ESCR - Mournes - Bloody Bridge Cone-sheet

Earth Science Conservation Review

Summary Full report
Mournes - Bloody Bridge Cone-sheet	Down

Site number:	1102
ESCC:	EF
Locality Type:	Coastal section	Status:	ASSI
Grid Reference:	J389276,J390271 End(s) of linear site, Not Entered
Google maps:	54.17904,-5.8726
County:	Down	Council area:	Newry, Mourne & Down District Council
Period:	Tertiary, Silurian
Stages:	Eocene, Silurian Undifferentiated
Lithostrat:	Granite

Site Description
Vertical cliff of Quartz Porphyry, cone-sheet, above Silurian hornfels, which makes the wave-cut platform below and just north of the Bloody Bridge carpark, Mournes, Co. Down.
Highlights:
Member of the Mourne Granite cone-sheet system.
Introduction:
A quartz porphyry sheet intruded into the Silurian hornfels dips westwards underneath the neighbouring granites of the Eastern Mournes. Silurian hornfels at footwall of cone-sheet, north of Bloody Bridge carpark; contact with acid porphyry is irregular and could be largely xenolithic. Hanging wall contact of cone-sheet at northern end of outcrop, well north of Bloody Bridge carpark; contact is irregular and can be traced down the centre line of photo. At least two pulses of acid magma entered the cone-sheet fracture which cuts earlier members of the Mourne Dyke Swarm.
Description:
The Bloody Bridge porphyry is a relatively coarse-grained granitic rock with large phenocrysts of quartz and alkali feldspar. The quartz is somewhat smaller in size than the feldspar, rounded, embayed and full of minute bubbles. The feldspar is also rounded and partly altered to kaolinite and fine-grained mica. Anorthoclase and microcline have been identified. An acid plagioclase, albite- oligoclase, is also present, as are small prisms of hornblende along with small grains of iron ore, biotite and epidote. The groundmass of the porphyry varies from microgranitic to micrographic-spherulitic; quartz was often the last mineral to crystallize filling the final mesostasial cavities. Quartz is thus present in three generations of crystals. The chilled margins are of finer grain size with smaller phenocrysts. A later and finer-grained porphyry invades the main sheet and shows chilled contacts against it. $image: 1102d.jpg Internal contact within cone-sheet, north of Bloody Bridge carpark; darker acid porphyry crowded with feldspar phenocrysts (up to 2cm) is invaded by light-coloured and more sparsely porphyritic component which appears to fill the fracture further north.$ Internal contact within cone-sheet, north of Bloody Bridge carpark; darker acid porphyry crowded with feldspar phenocrysts (up to 2cm) is invaded by light-coloured and more sparsely porphyritic component which appears to fill the fracture further north. Patterson (1943) described late-stage veins cutting the cone-sheet, one of these is a porphyritic granophyre which penetrates the cone-sheet close to, and parallel to its southern margin; the rest are quartzose veins some 100m further N along the rock platform. Akiman (1971) also mentions a 30 cm thick aplitic dyke with a NE trend cutting the porphyry at its S contact but this section is difficult to access. The rock platform further N offers good evidence of this later intrusion. Silurian grits and slates are common as xenoliths, ranging from a few cm to several metres in size; many are almost completely digested or permeated by the host rock.
Importance:
A well exposed portion of the principal cone-sheet associated with the Mourne Granites which contrasts markedly with the Glasdrumman Port section in being multiple instead of composite and in establishing an age relationship with the Mourne Dyke Swarm.
Interpretation:
The structural disposition of this porphyry sheet marks it as a member of the cone-sheet system which encircles the Mourne Granite ring-dyke intrusions. Multiple intrusion of acid magma indicates the repeated use of the one cone-sheet fracture.
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
Rocks:	Hornfels, Porphyry, Quartz porphyry
Structures:	cone-sheet
Relations:	Intrusion

Access
Approach:	Access to the S contact of this cone-sheet is hazardous; an attempt could be made from the Bloody Bridge Car park by taking the path down to the rocky shore from its northernmost corner. Relatively easy access to the centre and upper, N contact can be found by following the main road towards Newcastle and a little used footpath over a gap in the low stone wall just N of the cottage in wooded ground, at J38852722. This path descends steeply to the rocky shore and care must be taken to avoid the deep trench on its S side. The rock platform is high above sea level.

Access

Approach:

Access to the S contact of this cone-sheet is hazardous; an attempt could be made from the Bloody Bridge Car park by taking the path down to the rocky shore from its northernmost corner. Relatively easy access to the centre and upper, N contact can be found by following the main road towards Newcastle and a little used footpath over a gap in the low stone wall just N of the cottage in wooded ground, at J38852722. This path descends steeply to the rocky shore and care must be taken to avoid the deep trench on its S side. The rock platform is high above sea level.


Map(s):	Tomkeieff & Marshall (1935) mark its position and width, Plate XIX. The shore section between HWM and LWM from just S of Broad Cove, Irish Grid Ref. J389276 to just N of Bloody Bridge River, J38952715 would include the whole exposure.
Record type:	ESCR report
Entered by:	E M Porter

Enlander, I., Dempster, M. & Doughty, P., 2026. Mournes - Bloody Bridge Cone-sheet, County Down, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=1102. Accessed on 2026-11-06.