Antrim Coast Field Trip 9th-11th September 2011 Joint trip Walton Hall and Ireland OUGS Branches Led by Rosalie Grainger and Phyllis Turkington

This was a trip I had waited a long time for. It had been seven years since I had visited the lovely coastline of County Antrim, Northern Ireland. In September 2011, with the help of Rosalie Grainger and Phyllis Turkington, Walton Hall Branch returned to the region to share the visit with members of Ireland Branch.

Rosalie and Phyllis, knowing beforehand the route we would take from Belfast International Airport to reach our accommodation near the Giant’s Causeway, gave us some suggestions of places we might visit along the way. Phyllis passed to me a report she had prepared from a previous trip that took in interesting locations on the east coast at Sallagh Braes (near Carncastle, Ballygally), Minnis North and Garron Point. Rosalie suggested we take lunch at Carnlough and then visit Cushendun, Loughareema and Ballycastle. Great place names. It worked really well. We saw it all and the trip proper hadn’t even started!

Sallagh Braes

We drove (without stopping) from the airport out via Templepatrick to Larne, then turned north along the coast road. After Ballygally, we turned off the main road towards Carncastle and quickly found the location referred-to by Phyllis, the Sallagh Braes landslip complex.

Figure 1 The Sallagh Braes Landslip Complex (photo: Tom Miller)

The high ridge above Sallagh Braes has large slumped blocks of basalt and cretaceous chalk, interpreted (by Mark Cooper of the GSNI) as an end glacial waterfall that was fed by Lough Neagh meltwater. The ridge itself was too far to reach on that day, but we could clearly see the rotated blocks and slump structures from afar.

Minnis North

Continuing north, we soon reached the obvious landslip location at Minnis North. We stopped briefly for a look. Here, chalk and greensand on soft Jurassic rocks had landslipped down to sea level. There was a bowl-shaped area of subsidence some 100m wide, where Jurassic mudstone had mobilised and effectively squeezed from the ground. Landslipping occurs after heavy rain and winter weather. The local authority need to clear the road regularly by pushing the debris onto the beach opposite. Supposedly, there are Jurassic ammonites, belemnites, bivalves, sharks teeth and gastropods of Cretaceous age to be found here.

Garron Point

After a splendid lunch at Carnlough, we continued north to Garron Point where the road, almost at sea level, skirts the base of chalk cliffs. This is a popular stopping point. It has striking views of cretaceous chalk overlain by black Palaeogene basalt lava flows. Large blocks of the chalk, capped by the basalt, have slumped and rotated to show bedding steeply dipping to landward.

Figure 2 Garron Point, Large blocks of Chalk and Basalt, slumped and rotated (photo: Tom Miller)

Cushendun, Conglomerate

Along the coast north of Garron Point, the lavas and Mesozoic rocks recede to reveal Palaeozoic and Dalradian rocks. At Cushendun, a very coarse conglomerate is revealed of Lower Devonian age, similar to that at the Highland Boundary Complex in the vicinity of Loch Lomond. This is a very impressive outcrop. We stopped for a while at Cushendun, to look at this conglomerate and other rocks nearby.

Figure 3 Cushendun, Lower Devonian Coarse Conglomerate (photo: Tom Miller)

We studied the large well-rounded clasts in the Cushendun conglomerate. Some of the pebbles were huge, best described as boulders. We speculated on the context and environment of deposition. Glacial? Beach Deposit? We could not imagine the conditions under which such a deposit could occur. Nearby, but not in situ, were large blocks of schist and quartzite, used as sea defences. These are supposedly Dalradian, reminiscent of the Highland Boundary Complex suite of rocks in Scotland, just across the Irish Sea.

Eventually we left Cushendun and continued past the vanishing lake at Loughareema, towards Ballycastle. Stopping briefly, we noted that the lake had “vanished” but its margin was clearly visible in the peat. Running short of time (and energy), we proceeded towards the Giant’s Causeway and our hotel.

The Trip

That evening at the hotel, Rosalie gave us a good presentation and briefing on what we would see in the course of the weekend.

On Saturday, starting at Portrush, we would study the Portrush Sill and its intrusion into (and effects on) the Lower Jurassic rocks at outcrop there. We would look at the visible succession of Mesozoic sedimentary rocks beneath the Tertiary basalts.

We would look at the juxtaposition (and effects) of the Tertiary basalt eruptions on and against the Cretaceous chalk (Ulster White Limestone Formation) at Whiterocks to the east of Portrush. Leaving the geology aside for a spell, we would visit Dunluce Castle and learn something of the Earls of Ulster.

In the afternoon, we would visit Ballintoy and examine the chalk, volcanics, raised beach and faulting.

Sunday morning would be spent at the Giant’s Causeway, walking the lower path and looking at the basalts. After a lunch break, we would visit the Bushmills Distillery.

