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Earth Science Conservation Review

Summary Full report
Portmuck East - MineralogyAntrim
Site number:539
ESCC:EC
Locality Type:Coastal section Status: ASSI
Grid Reference: D463021 Approximate
Google maps: 54.84572,-5.72243
County: AntrimCouncil area:Mid & East Antrim Borough Council
Period:Tertiary, Cretaceous
Lithostrat:Antrim Lava Group, Ulster White Limestone Formation
Site Description
image: 0539a.jpg Coast opposite Isle of Muck, Islandmagee, Co.Antrim, June 1997.
Coast opposite Isle of Muck, Islandmagee, Co.Antrim, June 1997.
Highlights:
Magnetite-andradite rock at dyke margin, and a gobbinsite occurrence.
Introduction:
The localities of interest here are (i) a 4m wide dolerite dyke through Chalk southeast of Portmuck, at the margin of which the Chalk has been contact metamorphosed with the development of massive magnetite rock containing chalcedony and andradite garnet, and (ii) amygdaloidal basalt boulders near Two-Mouthed Cave along the coastline c.100m further south, in which gobbinsite has been recorded.
Description:
image: 0539b.jpg Coast opposite Isle of Muck, Islandmagee, Co.Antrim, June 1997: 4m wide dyke.
Coast opposite Isle of Muck, Islandmagee, Co.Antrim, June 1997: 4m wide dyke.
The 4m wide dyke is exposed in the intertidal zone at the southern margin of a storm beach and just south of the tombola which joins Isle of Muck to Island Magee. The tombola is itself an interesting geomorphological feature worthy of conservation (see Key Site 1136 - Muck Island Tombolo). The amygdaloidal boulders can only be reached at low tide. Their source is presumably in the cliffs nearby, but these exposures are inaccessible.

The localities are reached by either (i) a hazardous path around the seacliffs from the harbour, leading on to the storm beach, or (ii) crossing pasture fields adjacent to the farm buildings near to the car park. The latter involves crossing several fences and permission from the farmer would be required. The localities are not easily accessible and both would suffer if they attracted the attention of over-zealous mineral collectors.
Importance:
The 4m dyke exposure near Portmuck is probably the best of its kind in Northern Ireland. Two-Mouthed Cave is the second of only two known occurrences of the rare zeolite gobbinsite in Northern Ireland. Emeleus and Preston (1969) also record the occurrence of large pink apatites and well-formed ilmenite in veins in basalt adjacent to a dyke exposed in the intertidal zone at the SE end of the Isle of Muck. This was not examined due to the hazard of crossing the tombola.
Notes:
See also Key Site 1136 - Muck Island Tombolo, Island Magee.
Keywords
Minerals:Andradite, Gobbinsite, Magnetite
Rocks:Amydaloidal Basalt, Limestone
Structures:amygdales, dyke
Access
Approach:Access is via a hazardous path around the seacliffs from the harbour, leading on to the storm beach, or crossing pasture fields adjacent to the farm buildings near to the car park.
Management:
Coastal exposures, sensitive to collection. Improve access via clifftop path.
Record type: ESCR report Recorded by: R. Nawaz and N.R. Moles
Entered by: E M Porter
Last updated:07 MAY 68
 Enlander, I., Dempster, M. & Doughty, P., 2026. Portmuck East - Mineralogy, County Antrim, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=539. Accessed on 2026-11-08.

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