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Earth Science Conservation Review
| Devonian - Cross Slieve Group; Cushendun to Port Obe | Antrim |
| Site number: | 1146 | ||
| Locality Type: | Coastal section | ||
| Grid Reference: | D252326,D247289 End(s) of linear site, Not Entered | ||
| Google maps: | 55.12524,-6.03736 | ||
| County: | Antrim | Council area: | Causeway Coast & Glens Borough Council |
| Period: | Devonian | ||
| Stages: | Lower Devonian | ||
| Lithostrat: | Ballyagan Formation, Cross Slieve Group, Cushendall Formation, Cushendun Conglomerate | ||
| Site Description | |
|---|---|
| Highlights: | |
The outcrop of Lower Devonian rocks in Northern Ireland is confined to NE County Antrim. There the Cross Slieve Group comprises some 1300m of predominantly conglomeratic red-beds and is divided into 3 formations. The best exposures are found in coastal sections between Cushendun and Limerick Point, at Cushendall. All 3 formations are exposed between Cushendun and Port Obe. | |
| Introduction: | |
Spectacular cliff and shore sections between Cushendun and Cushendall expose a wide variety of lithologies that comprise the 3 formations of the Cross Slieve Group, namely the Cushendun, Ballyagan and Cushendall formations. The sedimentary rocks were deposited in a rapidly subsiding, fault-controlled basin that was bounded to the northwest by Dalradian metamorphic rocks and separated from them by a fault (or fault zone) with the characteristics and timing of the Highland Boundary Fault of Scotland. Location: The Lower Devonian rocks in NE Antrim form a wedge-shaped outcrop that extends for about 10km southwestwards from the coast between Cushendun and Cushendall. At Cushendun (D252326) impressive boulder conglomerates are exposed in the wave cut platform north and east of the Bay Hotel and also form precipitous cliffs, to the south, into which is cut the cave system leading to Cave House. Port Obe (D247289) is located some 3.5km south of Cushendun and is accessible via the precipitous path from the car park at Layd Church. The entire coastal section between Cushendun and Port Obe provides many opportunities for studying these sedimentary rocks. Inland exposure is poor and provides little additional information. Bibliographic Review: Wilson (1953) and Simon (1984) are the principal geologists to have studied these rocks and both agreed on their assignment to the Lower Old Red Sandstone facies and a probable Lower Devonian age. General Geology: The Type Area and sole outcrop of the Cross Slieve Group is in NE Antrim. Simon (1984) formalised the lithostratigraphical names in the group but because of the degree of lateral and vertical variability in the sediments he was unable to nominate stratotype sections or localities for the 3 formations. This report pinpoints selected sites, within the ASSI coastal section between Cushendun and Port Obe, where there are clean exposures of lithologies that are typical of each of the formations. The sedimentary rocks dip consistently to the southeast and rest unconformably on Dalradian metamorphic rocks in the green schist facies. The lowest beds of the Cushendun Formation are thin, schist-rich breccias and sandstones that are followed by about 600m of quartzite-rich conglomerates, sandstones and thin mudstones. These may be divided into a lower unit, comprising over 300m of coarse conglomerates, and an upper mixed unit of interbedded conglomerates, coarse sandstones and mudstones. Throughout the Cushendun Formation the sedimentary rocks fine upwards and are succeeded by the Ballyagan Formation consisting of about 160m of cross-bedded sandstones, with occasional thin conglomerates. Overlying and interfingering with these fine- grained sediments are some 600m of coarse conglomerates, containing almost exclusively igneous clasts, of the Cushendall Formation which are the highest beds in the Cross Slieve Group. At the southwest end of the outcrop of the Cross Slieve Group, the red-beds are overlain unconformably by a further sequence of red clastic sedimentary rocks, the Red Arch Formation. Both of these sequences are in turn overlain by the almost horizontal Cretaceous Ulster White Limestone Formation and Palaeogene lavas of the Antrim Lava Group. On the coast at Cushendall the contact between the Cross Slieve Group and the Red Arch Formation is obscured by the Cushendall Porphyry which is interpreted by Simon (1984) as a fault-bounded lava flow sequence of early Devonian age but which did not contribute the igneous debris contained in the Cushendall Formation. | |
| Description: | |
For site descriptions of the 3 formations of the Cross Slieve Group, see the associated Key Site; Key Site 307 - Bay Hotel to the tunnel entrance to Cave House Key Site 308 - South of Cave House Key Site 309 - Port Obe. | |
