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Earth Science Conservation Review
| Cashel Rock - Caledonide Igneous Complex | Tyrone |
| Site number: | 589 | ||
| ESCC: | EO | ||
| Locality Type: | Crags | Status: | ASSI |
| Grid Reference: | H600808 Centroid | ||
| Google maps: | 54.67218,-7.06997 | ||
| County: | Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Ordovician | ||
| Lithostrat: | Tyrone Volcanic Group, Laght Hill Tonalite | ||
| Site Description | |
|---|---|
| Highlights: | |
The rocks at Cashel Hill include penecontemporaneous tonalite and rhyolite and represent a maturing phase of acid magmatism during the development of an island arc in Central Tyrone. The site offers opportunities to view lithological and deformational variation within the principle lithologies and demonstrate complex contact relationships. | |
| Introduction: | |
Cashel Rock is a prominent hill in otherwise featureless country directly north of the main Omagh to Cookstown Road (A505). Access to the site is by walking northwards across moorland from Leaghan [2600 3810]. The tonalite at Cashel Rock forms part of the more extensive intrusion referred to as the Laght Hill Tonalite. On Portlock's geological map of the area, (1843) the tonalite at Cashel Rock was not distinguished from the surrounding rocks which were described collectively as "metamorphic rocks of hornblendic type". However, in the accompanying text, "syenitic" rocks were noted at several localities including Cashel and nearby Formil. These rocks were clearly distinguishable from the green schists of the surrounding area, and were described as consisting primarily of hornblende, quartz and feldspar. Portlock remarked upon the "confused and imperfect crystallization" of this lithology but stopped short of acknowledging its origin as an intrusive granitoid. On the first edition of the one-inch to the mile geological map (Sheet 26, (Draperstown)) (Geological Survey of Ireland, 1882) the rocks at Cashel Rock were not distinguished from the surrounding "Chloritic, Talcose & Micaceous Schists" and were loosely classified as being of "Lower Silurian age". In the accompanying memoir, Nolan (1884) described "Cashel Hill" as being composed mostly of "very peculiar rocks which show little schistose structure, and are usually massive having in many places the appearance of igneous rocks". Hartley (1933) produced the first detailed lithological map and description of the central Tyrone Ordovician volcanic plutonic terrane. He classified the rocks at Cashel rock as "hybrid rocks" and aplogranite. On the second (1:50,000 scale) edition of Sheet 26 (Draperstown) (Geological Survey of Northern Ireland, 1995), Cashel Rock was classified as tonalite and felsite and mapped as one of a series of a closely related elongated intrusions which includes the Laght Hill Tonalite and Crockyneill Summit. Rocks at Cashel Rock form part of the Tyrone-Girvan Sub-Terrane of the Midland Valley Terrane (Bluck, 1992) and are broadly equivalent in age to the ophiolitic-volcanic associations at Girvan-Ballintrae (Scotland) and Clew Bay (County Mayo) (Hutton et al., 1985). | |
| Description: | |
The main lithology present at Cashel Hill is a greenish grey coarse-grained granitoid (tonalite) composed primarily of white quartz, pale-grey plagioclase feldspar (heavily altered to white mica) and green hornblende crystals. The tonalite has a highly variable appearance across the site. This variability is almost exclusively related to degrees of deformation. In its least deformed state, the tonalite looks like a regular coarse-grained granitoid. In the most deformed state, the constituent minerals in the tonalite are stretched and mechanically broken down. In this state, the rocks have developed a penetrative schistose fabric, which strikes northeast to southwest across the entire site, and it is difficult to recognize any traces of original igneous texture in the tonalite. Close to the summit of Cashel Rock there are outcrops of pale pink and buff coloured, fine-grained acid rocks (felsite). These are weakly flow-bandedlaminated rhyolites or rhyodacites which are part of a series of outcrops (~7km long) extending between Slievemenagh (northeast of Cashel Hill) and Brackagh to the southwest. These rhyolites are very similar to many of the acid clasts which form a major component of the coarse volcanic breccias in the Tyrone Volcanic Group. Flow-banding and lamination in the rhyolite is oriented in a northeast-southwest direction and dips up to 75ยบ both to the northwest and southeast. Like the tonalite, the rhyolite is deformed and has a single, broadly coplanar foliation. Locally the rock is fragmented and forms a silica cemented breccia. The contact between the tonalite and rhyolite is poorly exposed. However a small area of the contact can be seen at the southwestern edge of the main crag, ~50m southwest of the rath near the summit of Cashel rock. This is an intrusive contact which indicates that at least in part, the rhyolitic is in intrusive contact with the tonalite. Tongues of rhyolite clearly cut and elsewhere, partially assimilated xenoliths have been noted within the rhyolite indicating that the rhyolite post-dates the tonalite. | |
| Importance: | |
Cashel Rock is a site of national importance that provides access to top quality outcrops of tonalite and rhyolite (Tyrone Volcanic Group). The site can also be used to demonstrate the high degree of heterogeneity in strain experienced by the tonalite. | |
| Interpretation: | |
The Cashel Rock site contains igneous rocks which formed as part of an island-arc during Ordovician times. Island-arc formation was driven at this time by the progressive closure of the Iapetus Ocean which had separated the north American continental plate (Laurentia) from southern Britain (Avalonia). Arc magmatism was generated by the north-westerly directed subduction of the Iapetus oceanic crust along the Laurentian continental margin. The tonalite and rhyolite at Cashel represent the maturing, cratonization phase of arc development during which acid magmas were generated by fractionation of more basic parental magma. The outcrop pattern of the Laght Hill-Cashel Rock-Slievemenagh Tonalite body is indicative of a lopolith type intrusion comprising three sill-like intrusions linked at depth to a common feeder. The fact that contact relationships indicate rhyolite intrudes and incorporates xenoliths of tonalite is an indication that tonalite emplacement was more or less contemporaneous with formation of the Tyrone Volcanic Group. | |
| Conclusions: | |
Cashel Rock is considered to be a site of national importance. It is a valuable educational and research resource providing access to high quality outcrops of rhyolite and tonalite which represent the maturing phase of development of an important volcanic-arc complex in central Tyrone. | |
| 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 sites within the Tyrone Volcanic Group see the following sites: Key Site 582 - Mveela More Key Site 583 - Creggan 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 other Granitoid sites within the Caledonian Complexes of N. Ireland see the following sites: 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 Cashel Rock, see the following site: Key Site 523 - Cashel Rock, near Creggan | |
| Keywords | |
|---|---|
| Rocks: | Rhyolite, Tonalite |
| Structures: | Caledonian, lopolith |
| Access | |
|---|---|
| Approach: | Access to the site is by walking northwards across moorland from Leaghan [2600 3810]. |
| Management: | |
This site encompasses a number of rocky knolls and outcrops in an area of partially fenced rough pasture. The land is used to graze stock. The exposed, irregular and rocky nature of the topography makes it unlikely that the site will come under direct threat from building development. The state of vegetation and overgrowth of the site is currently at an acceptable level. The exposed and windswept nature of the site is likely to prevent serious overgrowth. In general there is no requirement to restrict hammering and sampling at the site. The exception is the exposure of contacts between the tonalite and rhyolite. This is not extensive and should be preserved as far as possible. The rocks of the Tyrone Volcanic Group and closely associated intrusive granitoids such as the Laght Hill Tonalite 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. | |
| Uses | |
| Uses: | Rough pasture and grazing. |
| 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, 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. Cashel Rock - Caledonide Igneous Complex, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=589. Accessed on 2026-11-06. |
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