Dr Trevor Watts (UK)
This is the second and final part of an article on the volcanic highlights of Northern Ireland’s Giant’s Causeway and surrounds. For the first part, see Giant’s Causeway (Part 1): An introduction.) We were in the area for several days and the weather was fairly mixed, but there were glorious skies between the showers, and the high winds brought the waves up beautifully. Of the six highlights discussed below, we visited the first three in one day, as all were a few kilometres to the west of The Giant’s Causeway. Those to the east, we visited on another day.
They are all supremely interesting and give an idea of the range of volcanic features to be seen. You cannot see an actual, traditional volcano in Antrim, with its classic shape. However, you can visit many scattered and varied elements of the area’s vulcanicity, and so gain an appreciation of the overall picture.
The volcanic sill at Ramore Head, Portrush (about 13km west of the Causeway)
This is different to a lava flow because the magma was forced between layers of rock and not just spread over the top of them. There is easy parking and a well-made footpath around the peninsula, with good views of the irregular columnar jointing. There is one long, narrow inlet, perhaps 30m long by a couple of metres wide, which appears to have been formed where a dyke of softer material has been eroded away. Beneath the walls of dolerite, there is the country rock into which the magma was intruded: Lias shales. These are heavily metamorphosed and jointed in places, but they contain an abundance of ammonites in others. We didn’t find any, but we didn’t look very hard. However, there are many birds – herring gulls, terns and oyster-catchers aplenty – along with a smart playground for children and a very welcome cafe on a rainy day.
The volcanic vents at White Rocks Beach (about 10km west of the Causeway)
The cliffs that back this lovely beach are white chalk or white limestone (both names are used) and cutting through them is a series of volcanic vents. These are explosion necks through which lava rose. It is thought that they were rather sporadic in performance, as they seem to have become blocked periodically and then re-erupted, sending the blockage material exploding upwards, along with segments of the side walls and other more recent, cindery scoria and agglomerate.
Some of them appear to be filled with coarse basaltic agglomerate that has tumbled back into them. In some areas, a few such features are not considered to be volcanic in origin, but may be merely collapsed solution caves and widened joints in the white rock. However, the ones here are considered to be genuinely volcanic in origin. The sporadic metamorphosis of the country rock is a good indication of the molten activity of earlier times.
It is a beautiful walk along the creamy clean sands towards Dunluce Castle and is well worth the visit for just the stroll. In the cliffs, there are many examples of re-cemented agglomerates with a mixture of black volcanic boulders in a white limestone matrix. The vents themselves are mostly around 20m high, to the top of the cliff. They vary in width from a metre or so to 30m or more.
The lava flow at Dunluce Castle (about 8km west of the Causeway)
The castle is built on a striking headland composed of basalt, 30m high. It is the same series as the Lower Basaltic flows found at The Giant’s Causeway. Here, it lies on top of the white limestone that it burst through. Because of minor faulting very close by, it is also butted up against the limestone in places. The lava around the base of the castle is very varied – as well as the masses of undifferentiated basalt, there are extensive deposits of lava that was actually running just before it solidified, rather than having been settled and unmoving for some time.
There are signs of semi-solidified lava with angular blocks in it, some looking like pillow lavas formed under water, and some areas of agglomerate where lava and rounded lava blocks have been re-worked. Some patches are very distinctly different from one to the next, particularly where black solid basalts overlie dribbling iron-stained lavas.
The name ‘Dunluce’ means ‘strong fort’ in Gaelic and it was once besieged by Edward Bruce, brother of Robert the Bruce. There is easy parking and a small entrance fee to the castle. There has been a castle here since at least 1300 AD. The original early Christian and Viking castle was substantially extended in the early sixteenth century, and again in the latter part of the century, thanks to the bounty from The Girona, a shipwreck from the Spanish Armada.
