 | | Pollnaclanawley: stalagmites in Fossil Series. |
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| Site Type: | Karst | | Council area: | Fermanagh & Omagh District Council | | Grid Reference: | H1629 | | Bing maps: | 54.21016,-7.75476 | | Google maps: | 54.21016,-7.75476 | | Rocks |
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| Rock Age: | Quaternary, Carboniferous (Asbian, Holocene, Visean) | | Rock Name: | Dartry Limestone Formation, Glenade Sandstone Formation, Glencar Limestone Formation, Meenymore Formation | | Rock Type: | Chert, Limestone | | Interest |
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| Other interest: | bedding, fault, joints, rift, cave, doline, dry valley, flowstone, karren, limestone pavement, pothole, sinkhole, speleothem, stalactite, stalagmite, straw, sump |
| Summary of site: |
| The major and many minor components of the karst of this area are described individually under their own names (Polliniska, Key Site 321; Pollnatagha, Key Site 322; Peter Bryant’s Bullock Hole, Key Site 323; Black Pot, Key Site 324; Pollnaclanawley, Key Site 325; Peter Bryant’s Hole, Key Site 326; Pigeon Pots, Key Site 327; Goat Pot, Key Site 328; Sheep Pot, Key Site 329; Badger Pot, Key Site 330; Tea Pot, Key Site 331; Long Pot, Key Site 332; and Aghatirourke Pot, Key Site 333). | | This overview is important because it places these sites in their setting—a setting unique in Northern Ireland. Despite their proximity to the other caves and karst features of the Marlbank and Cuilcagh area, this group in the east demonstrates an association of unusual features which are at least partly explained by their geological context. | | The Knockmore Limestone Member of the Dartry Limestone Formation (essentially homogeneous, steep-sided Carboniferous mud mounds) was, and still is, widely exploited by the weathering and erosive forces operating in the area. These processes have been active from when the glaciers melted at the end of the last pulse of the Pleistocene (the period of glaciations commonly known as the Ice Age) through to the present day (and possibly even earlier). The major cave systems in this limestone have extensive horizontal networks of passages and chambers that are entered either by stepped passages (as in Pollraftra, Marble Arch and Boho Caves) or by a few long, vertical shafts (as at Noon’s Hole and Reyfad Pot). | | In the east of the area the Knockmore Limestone Member is absent. The surface streams flowing off the Meenymore Formation cross on to a zone of faulted (fractured and displaced) rocks with a thin development of the Carn Limestone Member on the surface and an equally thin sequence of Cloghany Limestone Member beneath that also outcrops just to the east. The Carn Limestone is thick bedded with black shales separating the beds. The Cloghany Limestone is a medium grey coloured lime sand, its grains being composed of a variety of shell and skeletal particles of marine invertebrates, with thick shale and limestone at the base. All the major caves in the area drain through these rocks before entering the Dartry Limestone below, here typically strongly bedded with beds of varying thickness containing local and heavy concentrations of insoluble chert (fine-grained quartz) in the form of sharp edged crusts and irregular nodules. | | These circumstances tend to create potholes with narrow entrance shafts in the Carn and Cloghany Limestones, widening with depth into the Dartry Limestone. There is a strong tendency, particularly in the Dartry beds, to vertical shaft development down rifts that exploit deeply persistent joints. Passages appear to be fewer and shorter than in the areas where the Knockmore Limestone is the dominant formation, and they are preferentially developed in the Carn and Cloghany Limestones where shales and impure limestones predominate, probably encouraging lateral flow. Once these are breached, long vertical shafts extend through the Dartry Limestone proper. This pattern of cave development, with near surface passages above deep potholes, is common in the area. | | Another factor is the presence of the massive, south-east trending Cuilcagh Dyke—a vertical sheet of dolerite (injected in a molten state) passing just north of Goat Pot and isolating all the East Cuilcagh caves on its southern side. This appears to have had profound and complicating effects on the flow of underground drainage. Only the water flowing into Goat Pot itself bucks the general trend, somehow crossing the dyke to resurge at Cascades Rising in the Cladagh Glen below Marble Arch, a distance of over 5km to the north west. | | Totally unexpected is the site where the waters flowing into Pigeon Pots reappear. Dye testing proves them to rise in Shannon Pot, acclaimed as the source of the River Shannon, 12km to the west in County Cavan. They have therefore passed east to west under the complete mass of Cuilcagh Mountain. A clue as to how this happens may be the dip (tilt) of the strong bedding in the Dartry Limestone which is to the west and south west. Once the flow is under the mountain, there are significant east-west faults that slice through the northern face of Cuilcagh, possibly encouraging a more westerly movement. Most other caves in the area appear to drain to a line of resurgences to the east. Only further research will explain this bewildering drainage pattern. | | The area has some interesting surface features, including limestone pavements around Pigeon Pots and a large area of collapse and solution features north of the Legacurragh Gap. This doline field is perhaps the best example of its kind in Northern Ireland. | | Legacurragh Gap itself is a deeply incised valley, completely dry, with an abrupt termination at its head. It is thought to date from the closing stages of the last glaciation, when the ground was frozen to depth (permafrost) and the ice was beginning to melt. Torrential meltwaters flowed over the frozen ground surface, cutting the valley as they went. The abrupt upstream termination suggests that a considerable flow was released at that point, possibly off an ice margin. Later, as the ground thawed, water was able to penetrate the limestones and, as the cavities below ground enlarged into cave systems, the entire flow became subterranean. The Pound, a similar feature, originated in the same way. | | Greenan Arch is a natural bridge in the upper member of the Dartry Limestone Formation, just below the contact with the impervious Meenymore Formation. It consists of a deep, basin shaped depression, breached into the adjacent valley on one side leaving the arch poised above. It is interpreted as another late glacial feature, formed during permafrost conditions, and is thought possibly to be the result of stream capture. | | The entirely different style of cave architecture in the East Cuilcagh area, its complex hydrology, the Legacurragh doline field and the other glacial karst features, all mark out this area as uniquely interesting and important, with considerable remaining research potential. Its new claim to fame, as the source of the River Shannon, also adds an Irish cultural interest. |
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