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Earth Science Conservation Review
| Moyola River | Londonderry |
| Site number: | 115 | ||
| ESCC: | EO | ||
| Locality Type: | Stream section | Status: | ESCR |
| Grid Reference: | H796974,H804975 End(s) of linear site, Not Entered | ||
| Google maps: | 54.81855,-6.7617 | ||
| County: | Londonderry | Council area: | Mid Ulster District Council |
| Period: | Carboniferous | ||
| Stages: | Chadian, Tournasian | ||
| Lithostrat: | Altagoan Formation, Mormeal Member | ||
| Site Description | |
|---|---|
| Highlights: | |
The Lower Carboniferous rocks exposed along the Moyola River near Tobermore have yielded a rich and moderately diverse fauna of fragmentary fish remains, which was described by Portlock during his survey of the area in 1843. It represents one of very few fish-bearing sites of this age in Northern Ireland. A tetrapod jaw recently discovered amongst Portlock's original material is the only early Carboniferous tetrapod known from Northern Ireland and may well have originated from this site. | |
| Introduction: | |
Early Carboniferous rocks are rather patchily exposed in the bed and low on the banks of the Moyola River to the south of Maghera. The exposures in question lie mostly downstream, to the east, of the confluence of the Altagowen River with the Moyola River, between Grid Refs 23/796974 and 23/804975. The outcrops have been assigned to the Mormeal Member of the Altagowen Formation, of late Tournaisian (early Chadian) age. Portlock (1843) described the site and that of the adjacent Altagowen River though in no great detail. He provided figures of several sections along the more deeply incised Altagowen River but only one (Portlock 1843; Plate E, fig. 6) for the Moyola River. The material collected and figured by Portlock appears to have been restricted to isolated teeth, scales and fin spines but encompasses a range of species; recent re-examination of some of Portlock's material in the collections of the British Geological Survey has revealed one specimen, figured by Portlock as Plate 13, figure 13, to be part of the jaw of a tetrapod (J. A. Clack, University Museum of Zoology, Cambridge; pers. comm.), although it is uncertain that this specimen is from the Moyola River. Scales of a rhipidistian collected by Portlock were described by Agassiz (1833-44) as Holoptychius portlocki is an exclusively Devonian genus. The precise horizon of this, and the other species cited by Portlock, was not given in his account. The site otherwise has received no further mention in the literature but the fish remains are the subject of ongoing research by Mags Duncan in the Department of Geology at Trinity College Dublin. | |
| Description: | |
Parts of the Mormeal Member of the Altagoan Formation are sporadically exposed in the bed and banks of the Moyola River to the south of Maghera, Co. Derry. The exposures are more discontinuous than the stratotype section (ASSI-78) and the succession, much broken by small faults, is impossible to reconstruct. Some of the thicker sandstone bands form ridges or small cascades across the river bed but the softer, mudstone units are largely overgrown or only intermittently exposed where the banks are scoured by floods. Within the succession a channel-lag conglomerate is composed, in part, of reworked pedogenic carbonate pellets and contains a rich assemblage of disarticulated fish bones, scales, teeth and fin spines. Higher in the sequence poorly exposed muddy shales have yielded drifted plant debris together with rare xiphosuran material. Samples of a fine sandstone with plant debris have also yielded fragments of scorpion and eurypterid cuticle. Portlock (1843) described the succession in the Moyola River as comprising "shales, grits and impure limestones [which] succeed immediately to the mica schist [=? Dalradian], and are followed by conglomerate beds". A few pages further on in his account he described the succession in more detail, instead considering it to rest unconformably on conglomerates which he assigned to the Old Red Sandstone, exposed in the core of a gentle anticline. The lowest units are hard, grey, thinly bedded coarse sandstones that are very micaceous in their lower part and once were extensively quarried in the area. On the west side of the bank adjacent to the outcrop of grey sandstone he described an exposure of more than 15 feet (5 metres) of shale containing calcareous concretions and divided into several beds by impure limestones. The lower portion of the shale was said to contain abundant scales and teeth of 'Holoptychius', while in one shale bed, rich in limestone concretions, the 'Holoptychius' remains occurred associated with the