Volcanic intrusions through the chalk at Kinbane Head (about 16km east of the Causeway)
There is an adequate little car park, but unfortunately the lower part of the path down to the castle has been closed for repairs for some time. Even so, the view from the first section of the path is worth going to see. To the east, there are about 92m high, near-vertical cliffs of intrusive dolerite and basalt from the Lower Basaltic Series, with at least one slender waterfall cascading down in a two-part descent. Below and to the west, there are less steep cliffs of white chalk. A chalk headland protrudes into the sea for about 200m. This is Kinbane Head (‘Kinbane’ means ‘White Head’). It has provided a home for the now-ruined Kinbane Castle since the middle of the sixteenth century.
In the immediate area of the promontory, there are at least six intrusive vents rising through the chalk. One that is easily seen is a volcanic neck immediately below the eastern wall of the castle, which reaches down to the narrow beach that joins the promontory from the mainland with its assortment of black volcanic boulders. Parts of the castle walls are made from these types of boulders. Much of the material within the vents is chalky fragments and tuff – volcanic agglomerate, ash, cinders and larger blocks that have been pyroclastically blasted out of the vents and fallen back in later. Half way along the eastern wall of the promontory, alongside a natural arch, there is another intrusion in which the rising magma solidified as fine-grained basalt (dolerite).
The volcano remnants at Carrickarede (about 14.5km east of the Causeway)
Here, within about one square kilometre, there are several volcanic features. These are unique in the region, as they are relics of the earliest phase of eruptive activity. They are complex and not easily discerned without spending some time and effort over the puzzle. Just offshore, there is an island, which is about 200m long and joined to the mainland by a swinging rope bridge. This island and parts of the nearby mainland are almost entirely intrusive dolerite, or fine-grained basalt that rose through the crust as the great neck of a volcano. Part of the island’s western flank is composed of various broken volcanic rocks, or agglomerates, which fill in a former vent. These also form parts of the adjacent mainland cliffs.
Along the coast to the west, there are white chalk cliffs rising vertically from the tiny beach and stretching to Larry Bane Head, just less than 2km away. They are topped by the semi-columnar dark cliffs of another intrusive feature, the Knocksoghy Sill. The sporadic rain showers that fell during our visit produced some spectacular rainbows over the offshore ‘Sheep Island’, which is also an intrusive sill formation.
The basaltic islands of Ballintoy Harbour
Here, there is a car park right on the sea front. Over the car park wall, there is a fault that separates the chalk of the mainland from the Causeway Basalts of the off-shore stacks, islands and peninsulas. These are columnar, but the columns and pavements are not as well defined as those around the Giant’s Causeway. The fault curves inland behind the car park, where it is seen splitting the roof of a sea cave that is perched on a raised beach from glacial times, 15,000 years ago. Besides sheltering from the lashing rain, our time there was mainly spent watching the rising winds, crashing waves, vivid rainbows and flights of geese overhead.
Overall, our break in Antrim was a great success and we saw many features that illustrate its volcanic past. They are subdued now, where once they were ragingly spectacular. However, it is very satisfying to stand on some of the most picturesque cliffs in the world and imagine the great volcanoes, pyroclastic flows and basalt floods that lurk behind the scenery.
We stayed at the Causeway Hotel, easily the most convenient accommodation for the Causeway. Tel 028 2073 1210 or visit: http://www.giants-causeway-hotel.com/.Other places to stay include The Bushmills Inn Hotel (Tel +44 (0) 28 2073 2339) at around £140/room a night, or many farmhouse B&B places such as Spring Farm (Tel +44 (0) 28 2073 1780) and Brown’s Country House (Tel +44 (0) 28 2073 2777), which provide excellent and inexpensive accommodation.
There are several excellent publications that are invaluable if you want more information:
|The Geology of Northern Ireland, Geological Survey of Northern Ireland, 2004.|
|A Geological Excursion Guide to The Causeway Coast, Paul Lyle, Environment and Heritage Service, 2005.|
|The Causeway Coast geological map (1:50,000), Geological Survey of Northern Ireland, 1998.|
|The Giant’s Causeway, Philip Watson, The National Trust, 2002.|
|There are also many Internet sites, including Wikipedia and individual ones for specific sites, although most of these are tourist information of a general nature and not specifically geological.|
|The parts of this article comprise:|
|Giant’s Causeway (Part 1): An introduction|
|Giant’s Causeway (Part 2): Other volcanic highlights|