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Earth Science Conservation Review

Summary Full report
Port Obe, CushendallAntrim
Site number:309File Number:D22
ESCC:EF
Locality Type:Coastal section Status: ESCR
Grid Reference: D24702882 Centroid
Google maps: 55.09125,-6.04686
County: AntrimCouncil area:Causeway Coast & Glens Borough Council
Period:Devonian
Stages:Lower Devonian
Lithostrat:Ballyagan Formation, Cross Slieve Group, Cushendall Formation
Site Description
image: 0309a.jpg Twin rock ridges of the Ballyagan Formation protruding from beach at Port Obe, Cushendall, Co.Antrim, with green mudstone forming upper layer of each ridge.
Twin rock ridges of the Ballyagan Formation protruding from beach at Port Obe, Cushendall, Co.Antrim, with green mudstone forming upper layer of each ridge.
Description:
Exposures of the Ballyagan Formation occur on the beach at Port Obe where the small river draining this steep gully emerges onto the beach. The beds dip seawards at about 40 deg. and are exposed in 2 ridges protruding through the shingle and thin sand cover. The rocks consist of lilac to purple-red coarse to medium-grained sandstones with low angle cross-bedding and parallel laminations. At the top of each low ridge is a 15cm thick bed of hard, pale green mudstone whose top surface shows a development of large scale desiccation cracks that are infilled with coarse sandstone from the succeeding bed.
image: 0309b.jpg Top surface of seaward-dipping rock ridge of Ballyagan Formation, showing large scale dessication cracks (ruler 0.14m).
Top surface of seaward-dipping rock ridge of Ballyagan Formation, showing large scale dessication cracks (ruler 0.14m).


Some 50m east of these exposures is a craggy 'island' composed of very coarse conglomerate containing mostly igneous debris that represents the basal beds of the succeeding Cushendall Formation.

On the north side of Port Obe are low cliffs in which the bedding is inclined seawards and the rocks consist exclusively of coarse pink sandstones of the Ballyagan Formation. Cross-stratification on a large scale is evident in these exposures.
image: 0309c.jpg North side of Port Obe showing cross-stratified sandstones of the Ballyagan Formation.
North side of Port Obe showing cross-stratified sandstones of the Ballyagan Formation.


The basal beds of the Cushendall Formation are also exposed some 450m to the north-northeast. Here they consist of very coarse, poorly sorted breccio-conglomerates with a maximum clast size of up to 1m, and a clast component dominated by intermediate volcanic igneous debris of dacitic composition.
image: 0309d.jpg Coastal exposure about 450m NNE of Port Obe, showing massive volcanoclastic breccio-conglomerates at base of Cushendall Formation.
Coastal exposure about 450m NNE of Port Obe, showing massive volcanoclastic breccio-conglomerates at base of Cushendall Formation.
Notes:
For general information and reference list on the Cross Slieve Group see Key Site 1146 - Cross Slieve Group; Cushendun to Port Obe.

For general information and reference list on the Devonian System in N. Ireland see Key Site 1147 - The Devonian System in N. Ireland - Introduction.
Keywords
Rocks:Conglomerate, Mudstone, Sand, Sandstone
Structures:cross-bedding, dessication cracks
Map(s): See the 1:50,000 O.S. Sheet 5 (Ballycastle).
Record type: Environment Serv. GIS, ESCR report    
Entered by: Doughty, P.J.
 Enlander, I., Dempster, M. & Doughty, P., 2026. Port Obe, Cushendall, County Antrim, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=309. Accessed on 2026-11-08.

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