Building the wall of life

Something there is that doesn't love a wall,

That sends the frozen-ground-swell under it

And spills the upper boulders in the sun.

In The icy winters of Robert Frost's New England, it is frostheave that unknits the top of a dry-stone wall. In Mayo, too, it could be trickling rain or the shock of a gale-gust that finally breaks the grip of one rock against another, so that, driving over the hill in the morning, people find a big dog's-head of a stone lying in the road. "You'd think someone would pick that up," says every driver in the townland, swerving and hurrying on.

But it could also be the big lorries, pressing between the walls with their 10-storey loads of sheep or hay or building blocks, that send bow-waves rolling through the soil. A nudge from a tractor can start a tumble, too. No wonder the bold, new, double stone walls at field gates along the road to the mountain have a cap of mortar to hold the top stones together.

A double dry-stone wall, built properly on flat-stone foundations, with plenty of through stones and a coping of stones laid on edge across the top, shouldn't need cement to help it keep its balance. I can think of particular walls (Clare Island has a great web of them) which nothing short of an earthquake will ever stir. It depends, of course, what you're building with. The awesome 1,500 kilometres of single dry stone walling on the Aran islands are jig-saws of limestone, which is both conveniently cuboid and densely heavy. In Connemara, Donegal, Wicklow and the Mournes, walls of granite use the coarse tooth of the rock as well as its weight.

In his timely manual Irish Stone Walls, stonemason Patrick McAfee describes the spectacular wall, built of cut granite, that climbs to the top of 15 mountains in the Mournes to mark the catchment of the Silent Valley reservoir.

Here in south Mayo the fieldrock is less obliging, most of it sandstone and gritstone dragged along in glacial till or water-rolled in a river to a smooth and slippery finish. There are, nonetheless, some fine walls built of it, and even the conical dog's-heads can be wedged to stay put.

For most of the new walls along scenic roads of the west, the county councils and FAS improvement schemes are using bedding stone, such as limestone, that is moderately quick and easy to build with, and many farmers building or repairing dry stone walls under REPS are recycling stones from ruins. I am so deeply appreciative that something actually good to look at is being added to the countryside that I can't bring myself to cavil (as Patrick McAfee might have to) at the expediency and economy of flat mortar copings.

If I were a proper naturalist, I would now be getting ready to study a suitable stretch of virgin wall to see how quickly nature proceeds to make use of it: how soon the lichens arrive, how soon the spiders and snails move in, and so on. For dry-stone walls are very special habitats, and one of Ireland's particular offerings to biodiversity.

An ingenious Canadian geographer called Holland, looking for a distinctive site-type in which to study patterns of species-density (read on: it gets clearer) picked on the stone walls of the west from Clew Bay to Galway Bay. For his purposes, they had to be about 100 years old and built of acidic rocks such as granite or sandstone. He chose 165 lengths and spent a summer on a survey of them. He found 133 flowering plants (not nearly as many, as it happens, as grow on the walls of some old towns). The more exposed to the south-west winds the wall-face was, and the nearer to the sea, the fewer wildflowers it had; indeed, many old walls along Galway Bay were completely bare. Holland seemed surprised by this: south-facing walls, he thought, should be warmer and thus more hospitable. But there it was: where plants were buffeted and dried out by "maritime air masses", there were far fewer species than on the northern, more sheltered side of the wall.

North-facing walls may lack the sun, but have fewer extremes of temperature and humidity. Sunbaked walls lose their heat fast at night (in Galway City, at dusk, quite startling waves of warmth can radiate from old limestone buildings), and they also dry out faster.

As an ecosystem, in fact, walls have been compared to the harsh conditions on the scree of high mountains. But plants adapted to them often grow luxuriantly in the moist, oceanic climate of the west. Walls that dry out in the wind are crusted with lichens and drought-resistant mosses, and grow plants like the stonecrops, that store water in fleshy leaves. The slowgrowing rusty-back fern is one that curls its leaves in dry weather to hold on to its moisture.

But plants are only part of the wildlife that takes possession of a dry-stone wall, so full of warm, dry cavities and crevices. For lizards, shrews and fieldmice, it is a Ballymun without lifts. Spiders weave webs across the balconies, or are specially adapted for particular prey. One hunts woodlice; another, with a dazzle-striped body that blends into shadows, jumps on to flies that are sunning themselves. Wrens come rummaging after spiders, or for hibernating butterflies tucked into cracks; stoats wait round corners for the wrens.

A natural dry-stone wall is a great addition to a garden: at one metre high, it adds twice its area for planting with all kinds of rockery species. Patrick McAfee's book tells how to pick stone at a quarry and how to build a wall without accidents: it's a really thorough manual, good on lime mortars, pointing, wet-dashing and all.

Walls do, however, have a down-side for the gardener: snails love them, and cluster in their cavities to hibernate. It's said that, after a night's foraging, a snail will often reappear at the same spot on the wall for its daytime rest (an affinity they share with limpets). You could mark them with nailvarnish and tell me if it's true.

Irish Stone Walls is published by O'Brien Press at £15 hardback