Earth Science Conservation Review
| Carrick Lough, Bunnahone Lough and the Sillees River | Fermanagh |
| Site Type: | Crag, Crags, Inland exposure, Stream section |
| Site Status: | ASSI |
| Grid Reference: | H0954,H1055 |
| Google maps: | 54.43492,-7.86129 |
| Rocks | |
|---|---|
| Rock Age: | Carboniferous (Asbian, Dinantian, Silesian) |
| Rock Name: | Glencar Limestone Formation, Tyrone Group |
| Rock Type: | Limestone, Shale |
| Interest | |
| Fossil Groups: | Arthropod, Bivalve, Brachiopod, Coral, Echinoderm, Gastropod, Polyzoan, Trilobite |
Summary of site:
An enduring problem in the study of fossils is knowing just how much of a community of plants and animals living in a particular place in the geological past is likely to be preserved as fossils and just what proportion of those are actually recovered in the collecting process. In other words, do we ever have an accurate picture of living communities in the geological past? There are very few places where circumstances preserve all of the shelly faunas in a way that allows them to be recovered intact, all delicate detail preserved. The area around Carrick and Bunnahone Loughs and a nearby short stretch of the Sillees River which drains them is one such place.
Small outcrops around Carrick Lough, below the outlet of Bunnahone Lough and in the nearby banks of the Sillees River expose parts of the Glencar Limestone Formation. The Glencar is Carboniferous in age and is part of the Asbian stage, dating to around 337 million years ago. At that time the area that has become modern Ireland was the submerged fringe of a supercontinent combining modern North America, Greenland, Europe and western Russia. It sat astride the equator on its infinitesimally slow drift northwards and the Glencar Limestone was formed in this tropical environment.
There is still discussion about the conditions under which the rocks were formed. They consist of shales and muddy limestones interpreted variously as shallow water deposits near a reef, slightly deeper water deposits but still close to a reef, deposits some distance from a reef or even in deeper water well beyond the reef fringe. Whatever the environment, following burial and the induration (hardening) of the sediment into rock, silica rich fluids percolated through the fossil-bearing layers and the original shells and other hard parts were selectively replaced by silica (a process known as silicification). From the viewpoint of the fossil preparator, this is an ideal situation because the rocks containing the fossils can be placed in acid baths to dissolve the limestone matrix while leaving the silicified fossils intact, with the finest and most delicate structures preserved.
The description of the remarkable fossil assemblages revealed by this process has been the work, principally, of three researchers who between them have described 56 lampshell (brachiopod) species, 69 sea moss (bryozoan) species, and a further 35 species including trilobites, ostracods, sea lilies (crinoids), blastoids, sea snails (gastropods), bivalves, sponges, corals and worms. 160 species in all have been described of which 29 are completely new to science and important details of most of the rest are described for the first time. There is no other Lower Carboniferous locality known presenting such diversity in such a confined space, let alone so beautifully preserved.
There is some disagreement about whether the fossils lived in the area in which they were found. The evidence from the brachiopods suggests that they were arranged as they would have been in life on a rather muddy sea bed but since the bryozoa needed a solid foundation for their holdfasts (a kind of securing ‘root') it seems unlikely that they could have lived perched on the mud. Since both are mixed together, it is likely that the brachiopods were in place and the sticks and fronds of the bryozoa were washed on to the same sea bed. The fragile nature of bryozoa and the fine detail preserved means that they could not have been transported far.
This area gives a rare insight into the diversity of invertebrate animals living on the sea floor during the deposition of the Glencar Limestone Formation in Asbian times. Its unique variety and the quality of its fossils have given rise to 20 academic papers and monographs and established the area in international studies of taxonomy and ancient environments. It has also contributed to the debate on the completeness of the fossil record, suggesting that most faunas are much richer than they appear. Its protection is therefore paramount.
There are no threats to the area at present except from plant overgrowth of outcrops which can readily be re-exposed with a little light gardening. Any other developments proposed for the area should be carefully monitored to ensure the survival of this truly exceptional site.
Small outcrops around Carrick Lough, below the outlet of Bunnahone Lough and in the nearby banks of the Sillees River expose parts of the Glencar Limestone Formation. The Glencar is Carboniferous in age and is part of the Asbian stage, dating to around 337 million years ago. At that time the area that has become modern Ireland was the submerged fringe of a supercontinent combining modern North America, Greenland, Europe and western Russia. It sat astride the equator on its infinitesimally slow drift northwards and the Glencar Limestone was formed in this tropical environment.
There is still discussion about the conditions under which the rocks were formed. They consist of shales and muddy limestones interpreted variously as shallow water deposits near a reef, slightly deeper water deposits but still close to a reef, deposits some distance from a reef or even in deeper water well beyond the reef fringe. Whatever the environment, following burial and the induration (hardening) of the sediment into rock, silica rich fluids percolated through the fossil-bearing layers and the original shells and other hard parts were selectively replaced by silica (a process known as silicification). From the viewpoint of the fossil preparator, this is an ideal situation because the rocks containing the fossils can be placed in acid baths to dissolve the limestone matrix while leaving the silicified fossils intact, with the finest and most delicate structures preserved.
The description of the remarkable fossil assemblages revealed by this process has been the work, principally, of three researchers who between them have described 56 lampshell (brachiopod) species, 69 sea moss (bryozoan) species, and a further 35 species including trilobites, ostracods, sea lilies (crinoids), blastoids, sea snails (gastropods), bivalves, sponges, corals and worms. 160 species in all have been described of which 29 are completely new to science and important details of most of the rest are described for the first time. There is no other Lower Carboniferous locality known presenting such diversity in such a confined space, let alone so beautifully preserved.
There is some disagreement about whether the fossils lived in the area in which they were found. The evidence from the brachiopods suggests that they were arranged as they would have been in life on a rather muddy sea bed but since the bryozoa needed a solid foundation for their holdfasts (a kind of securing ‘root') it seems unlikely that they could have lived perched on the mud. Since both are mixed together, it is likely that the brachiopods were in place and the sticks and fronds of the bryozoa were washed on to the same sea bed. The fragile nature of bryozoa and the fine detail preserved means that they could not have been transported far.
This area gives a rare insight into the diversity of invertebrate animals living on the sea floor during the deposition of the Glencar Limestone Formation in Asbian times. Its unique variety and the quality of its fossils have given rise to 20 academic papers and monographs and established the area in international studies of taxonomy and ancient environments. It has also contributed to the debate on the completeness of the fossil record, suggesting that most faunas are much richer than they appear. Its protection is therefore paramount.
There are no threats to the area at present except from plant overgrowth of outcrops which can readily be re-exposed with a little light gardening. Any other developments proposed for the area should be carefully monitored to ensure the survival of this truly exceptional site.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Carrick Lough, Bunnahone Lough and the Sillees River, County Fermanagh, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=112. Accessed on 2024-12-26 |
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