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Devonian - Red Arch Formation; Cushendall to Red Arch, GlenariffAntrim
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Summary Full report
Site Type: Coastal section
Site Status:
District: Moyle District Council
Grid Reference: D244276, D243261
Rock Age: Devonian (Upper Devonian)
Rock Name: Red Arch Formation
Rock Type: Conglomerate, Mudstone, Sand, Sandstone
Other interest: cross-bedding, dessication cracks, Unconformity, alluvial fan, braided stream

Summary of site:

This series of localities is the subject of a fundamental dating controversy. The Old Red Sandstone and the New Red Sandstone, Devonian and Triassic respectively, share more than a similarity in name. Although separated by more than 125 million years and two geological periods they were formed in similar desert environments producing rocks with a striking resemblance to each other. A further complication in this case is the complete absence of any kind of fossil evidence for dating purposes.

From Cushendall in the north, to a point on the eastern shore of Glenariff in the south, red conglomerates and sandstones can be seen all along the coastline. These are the rocks of the Red Arch Formation which have been the subject of at least two interpretations, by H.E. Wilson (1953) and J.B. Simon (1984). The rocks along this shore have been divided into units numbered 1 to 5 from the base upwards. The base of unit 1 is faulted and not seen but the visible section consists of conglomerates of rounded pebbles in a sandy matrix. Such rocks were formed by a type of flooding frequently seen in desert environments, where flash floods sweep material lying on the desert surface into channelled torrents that spread in broad alluvial sheets over low ground. Later, after the drainage pattern became more clearly defined, the streams became braided (i.e. with many dividing and merging channels). Above unit 1, and defining its top, there is an unconformity (a marked angular discrepancy) between the beds of unit 1 and unit 2. Below the unconformity the beds incline to the south-east at 26-35?; above it the beds are still inclined to the south-east but at the lower angles of 6-12?. Such a strong difference in dip would normally represent a long time interval, sufficient to allow tectonic tilting and then erosion to cut across the inclined beds before the later beds were deposited. This is significant because it is about this unconformity that the interpretations of Wilson and Simon differ. Wilson believes that the rocks below the unconformity are Old Red Sandstone (Devonian) and everything above is New Red Sandstone (Triassic), whereas Simon maintains that all the rocks along this coast are Old Red Sandstone and therefore of Devonian age. Unit 2, above this unconformity, comprises a series of thickly bedded, very coarse conglomerates with a larger proportion of quartzite and vein quartz pebbles than in unit 1. These too are the result of sheet flooding on to a fan spreading over lower ground. Unit 3 (seen north of Red Arch) is composed of pink pebbly sandstones and thin beds of overlapping pebbles arranged rather like fish scales. Many pebbles are of schists (presumed to be from ancient Dalradian rocks nearby) and the sands probably represent dune sands partly washed out in floods. There is no well-defined junction between units 3 and 4 and exposure is poor but a small area about 75m north of Red Arch shows unit 4 to be a massively thick conglomerate lacking bedding planes. There then follows another unconformity, less clear cut than the first, but with some angular differences. Unit 5 (seen at Red Arch) has a weakly cemented, schist-rich conglomeratic base, grading upwards into sandstones, some of which could be dune sands. The topmost beds are brown and green mudstones, deeply penetrated by desiccation cracks tha formed in a mud playa (a desert salt lake) baked under an intense sun. Until fossils are found, or some other means of dating the rocks in this section emerges, the age problem will remain. The raised beach caves eroded into unit 5 at the roadside near the entrance to the quay, and the cuttings on either side of the Red Arch spanning the coast road, have made the southern part of the section a landmark with considerable novelty and aesthetic appeal. There are no current threats.

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