June 30th - Murlough Bay, County Antrim.


Conductor; Peter Miller


Drive to sea level by the very steep and twisty road (suitable for cars but not larger vehicles). Park at the small car park, just where you reach sea level (ID 1955 4228). Walk along the lane southeast. Continue beyond the last house through rocks to where there is a small sandy beach: here descend to the shore (ID 2003 4186).


The grey, hard rocks here belong to the Dalradian Supergroup. These are sediments which were deposited in a long-vanished ocean which existed from 1200 to 600 million years ago. They were compressed and pushed deep into the crust in the Ordovician Grampian orogeny, about 500 million years ago, and metamorphosed by heat and pressure. The clays became schist, the sands quartzite and, and the limestones marble. The conspicuous “layering” is actually foliation produced by pressure but with a little imagination bedding can just about be discerned running at an angle to it. Sometimes the bedding is indicated by lines of weathered-out hollows. The foliation is partly due to flaky mica crystals which grew in the rocks under pressure and these give the rocks a rather attractive lustre in the sunlight.


Also conspicuous are masses of white vein-quartz. After the initial compression there was a phase of relaxation and cracks opened in the rocks. The quartz which must have originally been disseminated through the rocks migrated to fill the voids.  Later a further period of compression folded and sheared these veins, turning them into streaks and blebs of quartz.


The rocks dip to the northwest. The Torr Head Limestone – just a few km south - is an important marker as it is believed to be equivalent to the Loch Tay Limestone and therefore the sediments on one side must be Argyll Group and the other the overlying Southern Highland Group. It is therefore crucial to find which way up they are. This is done by looking for “way up” structures in the sediments. Unfortunately these are hard to find because of the type of sediment and of the extreme alteration. The general opinion is that the rocks are upside down which would imply that these Murlough Bay schists north of Torr Head are Argyll Group and therefore the oldest rocks in County Antrim.


Retrace your steps to the house. There is a dramatic change here on the shore from the hard grey Dalradian rocks to soft yellow or red Lower Carboniferous sandstones. These are about 330 million years old. They are folded only very gently: they dip west towards Fair Head. The sudden change is because the Carboniferous sediments have been faulted down to the north some 400 metres by a very large fault, the Great Gaw. The fault itself is not seen but its position can easily be deduced to within a couple of metres (ID 1995 4195). The Great Gaw runs south of Fair Head and strikes east-west: it re-emerges on the coast near Bath Lodge, just east of Ballycastle. The exact age of this faulting is uncertain: it may have actually been going on at the same time as the sediments were being deposited. 


The sediments were deposited in the deltas of rivers flowing off a continent which lay to the north in Carboniferous times.  Cross-bedding, characteristic of deltaic sediments, is very obvious as are patches of pebbles washed in by faster currents.

Most of the pebbles are white quartzite and almost certainly were derived from the Dalradian rocks, which must have been exposed and eroded by Carboniferous times. The red colour is due to oxidised iron and is characteristic of continental or continent-derived sediments. The harder dark lumps are iron concretions formed around washed-in plant fragments. The sandstones contain feldspar which does not survive exposure to the elements for very long so the material must have been eroded and deposited very rapidly.


A short distance northwest there is a group of large dark-coloured stacks, just below the cattle grid on the lane (ID 1986 4206).  These are of basalt lava. A large part of County Antrim is of course underlain by basalts of Palaeogene age (about 57 million years ago). However these Murlough Bay Basalts are much older and in fact are contemporaneous with the Carboniferous sandstones. They have more mineral veins and are more altered and sheared than the usual Antrim basalt. The base of the first flow can be seen resting on the sandstones at high water mark on the east side of the largest stack. There is a very small amount of ash below the flow. Go round to the west side of this stack. Here there are vague round structures in the lava. 

These were interpreted by the Russian geologist Tomkieff as lava pillows (submarine lava structures) but this is not generally accepted.


The stacks themselves are typical of many on the Antrim coast in that they were formed about 8500 years ago, along with the raised beach, when the sea level was higher than now. The smaller northern stack has a cave in it but this faces inland – not where one would normally expect to find a sea-cave.


Continue northwest along the lane. On the left side of the lane is a very well constructed lime kiln of with typical twin arches.  The kiln is mainly constructed of sandstone. Blocks of the very pure Ulster White Limestone or “chalk” are conspicuous in the landslip slopes above the lane. The chalk and coal would have been loaded into the top of the kiln in alternating layers and burnt. This produced quicklime (calcium oxide) which was removed through the stepped arches at the bottom along with the ash. The quicklime could be mixed with water to produce slaked lime (calcium hydroxide). Lime had many uses – among them the manufacture of mortar and cement, the treatment of fields to neutralise acid soils and the lime-washing of stone houses to waterproof the walls. As the west of Scotland is deficient in limestone there was a considerable potential export trade.


Return to the car park, cross a small stream and go northwest.  You come to a promontory [fig 1] formed by a jumbled mass of rocks (ID 1955 4252). This is Drumnakill: it is made of material from the Fair Head dolerite sill. The sill itself is far above this level but this whole mass has come down as a large landslip. Here you can inspect the dolerite at close quarters. Through a hand lens you can see the pale greenish-yellow crystals of olivine.


