjacent to the footwall is enriched in olivine, these picrodolerites are 1-2 m thick. Along its southern margin the igneous rock can be traced through a vertical distance of some 20 m and the fresh and weathered basalt lavas within 7-8 m of the contact have been thermally altered to glassy-looking rocks composed of hypersthene, plagioclase and cordierite. The northern margin is almost continuously exposed for some 18 m below the scarp top. Here the picrodolerite contains xenoliths of modified porcellanite and is itself altered to a picotite-olivine-plagioclase rock. The metabasalts are grey mullite-rich porcellanites derived from lithomarge. Below the dolerite and at the top of a narrow gully in the scree, coarse-grained dolerite veins the picrodolerite, both rocks contain porcellanite xenoliths and are themselves modified by contamination. Some of the structures seen here suggest slumping of this olivine-rich, basal layer down the inclined footwall of the conduit.
The main outcrop of metamorphosed laterite and lithomarge occurs some 90 m below the hill summit, at the base of the dolerite crags and above the grass-covered scree. Its size and nature would indicate an origin in the main interbasaltic horizon (The Interbasaltic Formation) of laterite and lithomarge which has been eroded from the lava succession in this region but which could have foundered into the active volcanic conduit and by sliding down the lower contact disturbed the early settled olivine-rich cumulates. This mass of porcellanite measures some 5.5 m x 1.7 m and was quarried by early man for his stone artifacts. Trenching revealed another 2.8 m of porcellanite below the scree.
The porcellanite
Porcellanite at main outcrop.
from the main outcrop falls into several mineralogical groups. Most of the porcellanite at Tievebulliagh was derived from original lithomarge and the mineral assemblage is characterised by iron-corundum/haematite/cristobalite or tridymite with accessory pseudobrookite and magnetite. The coarser-grained rocks at the igneous contact are more varied in their make up; assemblages of mullite/hercynite/cristobalite or tridymite with accessory ilmenite, magnetite, and sometimes pyrite, rutile or quartz are common and may contain essential cordierite or lack the hercynite. Original laterite is now an iron-corundum/haematite assemblage with accessory pseudobrookite and magnetite which changes to corundum/hercynite with ilmenite and magnetite of coarser-grain size against the igneous contact. A somewhat more aluminous laterite altered into a similar mineral assemblage but with essential mullite. The coarser-grained rocks show a lower oxidation ratio and some are obviously metasomatised and enriched in cordierite and bytownite.
The mineralogy is of special interest and is treated in detail in the literature. Mullite may occur as pure aluminium silicate or as an iron-rich variety. Corundum, too, may be the pure alumina or an iron-rich variety. Haematite, magnetite, ilmenite, and the spinel minerals - pleonaste and hercynite are the opaque iron oxides. Pseudobrookite, and rutile contain titanium oxide. Sapphirine, cordierite, bytownite are silicates. The silica minerals are represented by low-cristobalite, low-tridymite, or low-quartz.
Importance:
This site is the type locality for the first definitive account of the pyrometamorphism of laterite and lithomarge. The rock type, a tough fine-grained porcellanite, was first discovered by Neolithic man who established a stone axe factory site on the outcrop and scree, where an abundance of worked flakes can still be seen.
Porcellanite flakes in scree below main outcrop.
Similar material and another stone axe factory occur on Rathlin Island at the Brockley plug (D122523) but the material is not described in the same detail.
Interpretation:
Agrell and Langley (1958) have detailed the chemical reactions by which the metamorphic mineral assemblages have been generated. For the south contact the mineralogy of the weathered basalts (weathered in Eocene times), has been reconstructed. Hypersthene is derived from the alteration products of olivine and augite chiefly serpentine, iddingsite and saponite. Cordierite has grown from Mg-Al montmorillonites in the more aluminous rocks in the weathered profile of the lava flows, or from chlorite and halloysite. Mullite is the recrystallization at high temperature and low pressure of aluminium-rich minerals such as halloysite.
The original mineralogy of the main porcellanite outcrop was probably metahalloysite, gibbsite, haematite, and anatase, with minor amounts of goethite, magnetite, maghemite, rutile, chamosite, chlorite-type minerals, ilmenite, and quartz. The metahalloysite and gibbsite are replaced by mullite plus a silica mineral. Gibbsite could give rise to corundum. Haematite and anatase may have contributed towards pseudobrookite. Some of these minerals can be used as geological thermometers.
Conclusions:
The outcrop on Tievebulliagh Hill exposes a remnant of an unusual inclined volcanic conduit through the Tertiary basalt lavas. Fresh and weathered (during early Tertiary times) basalt are altered to high temperature and low pressure hornfelses and porcellanites. Porcellanites of varied mineralogy were formed from a mass of laterite and lithomarge derived from the main interbasaltic horizon. Mullite, corundum, cordierite, spinels, cristobalite and tridymite are some of the unusual minerals which occur in the rocks.
Notes:

