Summary of site:
The rugged and imposing cliff of Binevenagh, climbing above its forested skirts, presents one of the most striking panoramas to be seen anywhere in Northern Ireland. The cliff line extends from Downhill behind Castlerock on the north coast, around Binevenagh summit and south where it is breached by the Curly River valley, then discontinuously south on the western escarpments of Keady Mountain, Rigged Hill, Donald’s Hill and Benbradagh, 3km east of Dungiven.
The escarpment is a massive stack of basalt lava flows, 100m thick, almost everywhere lying on a surface of Cretaceous Ulster White Limestone. Behind the escarpment in the Binevenagh area they achieve a maximum thickness of 150m. The basalts are olivine-rich and belong to a group called ‘tholeiites’. Microscopically they show a range of mineral organisation, described as ‘texture’, from granulitic to ophitic (reflecting different cooling histories). Bands of basalt with sharply formed olivine crystals were formed in some lava flows and are similar to Upper Basalts in east Antrim on the Garron Plateau. Lava flows near the base of the pile have well preserved upper surfaces that show the ropy crinkling of their surfaces as they solidified.
The major debate about the Binevenagh basalts concerns their age. How do they relate to the Lower Basalt Formation, the Causeway Tholeiite Member of the Interbasaltic Formation and the Upper Basalt Formation? The prominent lithomarge red beds that differentiate the lavas to the east are entirely absent west of a line from Portrush to Ballymoney.
Several lines of evidence have been pursued in attempts to answer this question of which five appear to be important:
1. The thickness of the Binevenagh lavas (150m) far exceeds the thickest development of the Lower Basalts east of the River Bann (around 50m).
2. The absence of interbasaltic red lithomarges suggests that the Binevenagh lavas are probably later than them
3. At the very base of the lavas at Downhill there is a reddened laterite which, while not an interbasaltic bed like the Port na Spaniagh and Ballylagan Members of the north Antrim coast, could equate to one of them.
4. Another line of evidence relates to the presence of pebbles of a rock called rhyolite which only occurs in the Interbasaltic Formation (see Sandy Braes site description). The pebbles are found in the lowest lavas at Coagh, 8km east of Cookstown, and the argument is that these lavas would have to be later than the Interbasaltic Formation for the rhyolite to become incorporated into them. Presuming these lavas to be the base of the lavas seen around Binevenagh, 60km to the north, then the first eruptions were of late Interbasaltic age or later.
5. The presence of flow-banded basalt with large, sharply defined olivine crystals near the top of the Binevenagh lavas has already been mentioned and its similarity with basalts of the Upper Basalt Formation of the Garron Plateau in east Antrim noted.
Taken together, these observations strongly support the idea that the Binevenagh basalts are equivalent to the Upper Basalt Formation to the east and the Lower Basalts and Interbasaltic rocks are absent from the area.
The face of the western escarpment of Binevenagh has additional interest in the form of huge rotational landslips as massive slabs have separated from the main basalt pile, migrated downslope and come to rest. Obvious facets of the plateau at the top of these slabs show the amount of vertical displacement. The landslips show no glacial features and clearly occurred after the departure of the ice.
These lavas are of national importance as the maximum development of basalts at the western extremity of the Antrim Lava Group. They are significantly different from lavas on the Antrim Plateau and contribute to our understanding of how this volcanic event evolved in space and time. They are easily accessed but it is unsafe to descend the gullies from the summit due to the large volumes of rotten, loose rock and even approached from below watch should be kept for falling blocks that can bound unpredictably.