An introduction to the geology of Rathlin Island
The geology of Rathlin is characterised by white limestone covered by thick layers of basalt. In some places the basalt exists in hexagonal columns, much like those found in the nearby Giant’s Causeway.
At Brockley, in the west of the island, the basalt has been transformed into porcellanite, and this very hard rock was fashioned into axe heads by Neolithic man. There is also clear evidence of faulting, the vertical movement of large sections of the earth’s surface.
When viewing Rathlin from Ballycastle it is clear that the south facing cliffs of the island are made up of two layers. The lower white layer is limestone, the upper darker layer is basalt.
The picture of a limestone wall (right), contrasts the two types of rock – the construction date (1833) is outlined in basalt stones.
The white limestone was originally deposited beneath the sea during the Cretaceous Period about 90 million years ago. This sedimentary rock is formed from the compacted remains of the shells of small sea creatures. Pure white limestone is also known as chalk, but the Ulster variety is much harder than its English contemporary. This is a consequence of it having been overlaid with basalt.
Large quantities of flint nodules may be observed within the limestone. These nodules are composed of finely crystalline quartz, a form of silica. This material commonly occurs in the internal structure of sea creatures such as sponges.
When the limestone is being formed, the remains of these silica structures can become dissolved, and if conditions are right, the silica can precipitate out as a flint nodule (though there seems to be some uncertainty as to how this occurs exactly). When struck, the flint nodule shatters into sharp edged shards and many examples of ancient flint tools have been discovered on Rathlin.
The limestone also contains a significant number of fossilised belemnites. The belemnite was a squid like sea creature and the fossilised remains of its internal shell look like an amber bullet. In the past these remains were often referred to as ‘Thunderstones’ in the belief that they were lightning bolts. Fragments of these easily recognisable fossils are often seen in the limestone boulders that make up many of the island field boundary walls and in beach stones.
A wide swathe of raised limestone pebble beach runs between Church Bay and Mill Bay and in this area much of the soil is only a few inches deep. During the last ice age, which ended about 10,000 years ago, Rathlin would have been covered by a thick sheet of ice up to a mile thick.
The massive weight of this ice caused the land beneath it to sink and when the ice melted, limestone pebble beaches were formed along the still depressed shoreline. Over time the island slowly recovered its original position and the beaches that were laid down immediately after the melting of the ice were raised up.
For several years after the first world war the limestone in the cliffs above Killeaney was mined commercially. A jetty was constructed and a short section of narrow gauge railway track was laid along the shore to transport the quarried stone to the jetty. The limestone was then shipped to steel smelters in Scotland. The remains of the rail track and the jetty are still visible.
The black basalt that overlies the limestone was laid down during the tertiary volcanic period 60 million years ago. Huge quantities of lava flowed over much of the north of Antrim, and on Rathlin, near Altacorry, some of these individual flows reach a thickness of 40 metres.
At the west lighthouse viewpoint, which is open to visitors every spring and summer, you can get an excellent view of the basalt cliff face. The different basaltic layers are clearly visible and in some cases the flows are separated by a reddish brown stratum of weathered basalt known as laterite.
The presence of this laterite soil layer indicates that the rock immediately beneath it was exposed to the elements for a considerable period of time before it was covered by another outflow of lava.
The most well known geological features on Rathlin are the two volcanic plugs at Brockley. These plugs are formed from a type of basalt known as dolerite. Typically this occurs in the volcanic vents that carried the molten lava to the surface. At Brockley the vent heated the surrounding laterite (older weathered basalt) and recrystallised it into a much harder and denser type of rock known as porcellanite. A similar deposit occurs on the mainland at Tievebulliagh, Co Antrim.
Porcellanite is very hard and very heavy. When broken it forms sharp shards. Neolithic man discovered these properties and extracted the porcellanite to make distinctive stone axes. Examples of these Rathlin porcellanite axes have been found right across Great Britain and Ireland.
Along the eastern shore of the island the basalt sometimes occurs in hexagonal columns similar in form to those found at the Giant’s Causeway. At Doon Point the columns are heavily deformed. Some bend down into the sea, while others have been forced up, leaving behind them a thick swathe of fractured stumps.
Faulting is the downward movement of large sections of the earth's surface. On Rathlin faulting was responsible for the formation of the rock stacks off the west lighthouse. A separate fault was responsible for pulling the southern most tip of the island at Rue Point downwards into the sea.
By Jonathan Mitchell