ESCR - Devonian - Red Arch Formation; Cushendall to Red Arch, Glenariff

Earth Science Conservation Review

Summary Full report
Devonian - Red Arch Formation; Cushendall to Red Arch, Glenariff	Antrim

Site number:	1148
Locality Type:	Coastal section
Grid Reference:	D244276,D243261 End(s) of linear site, Not Entered
Google maps:	55.08056,-6.05208
County:	Antrim	Council area:	Causeway Coast & Glens Borough Council
Period:	Devonian
Stages:	Upper Devonian
Lithostrat:	Red Arch Formation

Site Description
Highlights:
At the southeastern margin of its outcrop in NE Antrim the Lower Devonian Cross Slieve Group is overlain unconformably by sedimentary rocks of the Red Arch Formation. This 400m thick, fining upward sequence of red-beds consists mainly of polymict conglomerates and pebbly sandstones and is divided into 5 informal units. Because there is no palaeontological evidence for the age of this formation, various authors have assigned either all or part of the sequence to an Upper Old Red Sandstone facies, of Upper Devonian age, and may or may not have recognised strata of early Mesozoic, Triassic age. The sediments were deposited on a small alluvial fan complex, with the detritus derived from the northwest. They have been compared with and likened to the Upper Old Red Sandstone of the west of Scotland.
Introduction:
Red-bed sedimentary rocks of the Red Arch Formation are exposed in an almost continuous section between Cushendall and Waterfoot. Only the very highest beds of Unit 5, at the top of the sequence, are not exposed on the coast; they primarily occur in stream sections on the west side of Glenariff. At the north end of the outcrop the Red Arch Formation is in faulted contact with the Cushendall Porphyry. Location: On the low-lying rock platform located between the Lifeboat House and slipway, some 800m south of Cushendall, and the Red Arch, 800m further to the south at the west end of Red Bay, are excellent exposures of the Red Arch Formation. Bibliographic Review: The distinctive nature of these sedimentary rocks and their classification as a lithostratigraphical unit, separate from the underlying presumed older Cross Slieve Group, has been agreed by all geologists who have studied the geology of NE Antrim. Although the age of these beds is a matter of some dispute, there being no biostratigraphical control, the principal protagonists, namely Wilson (1953) and Simon (1984) do agree on the existence of Devonian rocks of both Lower and Upper Old Red Sandstone facies. However Wilson (1953, figure 3) restricts rocks of the latter facies to the coastal section between the Lifeboat House and the position of Unconformity A of Simon (1984), some 400m north of the Red Arch. Thus according to Wilson's interpretation, Unconformity A of Simon is the sub-Triassic unconformity and the succeeding strata, which encompass all of Simon's Units 2 - 5, the red-beds in Glenariff and on the coast around Red Bay east of Waterfoot, would belong to the Triassic Sherwood Sandstone Group. In contrast Simon (1984, figure 3) included all the red-beds on the west side of Glenariff and the complete coastal section between the Lifeboat House and Waterfoot in the Upper Old Red Sandstone, and restricted the Mesozoic rocks to the east side of Glenariff, separated from the Palaeozoic outcrop by a fault. Both Wilson and Simon recognised the existence of the unconformity (A of Simon, p.111) but disagreed on its relative importance in chronological terms. It should be noted that Simon assigned the Red Arch Formation to the late Devonian (Upper Old Red Sandstone facies) on the basis of the lithological similarity, and comparable facies geometry, with reliably dated red-beds in the Firth of Clyde on the west coast of Scotland. In contrast Wilson based his distinction betweeen rocks of the Upper Old Red Sandstone facies (late Devonian) and New Red Sandstone facies (Triassic) on the consistent difference in the angle of dip of strata in the two parts of the outcrop. Bedding in Wilson's outcrop of the Upper Old Red Sandstone below the unconformity is inclined at between 26-35 deg. and, above the unconformity, within the New Red Sandstone outcrop, at between 6-12 deg. This matter is as yet unresolved. General Geology: Sediments of the Red Arch Formation dip towards the east and southeast at between 5 and 40 deg. (Simon, 1984) although the high angle dips only occur adjacent to faults or near the base of the sequence. The 5 informal units recognised in the coastal section are defined by vertical changes in clast and matrix composition. Within the sequence are two unconformities both believed by Simon to be intra-Devonian and of minor significance although Wilson postulated that Unconformity A was the base of the Triassic System in NE Antrim.
