The Giant's Causeway: Myth and Math

The Giant’s Causeway, found on the north coast of Northern Ireland, is a jagged promontory of neatly-packed basalt rock columns. Science, of course, holds the real answers to their formation, but for hundreds of years, storytellers have been coming up with their own ideas. The most ancient of these stories tells of Fionn mac Cumhaill’s epic escape from Benandonner.

The Legend of Fionn mac Cumhaill and The Giant's Causeway

Fionn was always in dispute with one of his rivals, another giant called Benandonner who lived in Fingal’s Cave on the island of Staffa in the Hebrides, Scotland. They were always arguing about who was the stronger of the two giants. Benandonner shouted to Fionn one day, ‘If I could get my hands on you I would make sure you could never fight again. Unfortunately I cannot swim so I am unable to get over to you and we shall never know who is the stronger of the two.’

Enraged, Fionn tore large pieces of rock from the nearby cliffs and rammed them down into the seabed making a causeway between Northern Ireland and Staffa. ‘Now you will have no excuse,’ he shouted across to Benandonner. When Fionn saw Benandonner coming over the causeway he realised how big he really was, so he devised a clever plan. After making himself a child’s cot and disguising himself in baby clothes he lay down and pretended to be asleep. On his arrival, Benandonner noticed the cot and the sleeping baby inside it. He was very afraid. ‘My goodness,’ he thought, ‘if this is the size of the baby, what size must his father be?’ Without waiting around to find out he raced back across the causeway, destroying it as he went so that Fionn could not come after him. And that is why only two fragments of the causeway now remain, one end on the coast of Northern Ireland and the other on the Hebridean island of Staffa.

Fingal’s Cave island of Staffa

The Maths

Most of the columns in the Giant’s Causeway are hexagonal but there are also examples of four, five, seven and eight-sided shapes. There are some 40,000 of these columns in total. They vary from about 38 cm to 52 cm in diameter and can be as high as 12 m. The tops of the columns form a series of stepping-stones that lead from the foot of the cliffs and disappear under the sea. The phenomenon was created some 60 million years ago by a flow of basaltic lava, which cooled rapidly as it ran into the sea, forming the distinctive shapes. Millions of years of erosion took place before the columns were revealed. This is thought to have happened during an Ice Age, 15,000 years ago. The region around the Giant’s Causeway has many examples of coastal scenery that have been formed by erosion caused by sea water. These include headlands, caves, arches, stacks and stumps.

Hexagon shaped stone basalt columns Irelands Giants causeway
Basalt columns: Giant’s Causeway

Things to do

Use atlases and maps to find out more about Northern Ireland. 

  • How would visitors travel to the Giant’s Causeway from Scotland or England?
  • What direction would they have to travel in?
  • How long would the journey last?
  • How much would it cost?

Investigate other physical features in the area like the Sperrin Mountains and Lough Neagh. Find out more about the province’s largest city, Belfast, its prominent tourist attractions and industries – especially shipbuilding.

Read through the legend of Fionn mac Cumhaill and the Giant’s Causeway outlined here. 

  • Make your own drawings and sketches of the central characters in the story from the descriptions given.
  • Retell the story in your own words or break up the story into sections and retell the events in the correct sequence in storyboard style.
  • Or you can really use your imagination and suggest other fictional reasons for the existence of the causeway.

Locate some small samples of igneous rocks like basalt and granite that can be examined under a magnifying glass. Igneous means made by fire. Compare them with examples of metamorphic* rocks like marble and slate and sedimentary* rocks like chalk and sandstone. How are the types of rock similar and how are they different? Have any of the samples been smoothed by the action of water? Try scratching them with a nail to see if they wear away easily.

* Metamorphic rocks are rocks that have gone through chemical and/or physical changes because of  intense heat or pressure. 

* Sedimentary rocks are rocks that have been formed by the build-up of materials on the Earth’s surface and on river and seabeds.

Main website for full details about the Giant’s Causeway is:

Giant Geometry

Examine the properties of regular and irregular hexagons. Regular hexagons will fit together exactly without help from any other shapes. 

Draw a large hexagon, divide it into six equilateral triangles (fig. 1) and use these triangles, like the pieces of a jigsaw, to make other two-dimensional shapes. 

Make a large equilateral triangle with four small triangles. 

Make a rhombus using two triangles. 

Make two parallelograms, one with four triangles and one with all six triangles. 

Make a trapezium with three triangles. 

Hexagonal shape with guidelines
Figure 1 Hexagon

Investigate nets that can be constructed into hexagonal prisms (fig. 2) of different lengths that will match the basalt columns found in the Causeway. Nets should include gluing flaps and accurate measuring, cutting and fixing will be needed. 

Figure 2

Find the properties of the six-faced hexadron (fig. 3.2). Sticking two tetrahedrons or triangular-based pyramids (fig. 3.1) together at the base is the easiest way to make this shape. 

Figure 3.1 and Figure 3.2

Did you enjoy learning more about the mystery and the maths of the Giant’s Causeway? Then you’re sure to love our Giants issue of AQUILA. Make sure you click here, to subscribe immediately!

Written by John Davis