island of the week

When Ailsa Craig's ethereal shape materialises out of the mist, soaring to a height of nearly 340m (over 1100ft) above the sea, one can understand why it was named ''fairy rock'' (aillse creag) by some ancient Celtic mariner. But it has also, more prosaically, been called Elizabeth's rock or Alastair's rock. Its popular name nowadays is Paddy's Milestone. It is more than 12 times the area and three times the height of the Bass Rock, which is a mere pimple by comparison, and it is so precipitous that even the sea birds can't nest on some of the cliffs.

Twenty-five thousand years ago, when Scotland lay smothered under a thick sheet of ice, a glacier flowing down the Clyde valley broke off pieces of Ailsa Craig and carried them south to the English Midlands where they still lie today scattered between Wales and the Pennines. The rock is mainly volcanic basalt but there is a seam of red, fine-grained micro-granite, which is the ideal material for curling stones. These were quarried and cut on the island then polished on the mainland and a few are still manufactured today for connoisseurs.

It's more than a decade since I landed on Ailsa Craig. We sailed there in a trusty bilge-keeled ketch - Jeananne - which belonged to my present-day sailing partners. She drew only one metre, which let us lie in shallow water alongside the small wooden jetty. Anchoring is not easy, as the sea bottom is steep and boulder-strewn.

A rusty, narrow-gauge railway line runs from the jetty past the quarrymen's cottages to the old quarry on the south side. A century ago almost 30 people - quarrymen, lighthouse keepers, and their families - lived here, but the quarrymen left and the lighthouse is now automatic. There are heaps of waste granite pieces, out of which the spheroidal curling stones have been cut, leaving fascinating shapes like miniature Henry Moore sculptures.

A zig-zag path starts near the lighthouse and climbs past the old square keep 100m up the slope. It was said to be a retreat for the monks of Crossraguel Abbey (near Maybole) and that the Catholics once held it on behalf of Philip II of Spain. Further up, the path passes over the shallow valley of Garraloo and beside the tiny Garra Loch, before making its way to the top. Here the world falls away in a sudden, vertiginous plunge to the sea far below and the view is enthralling. Beyond the white lace of the surf lie the wide stretches of the Firth of Clyde, with Arran, the Ayrshire coast, and the long dark shape of Ulster on the south-western horizon. Experienced climbers may prefer to go directly up the slope from the landing place. This is not difficult, but in places the route leads over steeply-inclined slabs.

Ailsa Craig is noted for its immense gannet colony, which accounts for about 5% of the world's total gannet population. There also used to be many puffins - an ornithologist reported in the 1860s that there were at least 250,000 pairs and that when he disturbed them ''their numbers seemed so great as to cause a bewildering darkness''. But in 1889 brown rats arrived off ships ferrying supplies to the newly-built lighthouse and, by 1984, they had wiped out the entire puffin population. Rabbits, incidentally, were introduced about the same time by the quarrymen to supplement their diet - and were later claimed to be breeding with the rats! In 1991 a massive rat eradication programme was instituted and, to date, it seems to have been successful. Puffins are, at last, visiting the island again.

Instead of climbing, it is possible to complete a relatively easy two-mile circumnavigation of the island. The exposed corner at Stranny Point in the south-west is the only minor obstruction. It has to be negotiated to reach the dramatic Water Cave when coming from the east past Little Ailsa, so try to time it near low water.

The names of features on Ailsa Craig are pure poetry - for example, Spot of Grass, Bare Stack, Doras Yett, Ashydoo, Rotten Nick, and Kennedy's Nags. Ailsa Craig itself is mentioned in the poetry of both Wordsworth and Keats but, strangely, not by Burns, and yet he grew up within sight of it.

l Access, by arrangement, from Girvan; Rachel Clare, 01294-833724; or, Martin McCrindle, 01465-713219. Crossing time approximately one hour.

l OS maps: 1:50000 Sheet 76 1:25000 Sheet 490 1:10000 (NX09). Admiralty Chart: 1:75000 No 2126.