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Earth Science Conservation Review
| Cladagh River, Marble Arch | Fermanagh |
| Site number: | 182 | File Number: | H13 |
| ESCC: | EO | ||
| Locality Type: | Stream section | Status: | local interest |
| Grid Reference: | H121346 Centroid | ||
| Google maps: | 54.26058,-7.8143 | ||
| County: | Fermanagh | Council area: | Fermanagh & Omagh District Council |
| Period: | Carboniferous | ||
| Stages: | Asbian, Visean | ||
| Lithostrat: | Dartry Limestone Formation, Glencar Limestone Formation | ||
| Site Description | |
|---|---|
| Highlights: | |
This is probably one of the best exposures of the contact between the top beds of the Glencar Limestone Formation and the conformable (? disconformable) base of the succeeding Knockmore Limestone Member of the Dartry Limestone Formation. | |
| Description: | |
The Glencar Limestone - Dartry Limestone contact is exposed in the west bank of the Cladagh River about 100m downstream from the Marble Arch on the north side of Cuilcagh Mountain. The main mass of limestone forming the 20m high cliff that dominates this bank is formed of unbedded to very thickly bedded (3-5m) pale to medium grey micrites with Stromatactis of the Knockmore Limestone Member of the Dartry Limestone Formation. Lying stratigraphically beneath the base of these limestones is some 8m of thinly bedded rocks that belong to the Glencar Limestone. The base of the Dartry Limestone projects beyond the top of the Glencar Limestone as an overhang and the contact is inclined upstream at about 15 deg. so that the Cladagh River emerges from the cave system entirely within the Dartry Formation. The top 4m of the Glencar Limestone are exposed at the base of the cliff in the west bank and in extensive bedding surfaces on the west side of the river. They consist primarily of interbedded pale to medium grey fine-grained calcareous sandstones with laminae of dark brown shaley siltstone and thin beds of nodular bioclastic argillaceous packstone. Thicker beds of dark grey argillaceous crinoidal limestone occur in the top 4m. Bioclasts in the arenaceous sediments are invariably broken, abraded and bored by algae and include abundant crinoid ossicles, bryozoa, brachiopods and small solitary corals. In the interval between 4m and 7.7m below the formation top the Glencar Formation consists of unbedded, pale grey micrite and brachiopod-rich micrite breccias which are virtually indistinguishable from the overlying Dartry Limestone mounds but which are underlain by the thin sandstones, siltstones and shales that occur in the top 4m of strata. Further examples of lenticular micrites in the top of the Glencar Limestone are found along the cliff line that borders the north face of Cuilcagh Mountain. For example, in the southern part of the Deer Park (H164 332), about 4.5km east-southeast of the Cladagh River, there are two micrite lenticles, about 3.5m and 5m thick in the top 13m of the Glencar Limestone. However, only 260m to the west, the Glencar-Dartry contact is exposed again but there are now no thick lenticular micrites in the top 4m of the Glencar Limestone, the strata consisting of thin-thick bedded crinoidal limestones with thin calcareous shales. Similar micrite mounds up to 8m thick also occur near the top of the Glencar Limestone in the cliff face on the southeast side of Belmore Mountain. | |
| Interpretation: | |
The overall environmental picture of the Northwest Basin during the time of deposition of the Benbulben Shale, Glencar and Dartry Limestones is of a reduction in the mud content and gradual establishment of clearer-water shelf conditions. The occurrence and thickness of the two older formations was thus wholly dependant on the relative cessation or continuation of the mud supply to various parts of the basin and thus the transition from mud- to carbonate sedimentation either commenced early in some areas, resulting in a thick Glencar Limestone, or late in others where the formation is thus attenuated. It is difficult to pinpoint the causes that influenced the development of micrite mounds, of Dartry Limestone type, in the top of the Glencar Limestone. Nevertheless their lenticular outline, apparently haphazard distribution and thin development suggests that the Glencar mounds, which merely presage the conditions of Dartry Limestone times, failed to develop into the massive micrite mounds of that formation when the prescribed conditions of contemporaneous, asymmetrical (fault-induced) subsidence did not accompany these initial micrite accumulations. The occurrence of sandstones and siltstones at the top of the Glencar Formation is uncommon in Co. Fermanagh and may be explained by enhanced erosion of rising land during late-Asbian times when Vis‚an tectonic activity reaches a peak in the north of Ireland. | |
| Conclusions: | |
The cliff exposure at the south end of the Cladagh River glen near the Marble Arch is an easily accessible and clean section that exposes the Glencar-Dartry Limestone contact and which contains lithologies in the former formation that are not known elsewhere in its outcrop. | |
| Notes: | |
For information on other sites in Subarea-1 see; Key Site 1174 - Subarea-1; Derrygonnelly-Marble Arch-Cuilcagh. | |
| Keywords | |
|---|---|
| Minerals: | Micrite |
| Rocks: | Breccia, Limestone |
| Fossil Groups: | Brachiopod, Coral, Crinoid, Polyzoan |
| Relations: | Marine sediments |
| Access | |
|---|---|
| Approach: | 100m downstream of the Marble Arch. |
| References (3) | |||
|---|---|---|---|
| Brunton, C.H.C., 1966a. Silicified productoids from the Visean of County Fermanagh. Bulletin of the British Museum (Natural History), Geology, vol. 12 pt. 5, pp.173-243. | |||
| Oswald, D.H., 1955. The Carboniferous Rocks between the Ox Mountains and Donegal Bay. Journal of the Geological Society of London, vol. 111, pp.167-186. | |||
| Taverner-Smith, R., 1973. Fenstrate bryozoa from the Visean of County Fermanagh, Ireland. Bulletin of the British Museum (Natural History), Geology, vol. 23 pt. 7, pp.389-493. | |||
| Map(s): | See 1:50,000 O.S. Sheet 26 (Lough Allen). | ||
| Record type: | ESCR report | ||
| Entered by: | Doughty, P.J. | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Cladagh River, Marble Arch, County Fermanagh, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=182. Accessed on 2026-11-06. |
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