 | | Blackslee Dyke at NE end of quarry section at Achameelan Viewpoint, Navar Forest scenic drive, Co. Fermanagh; central rib of hard dolerite with brown sand on either side marking rotten-weathered dolerite. |
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| Site Type: | Inland exposure, Quarry (disused) | | Site Status: | ESCR | | Council area: | Fermanagh & Omagh District Council | | Grid Reference: | H080557,H073565 | | Bing maps: | 54.45021,-7.87665 | | Google maps: | 54.45021,-7.87665 | | Rocks |
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| Rock Age: | Tertiary, Carboniferous (Asbian, Tertiary Undifferentiated) | | Rock Name: | Carboniferous Undivided | | Rock Type: | Dolerite | | Interest |
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| Minerals: | Tridymite | | Other interest: | Gravel, dyke, multiple intrusion, Intrusion, Some of the disused quarries are used as carparks |
| Summary of site: |
| Blackslee Dyke conforms to the classic description of a dyke. It is an almost vertical sheet-like structure of once molten rock, cutting through the gently inclined Carboniferous rocks of the area. These are the Dartry Limestone below and the Glenade Sandstone above and, at its most easterly outcrop, the dyket intrudes shales of the Meenymore Formation. It also shows another text book characteristic because it is injected along a pre-existing plane of weakness - a north-west / south-east trending fault (one of a network criss-crossing the area). It extends at least 1.25km through the eastern end of the Lough Navar Forest and may be even longer. | | The dyke can be seen in three quarries along its length. It is of considerable thickness, ranging between 6.5m and 15.5m, and has a number of interesting features. At its most southerly exposure (now somewhat overgrown) it intrudes shales and sandstones and is seen at its simplest, but at the second quarry (0.7km to the north west and now much overgrown) historical accounts describe molten rock invading the walls of the fault zone in the form of small sills along bedding planes and small dykes. Blocks of sandstone up to 3m in size were seen suspended in the dolerite (solidified lava), broken from the wall and transported as xenoliths, then frozen into position as the rock solidified. Smaller particles showed partial melting, a process known as fritting, suggesting high temperatures during intrusion, with confirmed pseudomorphs of tridymite. Tridymite is the stable form of quartz (one of the commonest of all rock forming minerals) at temperatures above 870ºC. It reverts to the low temperature form on cooling but the crystal shape of tridymite, which is diagnostic, survives. Some of the features can still be seen and later quarrying has shown that the molten lava passed around massive blades of wall rock in its ascent. In the same quarry, the dolerite of the dyke was seen to be crushed into a breccia - clear evidence of movement along the fault after intrusion. | | The limestone quarry at the north west limit shows a dolerite full of small quartz inclusions, demonstrating tridymite pseudomorphs perfectly. | | The Blackslee Dyke is one of a related series (or swarm) of similar trend crossing the area and recent quarrying suggests that there were several intrusions over a period along different but connected fissures. All are thought to be of Tertiary age, formed between 65 and 60 million years ago at the time the Antrim basalts were erupting. The size of this dyke provokes the question of what happened to all the lava that rose through it and adds substance to the suggestion of a former, much more extensive, lava plateau. | | The dyke is important because it shows a wide range of formation processes and is one of the few places where pseudomorphs after tridymite can be seen. | | The only threats are from quarrying and overgrowth of abandoned areas. |
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