Why the Causeway is Six-Sided

The short version

About sixty million years ago, this part of County Antrim was a rift valley with lava plains. Basalt lava erupted from fissures rather than from a single cone, so it spread in sheets rather than piling into a mountain. As each sheet cooled from the top and the bottom inward, it contracted, and because contraction has to relieve itself somewhere, the rock cracked.

The cracks, given a homogeneous material and uniform cooling, meet at about 120 degrees. Three of them meeting at a point produces a triangle; four produces a square; six produces a hexagon. Hexagons win because they are the shape that divides a surface into equal cells with the least total crack length. It is the same geometry as a honeycomb, arrived at by physics rather than by an insect.

What to measure

The columns here are mostly hexagonal, but not uniformly so. Count enough of them and you will find five-sided and seven-sided examples, and a few with irregular faces. That variety is important: it is what tells you the cooling was not perfectly uniform, which is exactly what you would expect from a lava flow that ponded against uneven ground.

The cross-joints along each column are a different story. They are not contraction cracks but unloading joints, formed as the weight above was removed by erosion and the column relaxed. They give the columns their segmented, stacked look, and they curve — concave downward on some columns, upward on others. The direction of curvature tells you which way the lava was flowing when it ponded.

Logging tasks

Send answers to the four questions below through the message centre. Do not put them in your log. A photograph of you with the columns is welcome but not required.

1. At the main causeway promontory, find five adjacent columns and count the sides of each. Report the five numbers. Do not choose the neatest-looking ones; take the first five you can stand beside.
2. Measure the width across the face of one typical column, and give the measurement in centimetres together with what you measured it against.
3. Find one column with visible curved cross-joints. State whether the curve is concave up or concave down from where you are standing, and explain in a sentence what that tells you about the direction the lava was moving.
4. Walk to the Organ, the cliff of tall columns on the east side of Port Noffer. Explain in two or three sentences why the columns there stand so much taller than the ones on the causeway itself.

Access and safety

• The stones are extremely slippery when wet, and they are wet most of the year. Wear shoes with grip and do not step onto the seaward faces.
• The causeway stones are the most visited rock surface in Ireland and are strictly protected. Walking on them is permitted; hammering, sampling and removing material are not.
• The path to the Organ involves a steep section and a set of steps. The return is uphill.
• The sea here is exposed to the Atlantic. Waves reach the promontory in any onshore wind and there is no lifeguard.

After the find

Portnaboe bay, behind the visitor centre, is a quieter beach with the same geology and none of the crowd. The clifftop path west toward Dunseverick runs for miles above the same lava sequence and gives the best view of how many separate flows built this coast.