fin spines and bones of Gyracanthus as well as with plant debris; the stems of some of these plant remains were described as "passing sometimes vertically through the beds". Less fragmentary material referred to as Amblypterus or Palaeoniscus was also said to have been found in this bed. It seems likely that this is the same exposure, or at least the same stratigraphic unit, as that described by Portlock in the Moyola River adjacent to its confluence with the Altagowen River. Here the fossiliferous strata were described as calcareous shales, in which were found fish remains of 'Holoptychius' portlocki, Gyracanthus formosus and Gyracanthus tuberculatus, together with an ostracod referred to as Cypris scotoburdigalensis, a xiphosuran referred to as Limulus trilobitoides, and crustaceans referred to as Dithyrocaris colei and Dithyrocaris orbicularis. The latter was encrusted with a minute Spirobis, as was the large flattened stem of a Calamites. Portlock also figured a small vertebrate fragment which he referred to as Cephalaspis ?, though the 'cephalaspids' [= Osteostraci] are an exclusively Silurian group and indeed all 'armoured' agnathans were extinct before the start of the Carboniferous; its identity, therefore, remains unclear. The bivalve mollusc Posidonomya complanata was cited as the only common fossil in these beds though even then only locally so. To the East of the coarse sandstone outcrop the sandstones and shales were described as appearing again at Forge Town Bridge (grid ref. 23/804975), where they were seen dipping to the east, but found to be less fossiliferous than those to the west. Material collected recently, and currently being studied by Mags Duncan in the Geology Department at Trinity College Dublin, was obtained from just upstream of the old corn mill near the confluence of the Altagowen and Moyola Rivers (grid ref. 23/795974). It comprises abundant small disarticulated fish scales and bones in thin, grey, platy sandstones with conspicuous muscovite mica flakes on bedding planes. Mitchell (1995) provides a fairly thorough description and interpretation of the Altagowan Formation at its stratotype locality in the Altagowen River, where the upper, Mormeal Member is particularly well exposed. The succession there is dominated by grey mudstones with some paler grey siltstones and greenish-grey sandstones which are often rippled and fill small channels. In the Altagowen River several micrite and evaporite replacement beds are present. A rich miospore assemblage recovered from the Mormeal Member of the Altagowen Formation (Owens et al. 1977) indicates a late Tournaisian age. Mitchell (1995) considered it to be post-Courceyan in age and to correlate with at least the early part of the Chadian Stage. | |
| Importance: | |
Although a considerable diversity of elasmobranch remains are known from the Carboniferous Limestone of Co. Armagh, vertebrate remains otherwise are rare in the Carboniferous of Northern Ireland. Other Carboniferous localities designated as potential ESCR sites on account of their vertebrate fossils (Glennasheevar and Ballycastle) have yielded only low diversity or monospecific faunas and hence that from the Moyola River is important for giving some indication of real diversity levels in the marginal early Carboniferous deposits. The recent discovery of a tetrapod jaw amongst Portlock's original material considerably increases the importance of this site; Carboniferous tetrapods are very rare fossils and none have been found previously in Northern Ireland. Although the exact locality from which this particular specimen was obtained is not recorded, vertebrate remains are considerably more abundant at the Moyola River locality than any of the other correlative sites mentioned by Portlock. Hence, even if the specimen proves to have originated from one of the other sites, the Moyola River exposures offer the best prospects of finding further material. | |
| Interpretation: | |
Portlock (1843) provides the only published interpretation of the Moyola River fish occurrences, attributing their richly fossiliferous nature to accumulation in a quiet marine pool, probably estuarine. Certainly the low diversity of the invertebrate fauna, and the presence of Spirorbis, indicates that the environment was not normal marine salinity. The presence of plant debris and scorpion cuticle also indicates proximity to a landmass. Portlock's statement that the plant remains were found "passing sometimes vertically through the beds' is intriguing, suggesting the presence of palaeosols within the sequence though none have been seen on recent visits to the site. Mitchell (1995) considered the sequence seen in the Altagowen Formation to record