Laudable land artists or limestone cowboys?

Another Life Michael Viney The film people left a little tower on the headland, just above the rock where the surf comes splashing…

Another Life Michael VineyThe film people left a little tower on the headland, just above the rock where the surf comes splashing around the oystercatchers and purple sandpipers.

It's more of a cairn, really - somewhere for Isolde to lean with the wind in her hair while Tristan's funeral boat is drifting ashore. They brought the stones to build it from Wicklow, which might seem a bit superfluous, but it's really very well done and has made itself at home in the landscape. Fresh lichens are growing already on the cairn's northern side.

A couple of rocky headlands up the coast, there's something else to see: a shore armoured with tonnes of limestone boulders, lorried endlessly by the county council from some distant inland quarry. Granted, the boreen that loops down to the cove had been badly undermined by storms, but extending the defences to protect an empty headland does seem an oddly quixotic thing to do: part, perhaps, of some dogged, Canutian strategy against the rising tide.

Limestone will erode a good deal faster, of course, than the hard native sandstones of this coast. A handsome and fascinating new booklet from UCD's Environmental Resource Management Unit reminds me of some of the most spectacular evidence of this: the strange and beautiful "mushroom stones" one can come across in many midland fields. They are very special products of Ireland's landscape history, and can also have a special significance for the island's human record.

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One is used to seeing what weather and water can do to limestone in the so-called "karst" country of the Burren, where the rock has been honeycombed into all manner of fantastic forms. It is here and on the limestone shores of lakes such as the Corrib that the undercutting action of waves becomes clear. But the mushroom stones in the colourful booklet by Louise Dunne and John Feehan can be a long way from water.

As the ice-sheets melted, Ireland was left strewn with glacial debris. In the midlands, big limestone boulders dropped by glaciers stood at the edges of great lakes. As the whole island rebounded from the weight of ice, in what's called "isostatic uplift", much of the water drained off, leaving the boulders exposed high and dry on their wave-sculptured pedestals. The greatest concentration of wavestones is around the Shannon floodplain, where they mark the original extent of Lough Ree and of the Shannon below Clonmacnoise. Others stand at the edge of vanished lakes that nourished the midlands' raised bogs. They make great perches and hunting-posts for birds, whose droppings have fertilized a whole tapestry of lichens.

The brilliant orange rosettes of Caloplaca and Xanthoria are just two of the species whose ecological subtleties are explored in the booklet. Along with their intrinsic fascination, sometimes the stuff of local myth and legend, the mushroom stones have a keen archaeological value. Because they mark long-vanished shorelines, they are often a pointer to the camps and settlements of the first Mesolithic people. Some of the earliest radiocarbon dates have been taken from 8,000-year-old charcoal in the camp fires of people who fished Lough Boora in Co Offaly, now just a cobbled shoreline in the middle of a cut-away Bord na Móna bog.

Wavestones were first recorded at Lough Ree in 1867, but a recent survey has found them also in Offaly, Tipperary, Galway, Clare and Cork. More than 60 are located in the booklet, and many smaller ones, no doubt, remain to be recorded. In spite of their importance they have no special protection (some of the more portable stones have already been carried off as garden ornaments) and Louise Dunne and John Feehan are specially keen that local communities should know their historical and geological value, and look after them.

All this will strike a special chord with Brendan Dunford and Ann O'Connor, whose brilliant interactive website www.burrenbeo.com offers such inspirational insight into the limestone country of the Burren. One of the concerns they document currently is the contagious passion of coach tourists for tearing up stone with which to build their own mini-dolmens and other I-was-here monuments and cairns: last season left hundreds of new ones on Ballyallaban Hill. Is this vandalism or "land art?" Dunford and O'Connor invite your comments. But see also the exciting work they're doing with the kids of New Quay and Ballyvaughan - the very antithesis of vandalism.

Ireland's Mushroom Stones part-funded by the Heritage Council, costs 10 from Louise Dunne at UCD, Richview, Dublin 14 (E-mail louise.dunne@ucd.ie).