Earth Science Conservation Review
| Creggan - Caledonide Igneous Complex | Tyrone |
| Site Type: | Crags |
| Site Status: | ESCR |
| Grid Reference: | H623782 |
| Google maps: | 54.64855,-7.03488 |
| Rocks | |
|---|---|
| Rock Age: | Ordovician |
| Rock Name: | Tyrone Volcanic Group |
| Rock Type: | Breccia, Volcanic, Lapilli Tuff |
| Interest | |
| Other interest: | breccia, Caledonian, fiamme, Extrusion |
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).
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).
| Enlander, I., Dempster, M. & Doughty, P., 2024. Creggan - Caledonide Igneous Complex, County Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=583. Accessed on 2024-12-26 |
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