A cocoa-coloured church, tucked quietly away in the Welsh Marches, sports an unusual stone carving of a Sheela-na-Gig. Like a 12th- century Playboy centrefold, it leaves very little to the imagination and, though many come to gawp at its curves, most are shy about buying postcards. Why it was constructed may be more mundane than they think

On a temple in Kathmandu, it would not be out of place. But a carving of female genitalia seems unusual on any church in England. The entire building is decorated in a remarkable fusion of Viking, Saxon and Celtic styles. But it is the 12th-century equivalent of a Playboy centrefold - known from the Irish as a "Sheela-na-Gig" - that draws most visitors to the cocoa-coloured sandstone church of Kilpeck, eight miles south west of Hereford.

The sculpture, one of more than 50 on the outside of the church, is described in a poem by Seamus Heaney as "hunkered under the eaves", "twig boned and saddle sexed". More prosaically, she has the head of an alien, is pulling open a vulva the size of a tractor tyre, and has nothing but a strand of recent cobweb for modesty.

Joy Manning sells tea and Welsh cakes from her home across the lane. "To the locals the Sheela is just part of the scenery. But it's definitely what people come for," she says. "Even so, they are shy about asking for postcards. Well, except for an American woman. And what she really wanted was a Sheela-na-Gig T-shirt."

Competing theories claim to account for why the carving is there. Perhaps she caricatures lust (though there are no other sins portrayed). Perhaps the grotesque top half of the figure represents death and the lower half procreation. Perhaps she is a charm to ease the process of giving birth. But what visitors are probably seeing is Christianity incorporating into the fabric of its church a pre-Christian Earth Mother, just as it took into its religious calendar the pagan festivals of Yule and Easter. That said, her smile - leering, mocking, inviting? - is, in its own way, as enigmatic as the Mona Lisa's.

Whatever the Sheela's theological status, her survival is something of a miracle. First because she is as crisp as the day she was carved, despite more than 800 years exposed to the weather. And secondly because she escaped the vandalism of Victorian prudery. I don't generally believe Freud, but this story merits his interpretation: in the 1840s, Geoffrey Lewis published a full account of the Kilpeck sculptures, illustrated with his own drawings, which is now on sale in the church. Defying all common sense and knowledge of anatomy, he says the Sheela represents a fool "with a cut in his [!] chest". But more telling is that Lewis's otherwise impeccable draughtsmanship fails. Despite his written rationalisation, he draws the Sheela with her hands, not clasping the "cut", but innocently at her side.

There are Sheelas elsewhere, particularly in France and Ireland. Even so, I wondered whether her presence here might have had some local influence. Mrs Manning assured me that Kilpeck is not now a wife-swapping sort of place. But this wasn't always so.

A century after the Sheela was carved, the Bishop of Hereford's travelling ecclesiastical court came to check on church attendance and other aspects of morality. In a neat hand, a court clerk writes in medieval Latin that they heard Maiota Leduart confess to fornicating with John ap Gwilim ap Rhys. Margaret, daughter of Robin the Noke, was accused of the same. John, the chaplain, was said to "wander in the night with fantastic spirits." And two other extramarital, if not extra-corporeal, liaisons are chronicled. For a tiny and remote settlement on the Welsh Marches, it's quite a tally.

The original record of the church court's visitation of 10 May 1397 survives in the archives of Hereford Cathedral, though not on public view. What is on display (along with the magnificent, if slightly creepy library of chained books) is the rare medieval world map, the Mappa Mundi, produced at some point between 1289 and 1310 by a Lincolnshire workshop. In their time, such maps were a status symbol, perhaps the 13th-century equivalent of a wall-sized, flat-screen TV.

Reflecting its origin, the representation of Lincoln and its nearby rivers is precise, while Hereford is a smudge somewhere out to the west, and the rest of the world is more emblematic than cartographical. But the Mappa was intended to be an encyclopaedia as much as an atlas. It is not just the world, but a world view. It portrays both the Greek myths and the wanderings of the Israelites. At the top is the Garden of Eden.

Dead centre of the Mappa, the compass point from which the circle of the world is drawn, is Jerusalem. But it is dominated by the Mediterranean, the cradle of both Christianity and Classical civilisation. Even after seeing Kilpeck, I think I am not imagining things when I say the sea looks like a womb, with the fallopian tubes at the top right and left, and the birth canal the Straits of Gibraltar. There is a German Mappa that shows the world as the literal embodiment of Christ. The Hereford Mappa seems to be its female equivalent.

To complete my immersion in the Welsh Marches' ecclesiastical history, I stayed in Llanthony Priory, 25 miles south west of Hereford, where the Welsh hills crowd into the Ewyas valley. The 19th-century hotel is built into 12th-century ruins once occupied by the Augustinian order's Black Canons. The twin spiral staircases up the tower's three storeys to the four-poster beds are a vertical labyrinth. Little more than arm's reach from the lancet windows is the skeleton of the monastery, its walls light against the dark bulk of the mountains. The only sounds on the first night were of the river and a distant sheepdog barking. On the second, by a near full moon, they were joined by someone playing the flute.

On that night, the woman who helps out in the hotel took three of us to a village birthday party. Earlier, preparing tables for dinner, she was happy to make jokes about the size of men's candlesticks. Later, she told us something about herself. She is 65 and drinks brandy and lime, and her name is - Sheila.

Fact File

From 29 June to 3 October, the Mappa Mundi will be exhibited alongside a collection maps in the new library building at Hereford Cathedral (01432 359880 for details).

Drawn from England, Ireland and Germany the maps have never before been seen together and the Duchy of Cornwall fragment, owned by the Prince of Wales, has not previously been on public display.

Enthusiasts should visit on 30 June for the lectures (pounds 12.50 including lunch) and those attending the conference (pounds 180 plus accommodation) can see the Hereford map out of its case.

At Llanthony, the Abbey Hotel's five double rooms cost from pounds 46 a night (call 01873 890487 for reservations) but the bathroom is shared between guests.

Aside from the Llanthony ruins, the Ewyas Valley is of interest to visitors for Cwmyoy church, dislocated by a landslip, and Capel-y-ffin, where ancient yew trees, with the girth of budding giant redwoods, dominate the graveyard.