The Tertiary basalts of Antrim and the underlying Chalk
– The Giant’s Causeway.
by T. Rockett
(A short talk given to members of the Hull Geological Society on 18th October 2007.)
The Giant’s Causeway is on Northern Ireland’s north coast 5 miles north of Bushmills. It is a natural pavement of basalt columns eroded by weather, sea and ice to an almost level surface which projects into the Atlantic Ocean at sea level.
The basalts are Tertiary Volcanics which appeared some 60 million years ago at the start of the opening of the Atlantic. Vast volcanic flows occurred not only in Northern Ireland but also across the sea in Scotland and the Western Islands. Not all the flows resulted in basalt columns – these were dependent largely upon the rate of cooling. Alternatively, in ancient times two giants, Finn MacCool of Ireland and Benadonner of Scotland co-existed. They never met but for reasons unknown wanted to fight each other. MacCool built the causeway so they could meet and do battle. As he approached Scotland he saw the vast size of Benadonner in the distance. He decided to run home, chased by the Scottish giant. MacCool reached home and asked his wife to protect him. She disguised him as her baby. Benadonner saw the size of the baby and assumed the dad must be really gigantic. He fled home, tearing up the causeway so he could not be followed. This is why one end of the causeway exists in Northern Ireland and the other on the Scottish Island of Staffa – the two surviving ends of the causeway.
History of The Causeway.
Throughout history locals knew of its existence. In 1692 a Bishop from Derry who was also a Cambridge scholar visited the site and published a report about it in a " learned " journal. In 1739 Dublin artist Susanna Drury painted some of the views and an engraver Vivares reproduced them and they were circulated all over Europe. This resulted in many visitors to the area. In 1772 Sir Joseph Banks discovered the basalt columns of Fingal’s Cave on Staffa – the other " end " of the causeway. In 1786 William Hamilton, an amateur geologist, wrote the first account of the geology of the area. In the late 1700s Dr. Samuel Johnson said "It was worth seeing, but not worth going to see."
Other visitors include - Sir Walter Scott (1814), William Thackery (1842), Samuel Clements (Mark Twain) (1882), Charles de Gaulle (1996) and Prince Charles (1996). In 1987, Richard Branson ditched into the sea nearby at the end of his record breaking Trans-Atlantic flight. He came back by more conventional transport the following year.
In 1588 La Girona a Spanish Armada Galleon sank there. Only five of the crew survived. The wreck was discovered in 1967
During Victorian times many of the columns were cut and sold as curios.
It was declared a World Heritage Site in 1986
Some basic Geology maps show the most northerly chalk on the UK mainland along the Yorkshire coast. However, on the Scottish mainland to the north-west of Mull, beneath the lavas of Beinn Iadain and Beinn na h-Uamha are a number of small chalk outcrops. No doubt chalk deposits were widespread in the north-west, as indicated by widespread chalk clasts, but today small outcrops seem limited to Mull, Eigg and Skye. On Mull the chalk is up to 20 metres thick. All this chalk belongs to the Inner Hebrides Group, probably of the Cenomanian, Turonian and/or Lower Senonian stages of the Upper Cretaceous.
In Northern Ireland the Chalk belongs to the same Group. Here it is economically called the "White Rocks", but to geologists it is Ulster White Limestone. It was deposited in a warm shallow sea during the Cretaceous between 142 and 65 million years ago. It is underlain by beds of Hibernian Greensand. There are flint bands in the chalk, which is very hard and up to 120 metres thick. Reports state the chalk is rich in fossils. Belemnitella is most common. The first true belemnites appeared during the early Jurassic (200 m.y.a .) and flourished until the end of the Cretaceous. Other fossils include the ammonites: Menuites, Neancycloceras, Pachydiscus ,Pseudoxybeloceras and Turrilites, which is coiled like a gastropod. Unlike the Scottish Islands the Chalk has been preserved and protected by thick layers of overlying basalt flows.
The Causeway Basalts.
The Tertiary basalt flows of the Antrim Plateau commenced some 60 million years ago as the Atlantic Ocean started to grow. There were several large lava flows in the early stages. The flows were not regular or continuous. For a period of 2 million years a warm, wet climate weathered the surface rock and formed a deep red soil rich in iron – a laterite which is over 4 metres thick. Then the volcanic activity recommenced. Some of the lava flowed along the course of a river valley in the chalk and became quite deep. This lava cooled slowly to form basalt crystals/columns.
During the last Ice Age moving ice eroded the upper parts of the lava, and when the ice retreated from this area some 15,000 years ago the tops of the basalt columns were exposed. Melting ice caused sea level rise and today the tops of The Causeway columns are just above sea level.
Most of the columns are 6 sided but some have 4, 5, 7 or 8 sides. One is supposed to have only 3. Nearby are examples of "onion weathering" (exfoliation) of the basalt. In keeping with local legends they are called "Giant’s Eyes ". Weathering and erosion have resulted in many named shapes e.g. The Camel, The Organ, The Harp, The Book, etc. The Causeway itself is made up of over 30,000 columns each remarkably similar in size – between 30/40 cm across.
Three promontories make up The Causeway.
Overall it is quite a small geological feature. It is often said that the Causeway starts in N. Ireland and continues below sea level to the Scottish Island of Staffa. This is not the case – the basalt columns disappear a short distance offshore.
Judd 1878, The Secondary Rocks of Scotland.
Bailey 1924, The desert shores of the Chalk Sea.
Mortimore, Wood & Gallois 2001, British Upper Cretaceous Stratigraphy.
Thank you to Felix Whitham for information, comments and advice.
All my observations were made as a tourist during a one day visit and all counts/measurements are visual estimates.