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Earth Science Conservation Review
| Mournes - Gruggandoo Quartz Porphyry Sheet | Down |
| Site number: | 1106 | ||
| ESCC: | EO | ||
| Locality Type: | Inland exposure | Status: | ASSI |
| Grid Reference: | J202258 Approximate | ||
| Google maps: | 54.1676,-6.15955 | ||
| County: | Down | Council area: | Newry, Mourne & Down District Council |
| Period: | Tertiary | ||
| Stages: | Eocene | ||
| Lithostrat: | Granite | ||
| Site Description | |
|---|---|
| Highlights: | |
Age relationship between cone-sheet and granite of the Western Mourne centre. | |
| Introduction: | |
One member of the cone-sheet system surrounding the Mourne Granites is truncated by G4, the earlier granite ring-dyke of the Western Mournes. | |
| Description: | |
Tomkeieff and Marshall (1935) regarded the inclined acid and composite sheets as surrounding the earlier granites, G1, G2, G3, of the Eastern Mournes and being cut off by the G4 granite of the Western Mournes. Emeleus (1955) mapped a number of inclined sheets around the southern and western margins of the Mourne Granites. At Gruggandoo and on Slieve Roe he found porphyritic spherulitic granophyres similar in nature to the 'Hilltown Dyke'. Many of these inclined sheets are linked (see fig. 1 in Tomkeieff and Marshall (1935) and the 1 inch geological map (1978) covering sheets 60, 61, 71, & 72) into a single cone-sheet. The outcrop on Gruggandoo shows a small apophysis of the G4 granite chilled against the sheet intrusion. | |
| Importance: | |
This locality is the only one where cone-sheet and granite come into contact. | |
| Interpretation: | |
The rock relationships provide conclusive evidence that the cone-sheet predates the granite. | |
| Notes: | |
This site will require little management as long as the land use remains the same. Bracken especially will obscure the critical contact during the summer months. For Mourne Granites general information and reference list see; Key Site 1129 - The Granites - Overview. For general information on the cone-sheet intrusions associated with the Mourne Granites, and the Mourne Mountain dyke swarm, see; Key Site 1131 - Cone-sheet and Dyke Swarm - Overview. | |
| Keywords | |
|---|---|
| Rocks: | Granite, Porphyry |
| Structures: | cone-sheet |
| Relations: | Contact, Intrusion |
| NonGeol: | Rough moorland, sheep grazing. |
| Access | |
|---|---|
| Approach: | Access to Gruggandoo via track off Hilltown-Rostrevor road 3.5km south of Hilltown. |
| Map(s): | Emeleus's (1955) fig.1 and the 1 inch geological map of the Geological Survey (1978) illustrate the geology of the locality. | ||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Mournes - Gruggandoo Quartz Porphyry Sheet, County Down, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=1106. Accessed on 2026-11-06. |
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