ESCR - Brown's Bay, Island Magee

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Summary Full report
Brown's Bay, Island Magee	Antrim

Site number:	1197
ESCC:	EF
Locality Type:	Coastal section	Status:	local interest
Grid Reference:	D435028 Centroid
Google maps:	54.85281,-5.76564
County:	Antrim	Council area:	Mid & East Antrim Borough Council
Period:	Tertiary, Cretaceous, Triassic
Stages:	Eocene, Palaeocene, Rhaetian, Santonian, Scythian
Lithostrat:	Antrim Lava Group, Interbasaltic Formation, Mercia Mudstone Group, Ulster White Limestone Formation

Site Description
East side of Brown's Bay, Islandmagee, showing exposures of Ulster White Limestone Formation.
Highlights:
The exposures at Brown's Bay, Island Magee demonstrate the detailed structural and stratigraphic relationships between the sedimentary and igneous rocks. The main value of the site is as a teaching aid particularly for field mapping principles.
Introduction:
The Mesozoic and Palaeogene rocks at Brown's Bay outcrop on the northern end of Island Magee, Co. Antrim. These strata were mapped by the Geological Survey of Ireland in 1867 and described by Hull (1876). Remapping of the area (GSNI, 1994) confirmed the veracity of the earlier maps. Cole et al., (1912) described the Interbasaltic Formation.
Description:
On the eastern side of Brown's Bay, reddish brown variably calcareous mudstones (weathered to soft marls) of the Mercia Mudstone Group are exposed on the foreshore. Mercia Mudstone Formation exposed on foreshore at east side of Brown's Bay. They sometimes carry blocks of basalt, chalk and flint which could represent a small vent agglomerate. There is a faulted contact between the marls and the Ulster White Limestone Formation which is well exposed at low tide. Along the fault green glauconitic greensand, 2-3cm thick, and dark grey mudstones and limestones, 5-10cm thick have been squeezed up and represent slivers of Hibernian Greensands and Waterloo Mudstone formations. The limestones above, probably the Glenarm Chalk Member, are well bedded chalk with numerous large tabular flints. In places red clay with flints is exposed along the chalk-basalt contact. On the western side of the bay, foreshore exposures are an almost complete, 15m thick, section through the Interbasaltic Formation. Interbasaltic Formation exposed on foreshore at west side of Brown's Bay. Just below the first cottage, dark grey zeolitic basalts grade into purple-grey mottled lithomarge; eastwards this becomes red and yellow streaked turning into pale orange bauxite which passes into brick-red structureless bauxite. The Interbasaltic Formation is also exposed above the road to the west of the bay and the Upper Basalt Formation can be examined in a small quarry off the track leading to the SE.
Importance:
The site is of importance in local geology terms illustrating stratigraphic and structural relationships between the sediments and igneous rocks, and enabling the environments of deposition to be demonstrated.
Interpretation:
The age of the Mesozoic and Palaeogene strata at Brown's Bay has not been in doubt since they were mapped during the original geological survey. The rocks of the Mercia Mudstone Group show deposition in inland non-marine seas. They are brought into juxtaposition with much younger sediments, the chalk of the Ulster White Limestone Formation, by a major NNW trending fault, a fracture direction which defines the eastern coast of Co. Antrim. The fault is a complex structure with slivers of the Hibernian Greensands Formation and Waterloo Mudstone Formation caught up in the fault plane; such features are rarely demonstrable. The Ulster White Limestone contains fossils that indicate formation in a marine tropical environment. In places along its upper surface clay-with-flints, a probable remani‚ soil, was developed before the volcanic outpourings. The Interbasaltic Formation represents a period of sub-tropical weathering between the two main volcanic episodes of the Palaeogene of Northern Ireland.
Conclusions:
The site is of local significance as a teaching aid. The relationships demonstrated here can be used for training in field observations and contrasted with other more continuously exposed localities.
Notes:
For information on other Triassic sites see Key Site 1198 - Cloghfin Port, Island Magee Key Site 1199 - Collin Glen Key Site 1200 - Scrabo Hill - Triassic Key Site 1201 - Milltown Key Site 1202 - Drapersfield Quarry Key Site 1203 - Lissan Water Key Site 1204 - Curraghlane. For general information on the Triassic System in N. Ireland see Key Site 1192 - The Triassic System in N. Ireland - Introduction.

Keywords
Rocks:	Basalt, Bauxite, Limestone, Mudstone
Structures:	fault
Relations:	Lacustrine sediments, Marine sediments
Paleoenv:	lacustrine, tropical sea

Access
Approach:	Access to the site is from the public car park and along the shore.
Management:
The only danger to the exposures would be marina and slipway development at the eastern and western sides of bay.
Threats:	Marina and/or slipway development.
Uses
Educ. Level:	Undergraduate

References (2)
Cole, G.A.J., Wilkinson, S.B., McHenry, A., Kilroe, J.R., Seymour, H.J., Moss, C.E. & Haigh, W.D., 1912. The Interbasaltic rocks (Iron Ores and Bauxites) of NE Ireland. Memoirs of the Geological Survey of Ireland.
Hull, E., 1876. The country around Antrim, Larne and Carrickfergus. Memoirs of the Geological Survey of Ireland.

Map(s):	GSNI (1994) 1:50,000 Geological Sheet 21 (Larne).
Record type:	ESCR report
Entered by:	E M Porter
Last updated:	8 Jan 2005

Enlander, I., Dempster, M. & Doughty, P., 2026. Brown's Bay, Island Magee, County Antrim, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=1197. Accessed on 2026-11-05.

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