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Earth Science Conservation Review
| Mountfield Quarry - Structural | Tyrone |
| Site number: | 646 | ||
| Locality Type: | Quarry (disused) | Status: | ASSI |
| Grid Reference: | H535787 Centroid | ||
| Google maps: | 54.65405,-7.1711 | ||
| County: | Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Ordovician, Precambrian | ||
| Stages: | Dalradian | ||
| Lithostrat: | Mullaghcarn Formation, Southern Highlands Group, Tyrone Volcanic Group | ||
| Site Description | |
|---|---|
| Highlights: | |
The site contains unrivalled exposure of the Sperrin Fault and its associated mylonite. | |
| Introduction: | |
Mountfield Old Quarry is located on the SW slope of Mulderg, about 0.8km west of a large working quarry and 0.7km north of Mountfield village (Fig. 12). Access to the site is via the A505 (Omagh to Cookstown) and the B category minor road that runs northwards from Mountfield towards Gortin. | |
| Description: | |
At the top of the northern face of the quarry, a 2-3m thick mylonite marks the major stratigraphic and low-angle structural break, which is the Omagh Fault. In the hanging wall there is a well-developed chloritic mylonite with fragments of semipelite and psammite from the Mullaghcarn Formation (Southern Highlands Group, Upper Dalradian). The footwall consists of massive dark green and grey basic lavas and tuffs of the Tyrone Volcanic Group (Ordovician). The Omagh Fault formed during the relatively late-Caledonian (D3) deformation, which is responsible for the zonal S3 cleavage in the Sperrin Dalradian and the single penetrative cleavage which permeates the Tyrone Volcanic Group. Safety Warning: Close inspection of the mylonite involves potentially hazardous access close to the edge of the quarry. | |
| Notes: | |
For general information on the Structural Geology of Northern Ireland, directions to other Structural Geology sites and an extensive reference list see Key Site 641 - Introduction to the Structural Geology of N. Ireland. See also Key Site 140 - Mountfield Old Quarry [Precambrian Stratigraphic block]. | |
| Keywords | |
|---|---|
| Minerals: | Chlorite |
| Rocks: | Basalt, Mylonite, Psammite, Semi-pelitic rock, Tuff |
| Structures: | fault, mylonite |
| Access | |
|---|---|
| Approach: | Access to the site is via the A505 (Omagh to Cookstown) and the B category minor road that runs northwards from Mountfield towards Gortin. |
| Record type: | ESCR report | Recorded by: | T.P. Johnston |
| Entered by: | E M Porter | ||
| Last updated: | 23/09/03 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Mountfield Quarry - Structural, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=646. Accessed on 2026-11-06. |
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