A sill is a horizontal sheet of igneous rock intruded into other rocks. When magma comes up from a deep magma chamber it can eventually burst out on the surface forming a lava flow. However sometimes it reaches a level where the pressure in the magma is greater than the weight of the overlying rocks: and in that case it can spread out sideways, jacking up the overburden. Being well insulated, a sill takes a lot longer to cool than a lava flow and so develops larger mineral crystals – well seen here. Dolerite has essentially the same mineral makeup as basalt: the only difference is the crystal size.


Igneous masses form joints perpendicular to the cooling surface as they cool and contract. (“Joints” are cracks which show no movement, unlike faults). Both in the case of a lava flow and of a sill, the cooling surfaces are horizontal and so the joints are vertical, giving the well-known “columnar jointing”. Although they are now rather jumbled and have mostly tipped over on their sides the columns are very clear at Drumnakill.


Proceeding northwest along the coast you have the choice of tramping across some very wet muddy slopes or scrambling along a very bouldery shore, eventually joining a grassy track. The Carboniferous sandstones are well seen along this stretch: there are also shale horizons representing marine incursions and some spectacular large spherical concretions [fig 2] (ID 1932 4270).  These are patches within the sandstone where carbonate has become concentrated by some sort of migration process, making the rock that bit more resistant to erosion. At this end of Murlough Bay there were coal seams and these were extensively mined in the past and you will pass the ruins of miners’ cottages.


From here Fair Head is a spectacular sight [fig 3] and can clearly be seen to have two components: the main sill and the lower, thinner, Binnagapple sill.The track ends at the Arched Mine [fig 4], with its twin adits, now closed (ID 1885 4317). (An “adit” is a horizontal tunnel).

Beyond the mine is the block scree, a great apron of truly gigantic blocks which runs right round the base of Fair Head.

Following the track southeast 100m from the mine there is a small dump where you can find samples of coal (ID 1889 4308). The Ballycastle coals are Lower Carboniferous and so older than the major coalfields of England. The Ballycastle coal seams were thin – none thicker than four feet – and were high in sulphur, which limited their usefulness for some industrial purposes. Drainage in the mines was also a perennial problem. In this rather inaccessible spot, the coal would have been loaded directly onto small boats for shipping out. Return by following the track which angles up to the south and joins the metalled road at the “middle car park” (ID 1917 4255). This involves a bit of uphill walking but is easier than the scramble back along the shore.


Drive up to the top car parks – these are just where the road levels out (ID 1909 4179).  On the way the road winds round many large chalk blocks – all this material is landslip.


Walk back down the road for 150m. Ascend the knoll on the right (ID 1919 4178). This has got to be one of the finest geological viewpoints in Ireland. Far below are the basalt sea-stacks and the sandstones. Above the shore is a huge amphitheatre filled with hummocky, jumbled landslip [fig 5] – very typical of many of the slopes round the Antrim Coast. It is thought that these vast landslips occurred about 18 000 years ago when the major Quaternary ice sheets melted and left the over-steepened slopes unsupported.


Further up is a small grey cliff which is made of the Dalradian sediments. Just above this is a conspicuous deep red formation – this Sherwood Sandstone which is a terrestrial desert deposit of Triassic age, about 240 million years old.


Overlying this, and most conspicuous of all, are vertical cliffs in the Cretaceous Ulster White Limestone (“chalk”) [fig 6], representing a return to marine deposition 90 million years ago.  These are the source of the many white blocks seen on the drive up. Since the Jurassic system normally comes between the Triassic and Cretaceous systems this means that the entire Jurassic is missing here. Was it never deposited, or was it deposited and then completely eroded before the late Cretaceous? A small stream emerges at the base of the chalk: the water can easily pass through the much fissured chalk, but cannot so easily pass through the Trias.


Beyond the range of white cliffs is a rounded ridge. This is almost entirely made of the Dalradian rocks, but there is a small flat cap of chalk on top which is much greener than the rest of the hill, because of its better drained and less acid soil. This rests directly on the Dalradian, so that the Carboniferous and even the Trias are missing at that locality. The Dalradian mountain is an expression of the “Highland Border Ridge”, which extends to the south-west and disappears under the Antrim basalts.  It seems to have been a recurrent axis of uplift throughout geological time. The same feature can be seen 20km to the north-west across the North Channel where it forms the Mull of Kintyre. The Mull lighthouse is clearly visible.


Looking west of course is the Fair Head sill, already mentioned, of Palaeogene age, and 57 million years old. It is very obvious from here that although it dips in general to the south (left) it also steps up periodically. This is quite characteristic of sills.


Just across the road (on the west side) are some small pits (ID 1914 4187). This is where the sill finally peters out at its eastern extremity.  In these pits both the dolerite of the sill and chalk can be seen in a rather complicated arrangement. At one point the chalk in contact with the dolerite is seen to have been altered by the heat from the sill: it has a grey sugary appearance quite unlike the usual pure white chalk. This demonstrates that the dolerite is indeed an intrusion – and not just a very thick lava flow – and also that it is later than the chalk.


In the course of this excursion to a limited area you will have seen rocks of a variety of types and of a range of ages which can scarcely be equalled anywhere else.

 
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Fig 1 - Drumnakill

Fig 2 - Spherical concretions

Fig 3 - Fair Head

Fig 4 - Arched Mine

Fig 5 - Jumbled landslip

Fig 6 - Ulster White Limestone

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