Lighthouses & Geology
Pauline Butler
Geology means 'the study of the earth'. The study of geology
deals with many aspects of our physical environment ranging from
topics as diverse as the eruption of volcanoes to the location of
oil, coal, and gas reserves, or estimation of the age of the earth.
It is not surprising that at various times geologists have turned
their attention to the places where lighthouses are situated or to
the materials from which they are built. Having lived all my life
at lighthouses this aspect of geology has always fascinated
me.
When I retired from Galley Head Lighthouse I knew I had to do something to occupy my time. I applied for a two year adult diploma course in geology run by Dr Bettie Higgs at University College, Cork and, to my delight, was accepted. I graduated with my diploma in geology in September 2000.
Hook Head is the oldest lighthouse in Ireland, and one of the oldest in Europe still operating. In the 5th century St Dubhan set up a fire beacon on the headland as a warning to mariners. After his death his monks kept the beacon going for another 6 centuries. Between 1170 and 1184 the Normans built the present lighthouse. It was built from local limestone and burned lime mixed with ox's blood. Even today traces of the blood-lime mix can be seen coming through the paintwork. The walls are 9 to 13 feet thick and 80 feet above the ground.
The rocks around Hook Head are hard limestone and calcareous shales of the Carboniferous period. These limestones are highly fossiliferous and contain a rich fauna of shelled invertebrates such as rugose and tabulate corals, bryozoans, brachiopods, crinoids, gastropods, and rare examples of trilobites.
The Fastnet Rock Lighthouse is the most famous of Irish lighthouses and everybody's idea of what a lighthouse should look like. It is a magnificent finger of granite rising from the base of the rock and clinging to it until at last it thrusts skywards. The rock itself if a jagged pinnacle comprised of hard clay slate, with the strata included at a high angle of varying degrees of harness, some of them converted into a hard quartzite or helvin. It contains veins of quartz, some large crystals, and probably belongs to the Lower Silurian System. There is also some coarse grained old red sandstone.
After a series of tragic accidents in the 1840s sanction was given to build a lighthouse on Fastnet Rock. A cast iron tower was built but in 1881 a severe hurricane hit the coast and gigantic waves smashed the lantern. In 1891 the Commissioners of Irish Lights decided, because of its strategic position, to build a new granite tower on Fastnet Rock. A total of 2,074 granite blocks weighing 4,300 tonnes were cut to precise specification at Penryn in Cornwall before being shipped to Crookhaven in Co. Cork. The granite blocks were of a superior stone, fine grained, hard, and uniform in colour, free from all blemishes or defects, and very finely dressed. The blocks were first assembled in Cornwall, then taken apart and built on the site. Each block was carefully keyed into the next one and into the rock itself, making the lower part of the tower a solid mass in order to strengthen the 51 metres tall tower against the onslaught of the sea. The building of this granite tower was a historic achievement and truly an Herculean feat.
At the entrance to the tower there are 2 bronze doors weighing a half tonne each, inside which is a large American oak door. The storm shutters and windows are made of bronze. The building of this lighthouse took 7 years, as work was dictated by wind, weather, and tide. At the other end of the country, Inishtrahull is the most northerly lighthouse in Ireland. This, the geologists reckon, is the oldest piece of land in Ireland.
Around the turn of the 20th century W.J. McCallion, a graduate of the University of Glasgow, was doing a geological survey of the gneiss rock of Inishtrahull and found that the basement rock on the island did not match that of the Irish mainland. His initial feeling was that the gneiss rock of Inishtrahull was similar to that of the Outer Hebrides.
This Augen Gneiss, so called because of the little pink eyes present, is a crushed granite and attracts a lovely blue-grey luminous lichen. In 1930 he published an article on this subject, but as time went by McCallion came to doubt his own theory.
In 1989 Roddy Muir, a young Scottish geologist working at the University of Wales at Aberystwyth with another geologist, Dr Stephen Daly, began work on a project funded by the Natural Environment Research Council. His brief was to sort out the origin of the basement rock of the islands of Islay and Colonsay. During his research he found the McCallion report and in a paleogeographic reconstruction he became convinced of the geological links between Greenland, Islay, Colonsay, and Inishtrahull.
By a careful study of the theory of plate tectonics and visits to all four locations, Muir confirmed his beliefs. His visit to Inishtrahull enabled him to fit into place the last piece of this intriguing geological jigsaw. The island of Inishtrahull is really part of the southern tip of Greenland and not geologically related to the Irish mainland. Eighteen hundred million years ago.
Inishtrahull, Islay, and Colonsay broke free of the southern tip of Greenland. Islay and Colonsay anchored off the coast of Scotland. Inishtrahull drifted further south to the Donegal coast, 800 miles from Greenland.
This island, the oldest land in Ireland has a modern reinforced concrete lighthouse tower, novel in design. It was built and came into operation in 1958 when it replaced the old lighthouse of 1813.
