On a visit to Chalgrove's ancient, half-timbered manor last summer, I was shown some burn marks in the oak timbering by a window. These, I was told, were witch marks', scorched by the carpenters into the building to protect the occupants from evil.

Such devices are fairly common in old houses.

Such devices are fairly common in old houses. It was once thought that witches could enter premises disguised as cats or frogs and cast spells on people while they slept.

To ward them off, ritual signs were placed near windows, doors, fireplaces and other places of entry.

Strictly speaking they are apotropaic marks'. The word apotrope is of Greek origin, literally meaning turning away' and applies to any amulet or other symbol designed to deflect evil. English house-builders employed a wealth of techniques to protect against witches and other ill-wishers. Sometimes they marked ceilings with magical symbols written in candle smoke.

Alternatively, a well-worn shoe or the dried corpse of a cat might be hidden in the chimney breast or roof cavity. Witch bottles' made of stoneware or glass were buried beneath the hearth or near doorways - filled with urine, bent pins and nail clippings.

The lucky horseshoe, seen today on greetings cards and wedding cakes, is a survivor from this tradition. In Tudor and Stuart times, it was set at the door specifically to protect against witches and, as Joseph Blagrave's Astrological Practice of Physick (1671) reports: to afflict the Witch, causing the evil to return back upon them'.

Today, when the idea of evil as a dynamic force is less fashionable than in times past, the lucky horseshoe has become a simple good luck charm.

Were Green Men - like horseshoes and witch marks - originally intended to ward off evil, protecting the house of God?

The mysterious faces are often seen in our older parish churches, either formed of leaves or disgorging foliage from mouth, eyes or nostrils.

As part of the Faces of Oxfordshire initiative in this year's Oxfordshire 2007 celebrations, we are exploring the examples that can be seen around the county.

Green Men are often described as pagan fertility symbols' and yet some faces are so markedly aggressive, glaring and baring their teeth, that it is hard to interpret them as emblems of fruitfulness.

High over the nave at Fulbrook's Church of St James the Great, a group of foliate faces burst from the timber roof beams. The church dates back to Norman times but was remodelled in the 13th century and the carved roof timbers may be from that period, or perhaps later. One grotesque head is disgorging leaves, another grins and bares pointy teeth. A third is a crop-haired medieval boy (left) poking out his tongue with all the crude comedy of a Beano cartoon.

A couple of miles to the north, at the Church of St Mary the Virgin, Shipton-under-Wychwood, is a medieval font adorned with further foliate heads. These are nobler, stone carvers' creations - but again, one face is conspicuously sticking out a tongue.

Shipton's sculpted font can be dated with greater precision than the Fulbrook wood carvings. It is decorated with eight images, three of which are Green Men.

Other motifs show the bear and ragged staff (badge of the Warwick family) and the rose en soleil (emblem of Edward IV). So the font dates in all likelihood to between 1461 and 1470 when the King and Richard Neville, known as Warwick the Kingmaker, were allies.

It seems deeply unlikely to me that the carvers in these two Cotswold churches were secret worshippers of ancient pagan deities. It would be even more extraordinary if the church authorities who commissioned the carvings were their collaborators.

More likely, the clerics and craftsmen were drawing on a general body of folk superstitions concerning fertility, magic and auspicious symbolism. These co-existed with Christianity, constituting a powerful sub-culture of beliefs.

No pagan priesthood promoted the superstitions. But from medieval times almost to the present day, every rural neighbourhood had its local herbalists and fortune-tellers, wise alike in natural medicine and astrology.

The widespread terms were cunning men' and cunning women.' They might be called on to heal sickness in the family, to cure ailing farm animals, to make love potions, cast horoscopes, and so on.

The cunning folk were not generally seen as witches or wizards themselves. But you could say that they dealt in Green Magic'. It was certainly their role to diagnose witchcraft, to identify evil-wishers and to thwart them through their own counter-spells. Witch bottles and lucky horseshoes were very much their stock-in-trade.

Especially widespread was fear of the evil eye. A belief that certain people can inflict disease or death simply by a glance was accepted even by educated people throughout medieval and Tudor times.

The malefactor might not even be aware of possessing the trait, but do evil accidentally. In churches, a grotesque face, baring teeth or sticking out a tongue, might well have served as a counter-spell to baffle ill-wishers; to show that the place of worship was protected by a magic of its own.

Tongue-pokers are recurrent motifs among the Green Men of Oxfordshire. Besides Fulbrook and Shipton, striking examples can also be seen in Standlake and Merton College Chapel. Those fiercely baring their teeth are also plentiful. The Green Men at Chipping Norton, Cuddesdon and Stanton Harcourt come immediately to mind. In a wider sense, it may be that the conspicuous display of carved vegetation was itself a form of counter magic.

The evil eye was especially said to desiccate - to wither crops, or to dry up wells - and a leafy face or head spewing foliage might signal defiance of the ill-wisher.

There was, in addition, a magic in the greenery itself. Many superstitions relate to the bringing in and taking out of boughs from the greenwood.

Hawthorn blossom was auspicious as decoration for the front door on May morning; but to bring it into the house at any time might bring a death. Holly, ivy, bay and mistletoe were all brought in at Christmas, but all evergreen boughs had to be taken out of the hall afterwards. The correct date for removal was Candlemas Eve (February 2) according to the poet Robert Herrick, who reported it must be done: That so the superstitious find No one least branch left there behind: For look, how many leaves there be Neglected, there (maids, trust me) So many goblins you shall see.

Who knows but that some Green Man faces were intended to represent the very goblins who might approach the church with mischief in mind. They would be confronted with their own image - and shrink back. I do not believe that there is any simple solution to the Green Man enigma. But I cannot help wondering whether some of the heads carved in rural parish churches were sited there after a representative of the church had a quiet word with the local cunning man.

You do not have to believe that the parties involved were in any sense pagan worshippers - they were just taking precautionary measures, like the church-going builders who scorched witch marks on house timbers, and farm workers who nailed horseshoes above the cottage door.

WARDING OFF EVIL Belief in the evil eye was deep-rooted and widespread in Europe. For the Ancient Greeks, protective devices often took the form of an eye, painted on drinking vessels, to prevent harmful spirits from being swallowed with the wine.

The evil eye was known as oculus malus to the Romans, and even today in the Mediterranean and Aegean, amulets and talismans are sold as protection against mal occhio.

It has been suggested that the evil eye belief is rooted in a primal aspect of human behaviour whereby dominance and submission are shown by gazing and averting the gaze. If we feel uncomfortable when people stare at us, it is because their glare implies a desire to gain control.

Protective devices often employ counter-magic to throw the glare back at the ill-wisher.

At Grey's Court, the great, gabled Jacobean mansion north-west of Henley-on-Thames, you can see a specimen of a witch ball'. These were manufactured in England in some numbers during the 18th and 19th centuries. The spheres of plain or coloured glass were hung in windows so that their lustre would attract passing witches who would then be neutralised by their own reflection.

The ferocious gaze of some Green Men carvings may derive from this tradition. Green vegetation evokes the vital life force in nature. To place a face with glaring eyes on a building would challenge and confuse the ill-wisher - all the more so if the face were streaming greenery from the mouth, eyes or nostrils.