Look to lichens for dating with a difference

At a distance Charles Fort in Kinsale looms up from the landscape in shades of grey stone but once you get closer you can see that its surface is actually multicoloured. Lichens crust the walls, including the bright yellow Caloplaca decipiens and the deep black Varrucaria maura. The tops of the walls are home to flowering grasses, mosses and heathers. Rather than a dead artefact it is a living habitat.

Of all the organisms now living among the 17th-century walls, the lichens have probably been there the longest. Lichens are an organism made up of fungus and algae and they can live for thousands of years, growing very slowly and regularly. Recognition of their age and their regular growth rate led to the science known as “lichenometry”, pioneered in the 1950s. It uses the growth rate of lichens to determine the age of the surface that they grow on.

Movement of glaciers

Roland Beschel, an Austrian scientist, first suggested the use of lichens for dating. He had noticed that larger lichens appeared on older glacial moraine (the rocks left behind when a glacier retreats) and proposed that a lichen growth pattern could be used to date the movement of glaciers in the Alps. Lichenometry continues to be an important way to study the retreat of glaciers, especially now in the context of climate changes observed at the poles. Other geological events can be dated using lichens including volcanic eruptions and landslides.

Lichenometry has also been extended into the realm of archaeology where it can be useful when radio carbon dating is either not possible or not likely to prove especially helpful.

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The technique is not without difficulty. Lichen growth rates can be affected by weather changes and the same species will grow differently in different environments (despite the fact that a single species of lichen may be found across the globe). Lichens on a surface may merge with one another, making it difficult to determine the precise size of a single specimen. Fire destroys lichen thereby “resetting” the lichenometric clock. The technique further assumes that a lichen immediately colonised the rock in question, but this may not be true.

Gravestones

Nonetheless, lichenometry can be helpful in dating particular kinds of sites and objects when other methods of dating prove difficult. Unmarked or degraded gravestones are one useful example. A study by Brandy Luthe et al in 2018 used lichen growth to date headstones in a cemetery in Arkansas that was associated with the advance of western settlement.

A rock that has lichens on a surface not exposed to the sun suggests that the rock has been recently moved

No documentary evidence existed about the establishment of the cemetery and some headstones were not dated. Dated headstones could be used to establish a growth curve for the relevant lichens. Lichenometry (combined with other methods) suggested that the cemetery was founded about 1800, earlier than the dated headstones.

The fact that lichens need sunlight to grow is also helpful in dating. For example, a rock that has lichens on a surface not exposed to the sun suggests that the rock has been recently moved. For archaeologists studying Native American sites lichenometry can help to establish how much the form of a wall, for example, has been altered by 21st century hikers or campers moving the stones out of place.

In Ireland, lichens have been used by Eoin Halpin to make an argument about the shape of portal tombs. Halpin supports the idea that the tombs were not fully covered with cairn stones by the observation of a lichen "tide mark" on the large stones forming the portal. Lichens above the mark are different than those below, suggesting that the lower parts were originally covered with cairn stones. Lichens only began growing below the tide mark after the cairn stones were disturbed.

Matilda Knowles

Irish botanist Matilda Knowles dedicated many years to the study of lichens, identifying new species and publishing a book on them in 1929. Her list of lichen species found in Ireland is a compilation of her own diligent work and that of many Irish naturalists over more than a century and a half.

Lichens have born silent witness to thousands of years of Irish history, colonising new surfaces as they have become available from trees in the deer park of the Castlemartyr estate, to the sea wall in Sutton, to limestone walls at Tara, to the Fort Hotel in Greencastle, to mud walls on the Circular Road in Dublin and apple trees at Ardnacrusha.

Dr Juliana Adelman lectures in history at Dublin City University