Pauline Butler comes from a lightkeeping family. She was married to a lightkeeper and on the death of her husband succeeded him as Attendant of Galley Head Lighthouse. She retired from this position in 1997.
When I retired from Galley Head Lighthouse I knew I had to do something to occupy my time. I applied for a two year adult diploma course in geology run by Dr Bettie Higgs at University College, Cork and, to my delight, was accepted. I graduated with my diploma in geology in September 2000.
Hook Head is the oldest lighthouse in Ireland, and one of the oldest in Europe still operating. In the 5th century St Dubhan set up a fire beacon on the headland as a warning to mariners. After his death his monks kept the beacon going for another 6 centuries. Between 1170 and 1184 the Normans built the present lighthouse. It was built from local limestone and burned lime mixed with ox's blood. Even today traces of the blood-lime mix can be seen coming through the paintwork. The walls are 9 to 13 feet thick and 80 feet above the ground.
The rocks around Hook Head are hard limestone and calcareous shales of the Carboniferous period. These limestones are highly fossiliferous and contain a rich fauna of shelled invertebrates such as rugose and tabulate corals, bryozoans, brachiopods, crinoids, gastropods, and rare examples of trilobites.
The Fastnet Rock Lighthouse is the most famous of Irish lighthouses and everybody's idea of what a lighthouse should look like. It is a magnificent finger of granite rising from the base of the rock and clinging to it until at last it thrusts skywards. The rock itself if a jagged pinnacle comprised of hard clay slate, with the strata included at a high angle of varying degrees of harness, some of them converted into a hard quartzite or helvin. It contains veins of quartz, some large crystals, and probably belongs to the Lower Silurian System. There is also some coarse grained old red sandstone.
After a series of tragic accidents in the 1840s sanction was given to build a lighthouse on Fastnet Rock. A cast iron tower was built but in 1881 a severe hurricane hit the coast and gigantic waves smashed the lantern. In 1891 the Commissioners of Irish Lights decided, because of its strategic position, to build a new granite tower on Fastnet Rock. A total of 2,074 granite blocks weighing 4,300 tonnes were cut to precise specification at Penryn in Cornwall before being shipped to Crookhaven in Co. Cork. The granite blocks were of a superior stone, fine grained, hard, and uniform in colour, free from all blemishes or defects, and very finely dressed. The blocks were first assembled in Cornwall, then taken apart and built on the site. Each block was carefully keyed into the next one and into the rock itself, making the lower part of the tower a solid mass in order to strengthen the 51 metres tall tower against the onslaught of the sea. The building of this granite tower was a historic achievement and truly an Herculean feat.
At the entrance to the tower there are 2 bronze doors weighing a half tonne each, inside which is a large American oak door. The storm shutters and windows are made of bronze. The building of this lighthouse took 7 years, as work was dictated by wind, weather, and tide. At the other end of the country, Inishtrahull is the most northerly lighthouse in Ireland. This, the geologists reckon, is the oldest piece of land in Ireland.
Around the turn of the 20th century W.J. McCallion, a graduate of the University of Glasgow, was doing a geological survey of the gneiss rock of Inishtrahull and found that the basement rock on the island did not match that of the Irish mainland. His initial feeling was that the gneiss rock of Inishtrahull was similar to that of the Outer Hebrides.
This Augen Gneiss, so called because of the little pink eyes present, is a crushed granite and attracts a lovely blue-grey luminous lichen. In 1930 he published an article on this subject, but as time went by McCallion came to doubt his own theory.
In 1989 Roddy Muir, a young Scottish geologist working at the University of Wales at Aberystwyth with another geologist, Dr Stephen Daly, began work on a project funded by the Natural Environment Research Council. His brief was to sort out the origin of the basement rock of the islands of Islay and Colonsay. During his research he found the McCallion report and in a paleogeographic reconstruction he became convinced of the geological links between Greenland, Islay, Colonsay, and Inishtrahull.
By a careful study of the theory of plate tectonics and visits to all four locations, Muir confirmed his beliefs. His visit to Inishtrahull enabled him to fit into place the last piece of this intriguing geological jigsaw. The island of Inishtrahull is really part of the southern tip of Greenland and not geologically related to the Irish mainland. Eighteen hundred million years ago.
Inishtrahull, Islay, and Colonsay broke free of the southern tip of Greenland. Islay and Colonsay anchored off the coast of Scotland. Inishtrahull drifted further south to the Donegal coast, 800 miles from Greenland.
This island, the oldest land in Ireland has a modern reinforced concrete lighthouse tower, novel in design. It was built and came into operation in 1958 when it replaced the old lighthouse of 1813.
Pauline Butler comes from a lightkeeping family. She was married to a lightkeeper and on the death of her husband succeeded him as Attendant of Galley Head Lighthouse. She retired from this position in 1997.