Description:
Several specific localities that occur between the Lifeboat House and the Red Arch have been selected as sites typical of the 5 units that make up the Red Arch Formation. A fifth site is located at Glasmullan Townland approximately 4km southwest of Waterfoot. For site specific information see the associated Key Site; Key Site 310 - Wave-cut platform at the Lifeboat House Key Site 311 - Small island 200m south of Lifeboat House Key Site 312 - Coastal exposures 180m north of Red Arch Key Site 313 - The Red Arch and coastal exposures Key Site 314 - Stream section at Glasmullen.
Interpretation:
Sediments of the Red Arch Formation were deposited on a small alluvial fan and consist mainly of polymict conglomerates and pebbly sandstones. Initial rapid uplift of the clast source area was achieved by a NE-SW trending contemporaneous bounding fault whose position cannot have been far from the present outcrops because of the coarseness of the conglomerates and local derivation of much of the debris, particularly the granodiorite clasts which probably came from Cushleake, 8km north of the Red Arch. It is both very difficult to ignore and probably unwise to underestimate the significance of the high angular discrepancy between the inclination of strata lying below and above Unconformity A, on the shore north of the Red Arch and to support without question Simon's contention that, in chronological terms, the time gap is only of minor importance. Nevertheless his interpretation of the palaeo-environment in which these rocks were deposited is still pertinent, and until substantive biostratigraphical control is available, all the rocks of the Red Arch Formation are included in the review of the Devonian System in Northern Ireland. Unit 1 conglomerates were deposited on an alluvial fan, initially under sheetflood conditions and later, when erosional channels become common, in a braided stream environment. Lying above Unconformity A are massive, very coarse conglomerates of Unit 2 (? basal Trias) whose clast content differs from Unit 1 (? late Devonian) conglomerates in the higher proportion of quartzite and vein quartz. Unit 2 sediments were also deposited by sheetfloods on an alluvial fan. Pebbly sandstones of Unit 3 are rich in schist clasts and have a quartz-rich matrix probably derived by reworking of aeolian dunes. Unit 4 conglomerates are full of quartzite debris. Both units originated from streamfloods on an alluvial fan. The nature of Unconformity B in outcrop is much less clear than that of Unconformity A although Simon (figure 7, p.117) illustrated a large, though variable, angular discordance between Units 3 and 4 below, and Unit 5 above the former surface. Once again he deliberately minimised the chronological significance of this, higher, unconformity and still included all units of the Red Arch Formation in the Upper Old Red Sandstone (late Devonian). Unit 5 conglomerates are loosely cemented and schist-rich and rapidly disappear higher in the sequence making way for red- brown, cross-bedded, possibly aeolian, sandstones. The highest strata are red-brown and green laminated mudstones, with desiccation cracks, which accumulated in playa lakes on an alluvial plain during a period of tectonic quiescence.
Conclusions:
The Red Arch Formation in the Cushendall-Red Bay area of NE Antrim offers an insight into the problems of interpretation that frequently confront geologists in the absence of definitive evidence for the age of the rocks. Although only 400m thick, the presence of 2 unconformities within this sequence of red-beds has led to the rocks being assigned ages ranging from the late Devonian (Upper Old Red Sandstone facies) to the Triassic (New Red Sandstone facies). Without the support of biostratigraphical evidence the problem appears intractable at present. Nevertheless the coastal section between the Lifeboat House, south of Cushendall and the Red Arch north of Waterfoot offers excellent opportunities for the study of a wide variety of rock types and geological features.
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
Relations:	Unconformity
Paleoenv:	alluvial fan, braided stream

References (2)
Simon, J.B., 1984. Sedimentation of a small complex alluvial fan of possible Upper Old Red Sandstone age, northeast County Antrim. Irish Journal of Earth Sciences, vol. 6, pp.109-119.
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. Sheets 5 (Ballycastle) & 9 (Ballymena, Larne).
Record type:	ESCR report
Entered by:	E M Porter

Enlander, I., Dempster, M. & Doughty, P., 2026. Devonian - Red Arch Formation; Cushendall to Red Arch, Glenariff, County Antrim, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=1148. Accessed on 2026-11-07.