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Mournes - Eagle Mountain, Slievemoughanmore, Pigeon Rock MountainDown
Summary Full report
Site Type: Cliff, Crag, Crags
Site Status: PASSI
District: Newry & Mourne District Council
Grid Reference: J250240
Rock Age: Tertiary, Silurian (Eocene, Silurian Undifferentiated)
Rock Name: Granite
Rock Type: Granite, Hornfels, Pegmatite
Minerals: Biotite, Diopside, Horneblende
Other interest: drusy, ring-dyke, stoping, Contact, Intrusion, Sheep farming, outdoor pursuits.

Summary of site:

The Mourne Mountain granites were formed by five separate pulses, three from a centre in the east (numbered G1 to 3) and two from another in the west (G4 and 5). This account describes the western granites G4 and 5 and their relationship with the surrounding country rock (the enclosing formation) the Silurian hornfelses, in the Eagle Mountain, Slievemoughanmore and Pigeon Rock Mountain area.

These granites were emplaced into the Silurian rocks of the area quietly, without large-scale disturbance and the evidence has been revealed as erosion has stripped off the overlying rocks to reveal the nature of the roof above G4. The best outcrops are around the summits of Slievemoughanmore and Pigeon Rock Mountain where the contact of the country rock with G4 is pin-sharp. There is an area of altered country rock around the contact, its thermal aureole, where the baking effect of the granite destabilised the original minerals, creating alternating bands of bluish biotite-rich and green diopside-rich hornfels. In this same area it can be seen that the complex of intersecting dykes that permeate the hornfels is abruptly truncated at the contact with the granite showing that this phase of activity was completed before the granites were intruded. G5 was emplaced below G4 and the contact between the two is again crisp. G4 is a pink to purple medium-grained rock whereas G5 is blue-grey and finer grained, a contrast that makes recognition of the two easy. The contact is spectacularly exposed in the cliffs on the east side of the ridge incorporating Eagle Mountain and Shanlieve but it can also be identified on the western slopes too. The nature of the junction shows no disturbance of G4 as G5 was emplaced. 51 million years ago after the eastern Mourne centre had been inactive for 5 million years the focus of igneous activity moved west. Above the deep-seated mass of molten rock a massive cylindrical fracture, over 10 km in diameter, propagated upwards within the country rock. Then the cylinder detached and subsided into the magma which surged around it through the fractures to occupy the space above. This first surge cooled to become the G4 granite, in contact with the roof of Silurian hornfels. A short time afterwards, geologically speaking, the process was repeated to emplace G5, except that this time the new roof was the underside of the solidified G4. On each occasion, volcanic gases and fluids gathered under the roof rocks creating local variations in the granites. This type of granite emplacement, called cauldron subsidence, is found throughout the world. These contacts can be seen elsewhere in the western Mournes but it is this area that shows them to best effect in a majestic landscape setting.

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