|Eastern Garron Plateau, County Antrim||Antrim|
|Site Type: ||Karst|
|Site Status: |
|District: ||Larne Borough Council|
|Grid Reference: ||D2923|
|Rock Age: ||Quaternary, Tertiary, Cretaceous, Jurassic (Eocene, Holocene, Palaeocene, Santonian, Toarcian)|
|Rock Name: ||Antrim Lava Group, Lias Clay, Upper, Ulster White Limestone Formation|
|Rock Type: ||Basalt, Clay, Limestone|
|Other interest: ||joints, No Data, cave, clastic sediments, sinkhole, sump|
Summary of site:
With few exceptions, well developed cave systems in the British Isles tend to form in limestones of Carboniferous age. It therefore came as a major surprise when, in 1984, a well developed cave was found in the Cretaceous Ulster White Limestone. It was discovered by a party of canoeists in the bed of the Black Burn, a stream flowing east from Craigatinnel to the sea, a little over 3km north of Carnlough. The possibility of significant karst features in the Ulster White Limestone had never previously been considered, so the discovery prompted a research commission for the entire eastern Garron Plateau, with exciting preliminary results.
The geology of the area is fairly simple. The earliest rocks seen around the plateau are the Triassic Mercia Mudstone Group, topped in places by the Jurassic Waterloo Mudstone Formation. There follows the Cretaceous Ulster White Limestone before the Tertiary Lower Basalt Formation, then the Interbasaltic Formation and finally, capping the plateau and forming the rugged flanking cliffs, the Upper Basalt Formation. The plateau drains radially from its summit and Black Burn is one of over 20 short streams/rivers on the east, north and west flanks.
Preliminary investigations of Black Burn proved 150m of passageway when progress was halted by flooding to the roof. Subsequent explorations with diving equipment have proved a further 350m of passages but this is not the end of the system.
The cave entrance is located in the bed of the burn, 110m downstream of the waterfall cascading over the Lower Basalts. Here a cluster of joints (natural rock fractures) are enlarged and a short climb down leads to a chamber with several holes in the roof and large collapse boulders of Ulster White Limestone on the floor. The passage from this chamber leads south-westerly following the joint direction for 15m, at which point the joint direction and passage veer to the south-east.
The immature nature of the cave can be appreciated in this passageway which is partly phreatic, partly vadose. Phreatic passages form entirely under water, allowing solution reactions to act equally in all directions. The result is usually a sub-circular or oval cross-section. Vadose passages are formed when the stream flows along the floor, with air space above, usually creating a tall, narrow cross-section. Most passages originate as phreatic tubes that become vadose as the water table falls in response to solution activity widening joints at lower levels. In such cases, key-hole shaped cross-sections may result but frequently, where cobbles and boulders are present, their collisions with walls and floor widen the passages at stream level. A clue to whether a passage is formed by solution action or by erosion and collapse is the presence or absence of scalloping (the continuous pattern of spoon shaped hollows on the cave walls), which provides indisputable evidence of flowing water.
The generally south-east trend of the cobble strewn vadose passage ends abruptly and reverses direction to the north-west, passing a point under the surface waterfall. It is 1m wide and 4m high here. This passage has five sumps (drowned sections) and the main surface stream enters the system after the first where the passage expands to 6m high and 2m wide. In this section the passage descends steadily through the Ulster White Limestone and includes a waterfall 4m high. The final sump has so far not been dived.
Dye tests have shown that the water entering the cave resurges at a small pool (2m by 3m) at Foaran, almost 4km north of Black Burn. This rising is less then 70m from the sea, making the so-called Foaran River a candidate for the shortest river in Northern Ireland.
Black Burn Cave is at an early stage in the cave development process, with mainly phreatic passages (passable in safety only in dry conditions) fed by vadose clefts in the bed of the burn. In wet conditions, the pattern of water flow appears to be complex, depending on the volume of water penetrating the system through its many access points. A significant erosive effort is contributed by the pebbles and cobbles that are washed in as part of the sediment load of the steep and aggressively active Black Burn. A surprise was the point of resurgence, demonstrating an underground flow direction at right angles to the burn itself.
Black Burn Cave appears to be unique in the British Isles - an extensive system embracing a wide range of classic karst features but developed in the Cretaceous Ulster White Limestone Formation (equivalent to the Chalk in England). Exploration and survey, essential preludes to serious research, are in their infancy. This, and related Cretaceous karst sites, should be designated to protect them from any adverse developments that could damage or detract from their exceptional importance.
Threats are largely natural, resulting from the coarse load of streambed debris from Black Burn in spate which tends to block the confined entrances. This is part of the natural cycle of development and is usually overcome for cave exploration purposes by limited digging to gain access to the cave. Of more long term concern could be overgrazing and any consequent acceleration of erosion rates in the Black Burn catchment area.