| Site Type: | Karst | | Site Status: | ASSI | | Council area: | Fermanagh & Omagh District Council | | Grid Reference: | H0850 | | Bing maps: | 54.39901,-7.8768 | | Google maps: | 54.39901,-7.8768 | | Rocks |
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| Rock Age: | Quaternary, Carboniferous (Holocene, Visean) | | Rock Name: | Dartry Limestone Formation, Glencar Limestone Formation, Knockmore Limestone Member, Meenymore Formation | | Rock Type: | Limestone, Mudstone, Shale | | Interest |
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| Minerals: | Aragonite, Calcite, Mirabilite | | Other interest: | bedding, fault, joints, slickensides, Glacio-fluvial sediment, breakdown, clastic sediments, doline, flowstone, gour pool, helictite, organic sediments, sinkhole, speleothem, stalactite, stalagmite, straw |
| Summary of site: |
| South of the bold prominence of Knockmore Hill there is a dramatic dry valley beneath which is Pollaraftra, a cave 2 km long. Both the valley and the cave owe their origin to an east-west trending fault which swings north east at the Screenagh River, passing Derrygonelly to the south on its way to Claragh. The rocks on the south side of this fault are displaced downwards, creating impressive limestone cliffs on the north side of the valley. | | There is no surface drainage in the limestone uplands to the north but the impervious cap of Glenade Sandstone and the Meenymore Formation beneath it on the south western side provide a southern catchment feeding two streams into the valley where they promptly sink into the bedded Knockmore Limestone Member of the Dartry Limestone, 500m and 30m west of the main cave entrance. | | Pollaraftra exploits the following local circumstances: a point source of water; a planar zone of rock crushed by faulting that offers reduced resistance to underground flow; and an exceptionally pure limestone that provides an ideal medium for cave development. | | The waters draining into and around the main sink, together with sub-surface water from the free draining limestone uplands to the north and south, flow along a simple linear cave defined by the fault, with only one major deviation. The water resurges at Legland Rising 2km to the east. | | The two main entrances are at the extreme western end of the cave. The Old Entrance is via a pothole descending in two pitches of 3m and 8m respectively into the Entrance Chamber, which can also be reached without equipment via Martin’s Entrance. This entrance 50m upstream leads into an obvious passage to the chamber. Slickensides (striations formed by the grinding of the rock during faulting) are clearly displayed in the Entrance Chamber. An obvious narrow passage carries the stream slightly south of east and in the roof here earlier phreatic tubes (passages formed completely under water), now abandoned as the water has exploited lower levels, are clearly seen. About 50m in, the passage becomes an active phreatic system with water to the roof. This sump can either be dived or bypassed using tight crawls at roof level. Beyond this first sump passages merge to form a canyon about 1m wide and 4m high. Here needle shaped crystals, tentatively identified as mirabilite (a hydrated sodium sulphate), can be found growing out of the crushed rock of the fault zone that forms the passage walls. The earliest high level passages are again seen here in the roof. There are many calcite formations (speleothems) in this section: stalactites, stalagmites, curtains, helictites, false floors (earlier cave floors of sand pebbles and cobbles cemented on their surface by calcite but later breached releasing their uncemented lower sediments) and cemented breccias containing bones. The canyon leads to Luncheon Shelf Chamber (15m wide, 2m high) and in a further 100m the Boulder Chamber (50m long, 30m wide and 10m high). Both these chambers are formed by the erosive and corrosive action of the stream widening the passage to the point where the unsupported beds above eventually collapse under gravity, forming a jumble of boulders at stream level. An inlet running parallel with the main passage enters Boulder Chamber in the north-west corner. | | A further 250m of canyon passage end at a second sump but a precarious climb up a wall coated in glutinous mud leads to an equally slimy upper series of now abandoned passages that bypass the flooded section. This upper series becomes progressively cleaner and, after a further 100m, enters an outstanding speleothem section particularly noted for a cascade of rimstone pools and unique, small, globular pellets of calcite around 1-2mm across, described as ‘sesame seed’ decorations. | | The active stream passage can be reached at several points from the upper series but there is a divergence of direction between the two. The upper series remains aligned with the fault, but the active stream loops northwards along an angular, joint-defined passage before returning to the east end of the upper series. There are two more sumps in this looped section of the active passage and the second has been dived for a length of over 100m without encountering a single air pocket. The stream continues in a narrow cleft about 60cm wide and 2m high and the dive was ended with the main stream continuing. | | The high level series continues along the line of the fault to a lake that can be waded, beyond which a further dry series with more speleothems leads to an upward climb through a huge loose boulder pile to emerge in the centre of a large chamber. A descent of 15m at the northern end of the chamber leads to the final section of the cave, The Canals. From this point the upper cave departs completely from the line of the fault in a largely northerly, joint-defined diversion. | | The Canals continue in high, narrow rifts for most of the final 900m. They consist of very cold, static water that appears to be stranded as a ‘perched’ water table. They can be waded or swum for the first 300m at which point the roof descends to 2m, leading into a 60m-long dry passage that ends in a boulder strewn rift chamber. A diversion here leads to other large boulder chambers thought to approach ground surface. Keeping to the left, a squeeze leads to the Second Canal which can be swum for a further 45m before water reaches the roof, where the passage again enters the fault zone and aligns with it. This sump has been dived, giving access in 170m to a further sump pool where the main stream resurges. There is then a further 200m of passage with five more sumps where exploration has ended at a boulder barrier. | | There are other cave sites in the area but none approaches Pollaraftra in importance. There are five short caves on the sandstone/limestone contact all of which originated as phreatic tubes that were later entrenched in a vadose (a cave passage with air space above the active stream) so that the passages now have keyhole-shaped profiles. 1.5km west of the entrance of Pollaraftra is Whitehouse Cave, also in the Knockmore Limestone, a rift leading to a boulder choked chamber; two further small caves, Poulnamuck and Pollnamansh, are in the same vicinity. | | Relict caves dot the southern flank of Knockmore, the most significant being Garrison Cave, with 75m of muddy crawl to a short climbable pitch. | | The limestone pavements on Knockmore and on the bench of its southern flank are the best to be seen in County Fermanagh, with bold clints and grikes and many fine solution features. This southern bench also has a line of dolines and potholes. | | Pollaraftra is Northern Ireland’s finest fault-defined cave system and incorporates a host of features and formations of particular importance. The speleothems (calcite formations) are of exceptional variety and interest and so far appear to have survived cave sport activity without major damage. Future cave guidebooks should, however, include a statement requesting care when passing through the decorated areas of the cave. The area also includes the finest development of limestone pavements to be seen anywhere in the province. |
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