LAND USE

Neolithic man first exploited the porcellanous rocks. The outcrop of the Cretaceous White Limestone to the north is pockmarked by numerous early nineteenth century chalk pits. Larger mounds in line with the Cretaceous outcrop at the base of the main scree
Hummocky ground below main scree (possible Neolithic spoil heaps).
may be natural features, neolithic spoil heaps, or a consequence of further exploitation of the limestone by the local farmers.
This site is also important for Mineralogy and Metallogenesis. See Key Site 535 - Tievebulliagh.

Keywords
Minerals:Anatase, Bytownite, Chamosite, Chlorite, Cordierite, Corundum, Cristoballite, Gibbsite, Halloysite, Hematite, Hercynite, Hypersthene, Ilmenite, Maghemite, Magnetite, Metahalloysite, Montmorillonite, Mullite, Olivine, Picotite, Plagioclase, Pseudobrookite,
Rocks:Basalt, Dolerite, Porcellanite
FossilGroups:No data
Fossil List:
Products:
Structures:plug
Relations:Contact, Intrusion
Geomorph:laterite
Paleoenv:
NonGeol:A Neolithic quarry and stone axe factory site. 19th century chalk pits are also located on the site.
Measurements
Length:No dataWidth:90 mHeight:No data
Depth:18 mArea:No data  
Access
Approach:This is an isolated, remote site and not often visited. Access is by a peat digging track. Transport can usually be taken to the intake some 200 m below the hill summit and 1.5 km away, and when the track is in good condition it is possible to drive to within 0.5 km of the summit, and only some 100-150 m below it.
Restrictions:Not entered
Planning: 
Management:This is an isolated, remote site and not often visited. The main scree yields ample examples of all the main rock types. The main porcellanite mass and associated metasomatised hornfelses are difficult to sample. The dolerite/basalt lava contacts are now obscured by peat and scree. Most of the archaeological material was removed by Knowles from the hill summit area and many of the roughout stone axes are housed in the Ulster Museum; the screes abound in flakes but roughouts are rare. Most porcellanite flakes occur by wastage of the grass covered scree. The underlying Cretaceous White Limestone has been quarried in a series of small pits along the outcrop and it is possible that this source of lime could be exploited in the future. Any exposure of the igneous rock in contact with the limestone could yield Scawt Hill type material and should be welcomed. Future research could add new minerals to those already listed. The metasomatic rocks and the picrodolerites have been the subject of initial investigations, and hold potential for further work.
Development: 
Threats: 
Uses
Uses:The site is wholly above the intake and the rough pasture is grazed by sheep. The base of the main scree is currently being worked for road gravel.
Potential:Further lime quarrying; excellent potential for further research.
Educ. Level:Undergraduate / Postgraduate
References

Map(s):

See the 1:50,000 O.S. Sheet 5 (Ballycastle) and the old (31.1.69) ASI map of / the area.

Map No:None entered
Rec Type Environment Serv. GIS, ESCR report Recorder:  
Enterer: Crowther, P.R.
Updates: 3 May 2003 / 24 JAN 01 / 21 FEB 97 / 03 FEB 97 / 28 JAN 97 / 23 DEC 96 / 12 NOV 96 / 05 NOV 96 / 07 JUN 96
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