ESCR - Creggan - Caledonide Igneous Complex

Earth Science Conservation Review

Summary Full report
Creggan - Caledonide Igneous Complex	Tyrone

Site number:	583
ESCC:	EO
Locality Type:	Crags	Status:	ESCR
Grid Reference:	H623782 Centroid
Google maps:	54.64855,-7.03488
County:	Tyrone	Council area:	Fermanagh & Omagh District Council
Period:	Ordovician
Lithostrat:	Tyrone Volcanic Group

Site Description
Highlights:
The Creggan site consists of a series of outcrops of fragmental volcanic rocks. These breccias and tuffs are part of a close stratigraphic association with the Copney Pillow Lava Formation which is a prominent stratigraphic level within the Tyrone Volcanic Group. The site offers a good opportunity for field study of textures and lithologies in fragmental volcanic rocks that are otherwise poorly exposed in the Tyrone area.
Introduction:
The site is located in fenced pasture beside the Creggan Road (B46) 700m south of the Filling Station at Creggan Crossroads. On one of the earliest geological maps of the area (Portlock, 1843), the rocks at Creggan and in the surrounding area were described collectively as "metamorphic rocks of hornblendic type". Portlock accurately mapped the combined extent of the Tyrone Volcanic and the Tyrone Plutonic Groups and clearly distinguished them from the mica schists of the nearby Sperrin Mountains. On the first edition of the one-inch to the mile geological map (Sheet 26 (Draperstown)) (Geological Survey of Ireland, 1882) the Creggan site was represented as "pyroxenic rocks" loosely classified as "probably of Pre-Cambrian or Upper Laurentian age". In the accompanying memoir, Nolan (1884), referred generally to Creggan site where, "in the vicinity of the RC Church is a peculiar breccia ... in which the pieces and semi-rounded pebbles are chiefly of hard green feldspathic rock with quartz blebs, highly altered slates etc". Nolan was in no doubt that this was a metamorphosed fragmentary rock which he compared with breccias at Broughderg. Geikie (1891) offered a more detailed description of the County Tyrone breccias (including Creggan) in a presidential address to the Geological Society of London. Hartley (1933) published the first detailed lithological map and description of the central Tyrone Ordovician volcanicplutonic terrane. Hartley included the rocks at Creggan in the division he referred to as the Tyrone Igneous Series. The stratigraphy of central Tyrone was revised in the light of resurvey work and the publication of the second editions of Sheet 34 (Pomeroy) and Sheet 26 (Draperstown) by the Geological Survey of Northern Ireland (1979,1995). The Tyrone Igneous Series was subdivided into two distinct components, the Tyrone Plutonic Group and the Tyrone Volcanic Group. These rocks are now interpreted as being part of an ophiolite complex and its volcanic cover, a fragment of Ordovician island arc or back-arc oceanic crust (Hutton et al., 1985). In their regional tectono-stratigraphic context, these rocks form part of the Tyrone-Girvan Sub-Terrane of the Midland Valley Terrane (Bluck et al., 1992).
Description:
The Creggan site consists of several closely spaced crags of weathered rock. The outcrops have a conspicuously fragmental texture which is most visible on weathered and slightly polished surfaces. The outcrops have a crudely bedded aspect with metre scale variations in fragment size. The bedding strikes at 75º and dips at 30º towards the northwest. The precise thickness of the breccia at Creggan is unknown because of lack of exposure however it appears to be at least several metres thick. It is one of several horizons which lie directly beneath and within the pillow lavas in the vicinity of Creggan. Recent field mapping and interpretation (Geological Survey of Northern Ireland, 1995) did not support Hartley's (1933) contention that the breccia lies stratigraphically above the pillow lava. The Creggan breccia consists mainly of fine-grained angular to sub-angular igneous clasts which range in size up to 10cm in diameter. Lithologically, this rock can be classified as lapillae tuff which may have undergone some degree of sedimentary reworking. The most abundant fragmental material is a pale and buff weathering "acid" felsitic lithology which may in part represent devitrified acid igneous glass. A few fragments have an ignimbritic appearance with flattened and deformed devitrified volcanic glass shards (fiamme). Less commonly, the tuff contains fragments of fine-grained greenish black basalt and angular clasts of red chert (jasper). The groundmass of the breccia is poorly sorted and is highly weathered and altered. Viewed in thin section, the rocks groundmass is composed largely of replacement chlorite. Amongst primary minerals still conspicuous in the groundmass are coarse-grained mechanically rounded feldspars. Towards the eastern edge of the outcrop, an undeformed dolerite dyke cuts the breccia. This intrusion is about 1m wide, strikes 164º and dips 74º west. This is one of a suite of similarly orientated, relatively late-formed dykes which cut the pillow lavas in the vicinity of Creggan. The rocks of the Tyrone Volcanic Group have been subject to regional deformation and metamorphism under lower greenschist conditions. Some parts of the outcrop at Creggan (particularly the finer-grained tuff lithologies) have a well-developed schistosity.
Importance:
Creggan is a site of national importance which offers easy access to high quality outcrops of volcanic tuffs and breccia. Volcanic breccia is an important lithological variant within the Tyrone Volcanic Group and appears to be closely associated both spatially and temporally with the Copney Pillow Lava Formation. The site offers a good opportunity to study the principle clast types and textures within the breccia. These features are particularly well displayed on weathered and wind abraded surfaces.
Interpretation:
The volcanic breccia at Creggan is one of several occurrences of coarse-grained fragmentary volcanic rocks closely associated with the Copney pillow Lava Formation in central Tyrone. The outcrops of these rocks are laterally impersistent indicating that they probably formed as a result of localized explosive volcanic activity in the vicinity of vents. The rocks at Creggan (Tyrone Volcanic Group) are part of an extensive sequence of mainly extrusive rocks formed in an island arc or back-arc environment. Arc formation was widespread in northern Britain and Ireland during Ordovician times. The Tyrone Volcanic Group has broad stratigraphic correlatives in the ophiolitic-volcanic associations at Girvan-Ballintrae (Scotland) and Clew Bay (County Mayo). Formation of the island arc and back arc was driven by the progressive closure of the Iapetus Ocean which had separated the north American continental plate (Laurentia) from southern Britain (Avalonia). The magmatism was generated by the north-westerly directed subduction of the Iapetus oceanic crust along the Laurentian continental margin.
Conclusions:
Creggan is a site of national importance. It is a valuable educational and research resource providing access to high quality outcrops of volcanic tuff and breccia within the Tyrone Volcanic Group.
Notes:
For information and references on sites within the Tyrone Basic Plutonic Complex see the following sites: Key Site 580 - Black Rock Key Site 581 - Scalp Hill For information and references on other sites within the Tyrone Volcanic Group see the following sites: Key Site 582 - Mveela More Key Site 584 - Copney Hill Key Site 585 - Beaghbeg Key Site 586 - Bonnety Bush, Altihaskey Key Site 587 - Tullybrick Key Site 588 - Stuhanleanantawey Burn For information and references on Granitoid sites within the Caledonian Complexes of N. Ireland see the following sites: Key Site 589 - Cashel Rock Key Site 590 - Craigballyharky and Craigbardahessiagh Key Site 591 - Craiganawork, Pomeroy Key Site 592 - Tintagh, Slieve Gallion Key Site 593 - Loughmacrory Key Site 594 - Cushendun For general information and references on the Caledonian Igneous Complexes of N. Ireland, and for directions to other regional sites, see the following site: Key Site 595 - Introduction to the Caledonian Igneous Complexes of N. Ireland For more information on the geology of the area around Creggan, see the following sites: Key Site 523 - Cashel Rock, near Creggan Key Site 1214 - Creggan Quarry

