ESCR - Mournes - Eagle Mountain, Slievemoughanmore, Pigeon Rock Mountain

Earth Science Conservation Review

Summary Full report
Mournes - Eagle Mountain, Slievemoughanmore, Pigeon Rock Mountain	Down

Site number:	1101
ESCC:	EO
Locality Type:	Cliff, Crag, Crags	Status:	ASSI
Grid Reference:	J250240 Centroid
Google maps:	54.15029,-6.08685
County:	Down	Council area:	Newry, Mourne & Down District Council
Period:	Tertiary, Silurian
Stages:	Eocene, Silurian Undifferentiated
Lithostrat:	Granite

Site Description
Highlights:
The mountainous area at the head of the Windy Gap or Pigeon Rock River is spectacular both for its scenery and geology. The summits of Slievemoughanmore and Pigeon Rock Mountain offer good exposure of the Silurian hornfels roof to the G4 Granite. The roof zone of the later G5 Granite against the earlier G4 intrusion can be found in the crags of Eagle Mountain.
Introduction:
Richey (1928) figured, in map and section, the hornfels roof to the granite of the Western Mournes but it was left to Emeleus (1955) to recognise two distinct granitic intrusions, G4 and G5, and to map in the roof contact between them.
Description:
The slightly domed roof of Silurian hornfels to the Western Mournes granites is preserved on Slievemoughanmore and Pigeon Rock Mountain. Its inclination is opposite to that which roofs the Eastern Mourne Granites on Slieve Muck. The Silurian sedimentary rocks appear to be undisturbed by the underlying intrusions and show only a limited degree of stoping by the granite. The contact is everywhere knife- sharp and in detail highly irregular. The Silurian sedimentary rocks are baked to hornfels in the vicinity of the granite and colour banding defines two contrasting types; a dark bluish biotite-rich and a pale greenish diopside-rich hornfels. Where the adjacent granite is drusy or pegmatitic the diopside may be altered to hornblende and to a coarser-grained rock. Numerous basic dykes intersect the hornfels but are truncated by the granite; this feature is best seen in the cap of Slievemoughanmore. The two granites of the Western Mournes are readily distinguished in the field. G4, away from its outer margin is typically a pink/purple coloured, relatively medium-grained, non-porphyritic rock; G5, by contrast, is blue-grey, finer-grained and non-porphyritic. Both granites, however, are similar at their chilled margins and may show a granophyric texture and conspicuous drusy cavities and irregular pegmatitic patches. The domed roof of G5 is slightly eccentric to that of G4, its highest point being a little further west. It is very well exposed on the east side of the Eagle Mountain - Shanlieve ridge at the foot of a cliff feature some 30-60 m high. The block scree below this precipitous wall yields fine examples. This southerly inclined contact can be traced to the south for about 1.6 km. On the westerly slopes of Eagle Mountain and Shanlieve this contact can again be traced and north of Windy Gap it again appears on Wee Slievemoughan. There is no obvious disturbance of the G4 Granite by the later intrusion.
Importance:
This area offers clear evidence for the ring-dyke/laccolithic top structure of the Western Mourne Granites and a little evidence for the westerly shift of igneous activity throughout the development of the Mourne Granites as a whole.
Interpretation:
The rocks of this area clearly illustrate the domed tops of two successive granitic intrusions. Emplacement post-dated a phase of dyke intrusion and took place with very little disturbance of the pre- existing rocks. Cauldron subsidence and ring-dyke formation is invoked with some evidence for a continuing shift of igneous activity westwards. Volcanic gases and vapours collected under the roof rocks of each granite mass and modified the rock on either side of the contact.
Conclusions:
Though other sites, as for example those on Rocky Mountain and below the Spelga Dam, offer similar features, this region is far more comprehensive, gives a better insight into the structural form of the Western Mourne Granites, and in its scenic relation to the underlying geology is probably the best in the area.
Notes:
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
Minerals:	Biotite, Diopside, Horneblende
Rocks:	Granite, Hornfels, Pegmatite
Structures:	drusy, ring-dyke, stoping
Relations:	Contact, Intrusion
NonGeol:	Sheep farming, outdoor pursuits.

Access
Approach:	The track from Brown Hill, near Attical, to Windy Gap and that from Leitrim Lodge to Pierces Castle afford relatively easy access. Further and more strenuous climbs are necessary to reach the crucial exposures.


Map(s):	Richey's (1928) and Emeleus's (1955) maps and sections can be consulted. The 1:50000 IGS map offers the most detail on the minor intrusions.
Record type:	ESCR report
Entered by:	E M Porter

Enlander, I., Dempster, M. & Doughty, P., 2026. Mournes - Eagle Mountain, Slievemoughanmore, Pigeon Rock Mountain, County Down, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=1101. Accessed on 2026-11-06.