The mountain that's upside-down

From many angles, Croagh Patrick is such an impeccable pyramid as almost to lose its own identity, becoming merely, if majestically, the signifier for mountain. It was this, and the sun's habit of rolling up and down the slopes a couple of times a year, that earned the Reek its druidic credentials.

I know somebody who climbed it three times to guarantee her Leaving Cert (once even in bare feet over that punishing quartzite scree), and somebody else who, living nearby, pops up and down quite regularly as a sort of jog before breakfast. The Reek has so many meanings for so many people, not least the would-be miners of the gold in its veins.

My own values for the Reek are all aesthetic. On winter mornings, against the light, the seaward scarp above Murrisk is deep purple, scalloped with pale, grey-violet scree. On summer evenings, from Old Head, the Reek is rosy and Italian, like a Bay of Naples watercolour. Liam Lyons, the Westport landscape photographer, has his own, memorable image: the mountain exactly reflected in a blue-and-silver calm at sunrise.

Now, however, my perception of Croagh Patrick is altered, refined - even, perhaps, a bit upset, like the mountain itself. I cannot look at it from the west without seeing this great band of quartzite rearing up and falling over backwards, propped by sediments that have also been turned upside down. I have to imagine the huge U-fold of rock curving under the ground and coming up 10km away, at Cregganbaun, the "white rock" at the gateway to Doolough Pass.

This monstrous rumpling of the crust, now eroded to the very symbol of stability, happened shortly after 400 million years ago, when the North American and European plates carrying the two halves of Ireland slammed together in the closure of the Iapetus Ocean.

I knew about that prodigious event, thanks to Frank Mitchell's incomparable books, but hadn't realised that its local consequences might be pointed out in the intricate texture of a rock.

The fingertip belonged to Dr Stephen Daly, a UCD geologist leading one of the field excursions for Irish Geology Week earlier this month. More than a score of such outings were organised all over Ireland, all for the general and amateur public, and all free. This recurring explosion of missionary zeal by the Irish Geological Association really is a most remarkable phenomenon: one doesn't find botanists or biologists rushing to celebrate their subject in this way. But I have never met an unenthusiastic geologist.

It's as if, in person, they are determined to live down the difficult jargon of their trade: synclines, cyclothems, sinistral transpressions and the rest. And they have such dramatic stories to tell, such primal pictures for the mind's eye.

The Louisburgh area, with Clare Island, has been a mecca for several generations of university geologists and their students: not for nothing is the main decoration of Gaffney's pub, at the little town's crossroads, a big, framed map of the bedrock of south Mayo. Together with the striking interest of the ancient structures and their really global sense of theatre, the sheer accessibility of rocks is what counts. Scotland shares many of the same geological features, but seals them off in private grouse moors, patrolled by officious keepers.

Even in Mayo, the freedom to roam is diminishing, and new bungalows and gardens are often carved out with scant regard for significant outcrops. But there was, it turned out, no shortage of the sedimentary rock which, on Kilgeever Hill as in the Reek, was demonstrably upside down. We could see the way the sediment had graded as it settled, layer by layer, in some ancient flood, and how the mineral particles within the rock had cleaved to new alignments in the crushing pressure of the fold. Not everyone can have an expert guide on hand to interpret the landscape's rocky armatures, and, outside of the national park guidebooks, there has been little popular literature to help. Recently, however, the Geological Surveys of Ireland and Northern Ireland came together to form a specialist group, "Landscapes from Stone", to further cross-Border tourism with the help of EU peace money.

The first products, a folding-map guide and accompanying book, A Story Through Time, have just been launched and are most impressive. Written by Dr Patrick J. McKeever, a geologist with the GSNI, they cover a new "northern" Ireland of 12 counties: those of Ulster, plus Sligo, Leitrim and Louth. They deal with the region in landscapes moulded by geology rather than politics.

Thus, the Erne lakelands are viewed as a whole, from Cavan to Enniskillen to Ballyshannon. The drumlin belt, an Irish glacial showpiece, is traced from the banks of the Shannon right across to Strangford Lough. The molten upheavals that accompanied the birth of the Atlantic are followed from the Mournes and Armagh's Ring of Gullion to the Cooley Mountains of Louth. The limestones and sandstones of west Breifne are pursued through successive horizons of flat-topped mountains, from Cavan and Fermanagh in the east to Sligo's Benbulben.

Grippingly written and visually seductive, map and book are aimed first at the general tourist and then, in market-speak, "to actively promote the 12 county region as a quality geology field study destination to the educational community overseas". The Geological Surveys stand ready to map out itineraries for schools and universities, or even, I suppose, jolly goldpanning clubs with wellies, shovels and personal woks.

None of this need discourage the Irish abroad in their own land. Details of the map and the book can be had from the Geological Survey of Northern Ireland at 20, College Gardens, Belfast BT9 6BS (08-01232666595) or the Geological Survey of Ireland, Beggars Bush, Haddington Road, Dublin 4 (01-6707444). The Irish Geological Association has a website at http://www.tcd.ie/ Natural-Resources /Geoscience/IGA.html