Keywords
Rocks:	Breccia, Volcanic, Lapilli Tuff
Structures:	breccia, Caledonian, fiamme
Relations:	Extrusion

Access
Management:
This is a relatively compact site encompassing a number of rocky knolls at the edge of well-managed pasture. The site is unlikely to come under any direct threat from development. The state of vegetation and overgrowth of the site is currently at an acceptable level. However any attempt to vegetate or cover the outcrop could degrade the site. The rocks of the Tyrone Volcanic Group are considered to be prospective for a number of minerals including base metals and gold. Licensed mineral exploration has previously been carried out throughout central Tyrone. The area which includes this site is likely to attract further exploration activity in the future. Some of the best parts of the site are those which have weathered and developed a wind blown and cattle rubbed patina. This is where the fragmental nature of the rock is most clear. Hammering and sampling of such surfaces should be restricted as far as possible. The area of interest at this site is not fenced from the surrounding pasture and site visits almost invariably will involve entering the field which may contain livestock. The site is in full view of the farmstead about 50m further up the lane. Careful management of issues such as safety and continuing access to the site are seen as priorities.

Access

Management:

This is a relatively compact site encompassing a number of rocky knolls at the edge of well-managed pasture. The site is unlikely to come under any direct threat from development. The state of vegetation and overgrowth of the site is currently at an acceptable level. However any attempt to vegetate or cover the outcrop could degrade the site.
The rocks of the Tyrone Volcanic Group are considered to be prospective for a number of minerals including base metals and gold. Licensed mineral exploration has previously been carried out throughout central Tyrone. The area which includes this site is likely to attract further exploration activity in the future.
Some of the best parts of the site are those which have weathered and developed a wind blown and cattle rubbed patina. This is where the fragmental nature of the rock is most clear. Hammering and sampling of such surfaces should be restricted as far as possible.
The area of interest at this site is not fenced from the surrounding pasture and site visits almost invariably will involve entering the field which may contain livestock. The site is in full view of the farmstead about 50m further up the lane. Careful management of issues such as safety and continuing access to the site are seen as priorities.


Map(s):	GEOLOGICAL SURVEY OF IRELAND, 1882. 1:63,360 Geological Sheet 26 (Draperstown). (Dublin, Ordnance Survey for the Geological Survey of Ireland). GEOLOGICAL SURVEY OF NORTHERN IRELAND, 1979. Pomeroy, Northern Ireland Sheet 34. Solid. 1:50,000. (Southampton: Ordnance Survey for the Geological Survey of Northern Ireland). GEOLOGICAL SURVEY OF NORTHERN IRELAND, 1995. Draperstown. Northern Ireland Sheet 26. Solid and Drift Geology. 1:50,000. (Keyworth, Nottingham: British Geological Survey).
Record type:	ESCR report
Entered by:	E M Porter
Last updated:	04 MAR 2001

Enlander, I., Dempster, M. & Doughty, P., 2026. Creggan - Caledonide Igneous Complex, County Tyrone, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=583. Accessed on 2026-11-06.