| Site Type: | Sinkhole | | Site Status: | ASSI | | Council area: | Fermanagh & Omagh District Council | | Grid Reference: | H09354754 | | Bing maps: | 54.37652,-7.85687 | | Google maps: | 54.37652,-7.85687 | | Rocks |
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| Rock Age: | Quaternary, Carboniferous (Asbian, Holocene, Visean) | | Rock Name: | Dartry Limestone Formation, Knockmore Limestone Member | | Rock Type: | Limestone | | Interest |
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| Other interest: | fault, joints, breakdown, cave, doline, sinkhole |
| Summary of site: |
| A substantial river sinks into the large, cliff-lined shake hole at Pollanaffrin in times of flooding. A scramble leads to an 8m pitch on to a floor. On the east side, a climb through boulders leads to a second, tight 8m pitch and squeezes down drops of 3m and 2m to a junction. From here a northerly rift pinches out after 20m. To the south, a tight rift, over 250m long with a series of right-angled corners, passes through a duck and two sumps. The stream drains into the floor 20m before the final, impenetrable choke. | | Sections of passage in the cave have a keyhole cross-section. This is the result of the early history of the cave when it lay below the water table and all the spaces were flooded; chemical weathering was able to dissolve the limestone equally in all directions along the preferred flow, creating a circular, tube-like passage. As the cave deepened and the water table fell, the residual stream flowed along the floor of the tube, eventually cutting a notch to create the keyhole profile. This feature can be seen in many caves. Cave development below the water table is described as the phreatic phase and that above, the vadose phase. | | The joints that cut near vertically through the limestone beds result from the stored stresses accumulated by the limestone through its history. Here they are in two sets crossing at right angles and the rifts in this cave seem to be controlled by these joint directions and are reflected in the cave plan. | | Dye testing of the water to establish its flow has not been conducted at Pollanaffrin but it is believed to enter Noon’s Hole. |
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