Moving mountains

At this point in autumn the sunset has moved south, its two halves - sky and ocean - hinged at the horizon by the calm, dark profile of Inishbofin. We are hard to impress after all these years, but the other night: "Just look at this!"

The clouds stepped westwards in a brilliant, gilded lattice of what the books call "billow-type altocumulus". From the north, an astonishing wedge of clear sky: cerulean, moonstone, those soulful Florentine blues. The sea was a deep violet, all the way to the strand. And the sun submerged beyond the island, but marked the spot with two fierce smudges of magenta.

What do you watch, after that? Hardly the box in the corner. Turn, instead, to the mountain and let the sunset's afterglow fade among its northern buttresses, the rock rose-red, then purple.

From Hallowe'en on, Mweelrea becomes a looming, insistent presence in our lives. Summer dulls it down with haze and drab colour and a high, flat light, but the four months of winter give it back every one of its 2,668 feet (mere metres, for once, will not do).

Any morning now will bring the special moment when the sun sneaks along behind the ridge to emerge, fully-fledged, at Mweelrea's dark shoulder, a sudden, incandescent Klieg-light catching every drop of water in the grass, every curling frond of fern. A crystal, hung on a thread in the window, fires shards of rainbow round the room.

Once, in an early fit of heresy (after, perhaps, some unrewarding trudge across the bog) I dared describe the mountain as "dull". That brought a letter from a professor of geology, Paul Mohr of UCG, which I kept for years as a reminder of the lyrical sensibilities that can lie beyond the jargon, often difficult and mind numbing, of the study of the fabric of Earth.

It led me back to the birth of Mweelrea's coarse sandstones at the margins of the ancient Iapetus Ocean; to the volcanoes that sent pulverised magma hissing into the shallows. Looking up at the flushed sunset mountain, I can picture those veins of ash and glass winding through its sedimentary flanks. I try to look beyond today's worn and folded mountain to the Ordovician landscape - a great flat delta, its braided, grit laden rivers - that laid down the rock at anything up to 500 million years ago.

This leap one has to make, this conceptual double-entry between the ancient landscape that formed the rock and its modern, iconic simulations, is another challenge for geology's PR. In the Wicklows, for example, the distinctive cone of the Great Sugar Loaf is one of the several peaks that says "volcano", and some vague knowledge that Wicklow granite is an igneous rock reinforces the whole dramatic image.

The peak of the Great Sugar Loaf was never a volcano, but started life as sand on the ocean floor. In the collision of continental plates that welded the two halves of Ireland together, a great arc of mountains was raised: the Caledonians, of which Mweelrea is a fragment. Their weight later forced magma to well up into the crust in areas such as Wicklow, where it cooled deep underground into crystalline masses of granite.

Above were the old sediments, now transformed by pressure and baking heat into slate and schist and tough quartzite. This last is the rock in the peak of the Great Sugar Loaf, and of many western mountains, such as those in Connemara and Co Donegal. Where the "roof" of older metamorphic rocks is worn away, the granite foundations are exposed, as in the outcrops in the heather-covered hillsides along the Wicklow Way.

And along the southernmost section of the Way, in the stone walls of the boreens, are tiny needles of green hornblende - evidence that there were, indeed, actively erupting volcanoes, but at a time when Wicklow was still beneath the Iapetus Ocean: their pulses of ash spread out as "pancakes" on the seabed.

This is one of the fascinating episodes explored in a series of five leaflets on the rocks and landscape of the Wicklow Way, researched by Dr Peadar McArdle, director of the Geological Survey of Ireland* and published with the help of Coillte. Each leaflet covers a day's walk, perhaps 20 to 30 kilometres, and has a separate theme - the granite story, say, or lead mines, or the glaciers and erratics of the Ice Age.

Many Dubliners like to carry some breath of their weekend excursions into their daily working lives, if only to glance up at a bird through the office window, or note a plant that has rooted itself in some unlikely corner. Some knowledge about rocks, absorbed in the Dublin and Wicklow Mountains, could add a whole new dimension to life in the inner city.

Christopher Moriarty, in his new paperback Exploring Dublin: Wildlife, Parks, Waterways (Wolfhound, £8.99) has a deeply interesting chapter on the building stones of Dublin and where they came from. The architects of the classical city looked naturally to the limestone and granite abounding in its hinterland and turned to England's pale grey Portland limestone only for the posher, more finely-cut porticos and pediments.

For homely Irish limestone, studded with fossils from that same Iapetus Ocean - brachiopods, crinoids, corals and the rest - study the walls of the Point Depot or the canal bridge at Dolphin's Barn. Granite slabs pave Molesworth Street and much of Merrion Square, as well as cladding the more important Georgian buildings. Moriarty gives an excellent guide to the different sorts of granite - the micas and feldspars - and notes, for the record, that the red granite so many of us have polished with our elbows while drinking coffee in Bewleys came all the way from Sweden.

The GSI is at Beggar's Bush, Haddington Road, Dublin 4, 01 6707444.