Saturday, 10th September

Lansdowne Crescent, Portrush

After breakfast we set off for Portrush and the promontory of Ramore Head. This promontory is the outcrop of a dolerite sill that intruded the Lower Jurassic shales in this area. We gathered at Lansdowne Crescent, a famous location on the east side of the promontory. Here, both the dolerite of the Portrush Sill and the Jurassic marine shales could be seen, in contact. Rosalie explained that this location was significant in the historic 18th century dispute that took place between the “Neptunists” and the “Plutonists” as they tried to understand rocks and their formation.

Figure 4 The Portrush Sill, Dolerite in Jurassic Marine Sediments (photo: Tom Miller)

The Neptunists believed that igneous rocks and hard metasediments were essentially ancient precipitations from a primordial ocean. Stratified rocks (containing fossils) were believed to be products of erosion from the ancient precipitates. Volcanic lavas were believed to be from the burning of coal beds.

In contrast, the Plutonists believed in volcanism and the perpetual rock cycle (led by James Hutton).

The Portrush sequence was, for a while, held as supporting the Neptunian argument because it contained definite marine fossils (ammonites) that could not have existed in a molten magma. Ultimately, the dispute was concluded in 1802 when John Playfair gave the widely accepted hornfels explanation, for the very hard-baked ammonite-rich rocks.

Interestingly, an information board close to the outcrop described the dolerite Portrush Sill as a ‘Dolomite’ sill in the Liassic Shales. Rosalie was quick to point out the error here.

Whiterocks

We set off eastward to our next location at Whiterocks. After parking the cars we descended to the beach. Rosalie pointed out the chalk cliffs (Ulster White Limestone) topped by black basalt. A striking contrast. We ventured onto the beach to look at the effects on the chalk of the proximity of the basalt lavas.

Figure 5 Whiterocks, Rosalie explains the Chalk Landforms (photo: Tom Miller)

Rosalie explained the coccolithic formation of the chalk, and some of the effects of local jointing on the formation of stacks/pillars and arches on this section of the coastline. She pointed out a local basalt vent structure, the presence of vent agglomerates and the effects of heat on the chalk from the nearby basalt. The normally very regular flint bands would become chaotic and jumbled close to the basalt intrusion.

Features in the chalk included occasional belemnite fossils and stylolites formed where calcite mineralisation had occurred, under pressure, in zig-zag stress cracks in the chalk. Figure 6 Whiterocks, Caves in the Chalk (photo: Tom Miller)

Rosalie described the significant variations that occur in the sand level along the coast. On other occasions, vent agglomerates (with zeolites) can be found in the chalk/basalt margins, but on that day they were buried and out of sight.

We left the beach and travelled east along the coast road towards our next location at Dunluce Castle. Rosalie stopped us briefly on the road to look at the profile of the coastline and to see how the NW/SE faults were expressed along the coastline.

Dunluce Castle

After a good lunch in the cabin beside Dunluce Castle, we entered the castle and took in an excellent video presentation of recent archaeological work done at the castle site. Very well presented and full of interest. While Dunluce Castle has the appearance of a defensive stronghold that's seen desperate times in the past, this is actually not the case for most of its history. Yes, it was built as a defensive stronghold, but when you walk around it you notice that it was actually a grand old house with (no doubt) a fine kitchen and banqueting hall.

Dunluce Castle was built in the 13th Century by the 2nd Earl of Ulster (one Richard Og de Burgh) whose wife is thought to have been a daughter of Count Arnold III of Guines (a familiar name today).

Figure 7 - Dunluce Castle (photo: Phyllis Turkington)

By 1513 it was held by the warlike MacQuillin clan, who were eventually displaced by the Clan MacDonald in 1583. Dunluce Castle remained in the hands of the MacDonalds (or later perhaps, the MacDonnells) until their impoverishment in 1690 after the Battle of the Boyne. After this, it fell into ruin and was scavenged for building stone.

It is believed that at one time in the Castle's history, its kitchen fell into the sea killing all those in it. Only a kitchen boy survived because he was sitting in a corner that did not fall.

We enjoyed an interesting tour around Dunluce Castle, led by the local guide. It must have been a splendid place in its heyday. Occasionally, one could find hexagonal and pentagonal stone slabs in the masonry. There was speculation as to the origin of these. Recent archaeological excavations (in 2009) located the foundations of a complete town, just to the east of the castle.

Ballintoy

After our visit to Dunluce Castle, we travelled east in warm sunshine to the small coastal village of Ballintoy, on the Ballycastle headland. We descended the winding road to the old harbour and parked in the car park beside the old twin lime kilns.