| Interpretation: | |
Sediments of the Cross Slieve Group were deposited in an early Devonian basin that was initiated and maintained by large scale contemporaneous faulting. At the base of the sequence are 300m of conglomerates of the Cushendun Formation which are overlain by a similar thickness of coarse sandstones with pebble beds. In order to accumulate a substantial thickness of coarse conglomerates it is necessary to have a sediment source area with steep slopes, which is near to the basin of deposition and that these characteristics are maintained over a long period of time. After 300m of sediment accumulation conglomerates of the Cushendun Formation still have a maximum clast size of more than 20 cm. The conglomerates are believed to have been deposited by successive sheet floods on an alluvial fan and the succeeding finer-grained sandstones and pebble beds represent a time when fault-controlled uplift was waning. The precise position of the faulted (Highland Boundary Fault?) margin of the Devonian basin is not known. The origin of the vast quantity of well-rounded quartzite clasts (boulder to pebble size) in the Cushendun Formation conglomerate, from a northwesterly source, is not completely understood although Simon (1984) has postulated a plausible theory. Quartzite beds (formations) are virtually absent in the Dalradian rocks of northeast Antrim, where schists are dominant. It is therefore all the more curious and somewhat inexplicable that schist clasts are confined solely to the thin basal breccia of the formation and are absent from the rest of the conglomerate. Simon believes that during deposition of this formation, the adjacent Dalradian high ground was blanketed by earlier Palaeozoic quartzite-rich conglomerates, thus preventing significant erosion of the underlying rocks and accounting for the absence of schist clasts in the Devonian conglomerates. At present the quartzite clasts are considered to have a polycyclic history but are embedded in a sand matrix of single cycle origin. The sandstones of the Ballyagan Formation are often cross-bedded on a large scale and were deposited by large, probably braided, rivers. Palaeocurrent directions in the sandstones show transport towards the east and northeast, trends which are normal to those obtained from the Cushendun and Cushendall formations. The lowest beds of the Cushendall Formation consist of very coarse breccio-conglomerates that were deposited by proximal braided streams on a steep alluvial fan surface. At higher levels in the formation finer-grained braided stream conglomerates are occasionally interbedded with sandstones of possible aeolian origin. Clasts in the coarse conglomerates consist almost exclusively of intermediate igneous debris. Palaeocurrent directions obtained from cross-bedding, clast imbrication and lateral changes in bed thickness are variable but indicate an overall flow to the northeast. The source of dacitic volcanic debris must therefore have lain to the south of the outcrop of the Cushendall Formation, from contemporaneous lavas which are not now exposed. | |
| Conclusions: | |
The exposures of Lower Devonian rocks, of the Cross Slieve Group, in NE Antrim are unique in Northern Ireland. They are superbly exposed in coastal sections between Cushendun and Cushendall and offer excellent opportunities for the study of a wide variety of sedimentary rock types and sedimentary structures. The Cross Slieve Group comprises about 1300m of largely conglomeratic sediments that are divided into 3 formations. Deposition of the greater part of this sequence was initiated and sustained by contemporaneous faulting at the northern and southern margins of the basin of accumulation. At present there is no biostratigraphic control on the age of the Cross Slieve Group. | |
| Notes: | |
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 |
| Paleoenv: | alluvial fan, braided stream |
| References (2) | |||
|---|---|---|---|
| Simon, J.B., 1984. Provenance and depositional history of the Lower Old Red Sandstone of northeast Antrim. Irish Journal of Earth Sciences, vol. 6, pp.1-13. | |||
| Wilson, H.E., 1953. The petrography of the Old Red Sandstone rocks of the north of Ireland. Proceedings of the Royal Irish Academy, vol. 55, pp.283-320. | |||
| Map(s): | See the 1:50,000 O.S. Sheet 5 (Ballycastle). | ||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Devonian - Cross Slieve Group; Cushendun to Port Obe, County Antrim, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=1146. Accessed on 2026-11-07. |
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