the spread of the early Carboniferous marine transgression across northern Ireland. Coarse alluvial-fan sandstones and conglomerates were succeeded by finer-grained red beds with pedogenic carbonate nodules, with a marine influence not becoming evident until deposition of the Mormeal Member. The presence of evaporite replacement beds and algal stromatolites suggests a hypersaline environment. The reworked carbonate pellets seen in some of the coarser units on the Moyola River may well have been derived from erosion of pedogenic carbonates. The evident proximity to land accounts for the abundance of plant debris and the presence of scorpion cuticle in some samples. The occurrence of tetrapod remains in such an environment is, therefore, not entirely unexpected. Both stratigraphically and lithologically the succession seen in the Altagowen Formation bears close comparison with that of the Lower Carboniferous (Chadian), Holywood Group exposed on the south side of Belfast Lough. The Mormeal Member bears some similarity to the Ballycultra Formation, which also comprises grey mudstones with evaporite replacement beds and a low diversity quasi-marine fauna including fish remains. The taxonomy of the fauna described by Portlock (1843) is in need of considerable revision. Several taxa which he cites are exclusively pre-Carboniferous, including Holoptychius [Devonian] and Cephalaspis [Silurian], while others are post-Carboniferous, such as Limulus [Recent]. Mags Duncan at Trinity College Dublin currently is investigating the vertebrate remains and her results should provide further information on this component of the fauna. | |
| Conclusions: | |
The sediments of the Lower Carboniferous Mormeal Member on the Moyola River were deposited in a marginal, quasi-marine environment in which fragmentary fish remains form a significant component of the fauna. A partial jaw of a tetrapod, the only early Carboniferous example known from Northern Ireland, may have come from this site. | |
| Notes: | |
For general information and references on the Palaeontology of Northern Ireland see Key Site 109 - Palaeontological sites of N. Ireland - Introduction For other site specific information and references on the Palaeontology of Northern Ireland see Key Site 110 - Ballycastle Pellet Chalk, Key Site 111 - Ballypalady Plant Bed, Key Site 112 - Carrick Lough, Bunnahone Lough and the Sillees River, Key Site 113 - Faragandoo, Key Site 114 - Minnis North Mudflow, Key Site 116 - Mullynaskeagh Clay Pit, Key Site 117 - Murlough Bay, Key Site 118 - Portnaloub, Key Site 119 - Sillees River, Glenasheevar, Key Site 120 - Tircrevan Burn, and Key Site 121 - Tullyconnell. | |
| Keywords | |
|---|---|
| Rocks: | Limestone, Mudstone, Sand, Sandstone, Shale |
| Fossil Groups: | Arthropod, Bivalve, Crustacea, Fish |
| Paleoenv: | estuarine |
| Access | |
|---|---|
| Management: | |
The extent of exposure of the Mormeal Member along the Moyola River at any one time is very much dependent on the interplay between erosion and sedimentation of the banks and riverbed. This is influenced by factors such as vegetation cover, local slips and temporary dams caused by stranded debris. Hence, it is highly probably that the exposures which were described by Portlock (1843) have long since been obscured as others have been revealed. No specific management practices are recommended for the site, although any future proposals to permanently raise river level, through creation of a dam or weir downstream of the site, should be resisted. | |
| Threats: | Creation of a dam or weir downstream would be detrimental. |
| Uses | |
| Uses: | The exposures lie in the riverbed and banks bordered by rough pasture and scrub woodland. |
| References (4) | |||
|---|---|---|---|
| Agassiz, L., 1833-1844. Recherches sur les poissons fossiles vol. 1-5, pp.1420, plates 369, Neuchatel. | |||
| Mitchell, W.I., 1995. The Carboniferous System in Northern Ireland - Earth Science Conservation Review (Unpublished Report) 6 vols, 430 pp, Department of the Environment. | |||
| Owens, B., Gueinn, K.J. & Cameron, I.B., 1977. A Tournaisian miospore assemblage from the Altagoan Formation (Upper Calciferous Sandstone), Draperstown, Northern Ireland. Pollen et Spores, vol. 19, pp.313-324. | |||
| Portlock, J.E., 1843. Report on the Geology of the County of Londonderry and of parts of Tyrone and Fermanagh. HMSO, Dublin. | |||
| Record type: | ESCR report | Recorded by: | Michael J. Simms, Department of Geology, Ulster Mu |
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Moyola River, County Londonderry, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=115. Accessed on 2026-11-06. |
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