The evening was beautiful. We climbed the steps up onto the kiln top and surveyed the old harbour and the basalt dyke in the chalk cliffs behind. At Ballintoy, there are several interesting geological features: a volcanic plug, indications of raised sea levels and well-defined local faulting.

Figure 8 – Basalt Dyke in the Chalk, Ballintoy Harbour (photo: Phyllis Turkington)

Walking north from the car park, we paused alongside the sea wall and assembled for a group photograph. All around us were outcrops of the “middle” causeway basalt flow (the Causeway Tholeiitic Member), distinct from the lower basalt by its columnar jointing. From the end of the harbour basin, one could see how the local north-south faulting had neatly partitioned the different rock types at outcrop.

Walking east from the car park, we crossed another narrow beach and walked beside the smooth edges of a huge, dark, bare basalt plug, marking the position of a local vent. The chalk/limestone platform to seaward of the plug was highly fossiliferous, containing hundreds of belemnites and the occasional ammonite. Rosalie pointed out the trace of a very large ammonoid shell, some 0.5m across.

Figure 9 – Group Photograph (photo: Phyllis Turkington)

We surveyed a “blow hole” wave-cut erosion feature in the limestone cliff facing the sea.

Figure 10 – Limestone (left) and Basalt Plug (right), Ballintoy (photo: Tom Miller)

This area was very popular for sea angling Rosalie commented. The anglers watched us with some dismay. This was our final location of the day. Somewhat tired now, we returned to the hotel.

Sunday, 11th September

Sunday was a duller day. A little wet with occasional showers and a steadily increasing blow from the west.

Figure 11 – Giant’s Causeway, Lower Road (photo: Tom Miller)

Descending the lower road, Rosalie took some time to explain the contexts of the basalt flows. This was easily done, as the different basalts are fully exposed and readily distinguished from each other. She explained that there had been three major phases of basaltic volcanism in Antrim around 60 Ma ago. The Lower Basalts of the first phase (beneath the causeway) had built a substantial lava plateau over the chalk. This had continued periodically for over 100,000 years before activity ceased and a time of tropical deep weathering and forest growth ensued. The deep weathering produced a form of red laterite soil of a type that can be seen today in parts of Africa. This was exposed as a now "interbasaltic" surface beside the road.

Figure 12 – Giant’s Causeway, Distinctive Lower Basalt (photo: Tom Miller)

After the prolonged period of inactivity, a much more extensive volcanic event followed, which produced the thick Middle (Causeway) Basalts. These are tholeiitic in chemistry and have the distinctive columnar jointing. The Causeway Basalts, although thick (~100m) were restricted to a small area, being controlled by the weathered topography of the Lower Basalts. They were also erupted into a river basin, which allowed for the slow cooling that produced the columnar formation. The Giant's Causeway basalt flows are only 2km wide.

Figure 13 – Giant’s Causeway, Red Interbasaltic Laterite Layer (photo: Phyllis Turkington)

We soon descended to near sea level and made our way towards the Grand Causeway. The columns and hexagonal/pentagonal paving out on the causeway is quite remarkable. In 1940, a Russian geologist named Tomkeieff suggested descriptive terms for the basalt formations, based on classical Greek architecture. He proposed a three-fold division of the columns of a causeway lava flow into Lower (Colonnade), Middle (Entablature) and Upper (Upper Colonnade), to differentiate between the different levels in the flow internal structure. These divisions became quite clear to see when the Giant's Organ came into view. The colonnade is remarkably regular in form. The entablature is less regular (in fact, quite chaotic) above the Giant's Organ. The upper colonnade returns to a more regular columnar form but has frequently curving columns.

Rosalie explained that the distinctive jointing is due to shrinkage after cooling. The orientation of the joints indicates the direction of heat flow through the basalt during cooling. She then explained that after the eruption of the Causeway Basalts, another period of dormancy ensued in which a second laterite soil formed. This quiet period (shorter than the first) came to an end with the eruption of the Upper Basalts. The Upper basalts marked the final phase of Tertiary volcanism in Antrim.

Figure 14 – Giant’s Causeway, Giant’s Organ Colonnade (photo: Phyllis Turkington)

In windy showers, we returned to the Causeway Hotel for a warming coffee, then on to Bushmills to visit the famous Bushmills whiskey distillery.

We arrived at the Bushmills Distillery in time to have a good lunch, and then to partake in an extensive tour of the whiskey making process. We were shown the great fermentation vats, distilling halls, sherry barrel halls and finally, the bottling hall. A large-scale operation. The whole tour took about an hour (seemed like 20 minutes). They are very proud of their triple-distilling process. At the end of the tour, we were offered a taster of the 10-year, 12-year, 16-year and one other that I can’t remember! (can’t think why...).

A lovely weekend. We thanked Rosalie and Phyllis heartily for their patience and willingness to share their knowledge with us. We shall return.

Tom Miller