Walk in springtime along the river at Godstow and you are likely to hear nesting rooks cawing high among the trees on Trout Island'. Or are they rooks? According to local lore, the age-old rookery's tenants are spirits of the nuns who, in mediaeval times, inhabited the neighbouring nunnery.

Lying on the banks of the Isis, about two miles north of Oxford, the ruined chapel is the only building surviving from the convent of St Mary and St John the Baptist. The chapel (dedicated to St Leonard) stood at the south east corner of its inner cloister. You can still see remnants of a squint high in the north wall, which once looked down into the chancel from a first-floor chamber in the nunnery building. Some decades ago, mediaeval floor tiles were uncovered along the bank of the nearby lock and stone coffins have also been revealed by erosion.

Godstow nunnery has a unique mystique.

It was founded in 1133 by the pious Dame Ediva who, while living a holy life at Binsey, one night heard a voice telling her to rise and go where a light from heaven touched the ground.

There she was to establish a nunnery for 24 of the moost gentylwomen that ye can find.' It was evidently at Godstow north of Binsey that she saw the light, as after telling Henry I about her vision she won royal support to build her religious establishment there.

The convent was dedicated in 1139 in the presence of the king, the queen and the archbishop Canterbury. It was always an aristocratic establishment to which many noble families sent their daughters. Godstow was the burial place of Henry II's mistress, Fair Rosamund' who died in 1176 poisoned, some suspected, by Henry's jealous queen, Eleanor.

Whatever the truth of the murder story, Rosamund was buried in the nunnery church and the grief-stricken king afterwards lavished riches on the convent. Rosamund was laid to rest before the altar where her tomb was revered almost like a shrine. This idolatry scandalised visiting churchman, Bishop Hugh, who later ordered her body to be moved from the church and re-interred outside its walls.

A wealthy convent, holding land in 17 counties, Godstow was, by and large, a respectable and well ordered establishment. But charges of licentiousness would be levelled against the nuns in the later Middle Ages. In 1432 an official report declared: Oxford scholars brag that with the nuns they may have junketing of every sort, to their heart's content clerks of Oxford are to be utterly excluded from the nunnery'.

Orders were further given that there should be no more exchange of letters and tokens between nuns and scholars. Nuns must not go into Oxford, or speak to strangers in church or chapel unless in the presence of another nun. Drinking should also be curtailed.

However, the new regime seems to have been hard to enforce. In 1445 a nun of Godstow eloped with a monk of Eynsham, and the abbess complained to visiting church officials that nuns were again going into Oxford, scholars were lurking in her cloisters and she could not stop it.

A gold ring of about 1450 was discovered at the site of Godstow nunnery and is now in the British Museum. It is inscribed Most in mind and in my heart, Lothest from you for to depart'. Was this one of the proscribed tokens exchanged by clandestine lovers at the convent?

In due course, discipline would be restored at Godstow, and by the time of the Dissolution of the Monasteries it had recovered its reputation. The nunnery was suppressed in 1539 and, fortified during the Civil War it was accidentally destroyed by fire in 1645.

What survives is the picturesque ruin of the chapel, and the memory of Fair Rosamund. Her fate inspired the Reverend John Brand's poem On Illicit Love written among the Ruins of Godstow Nunnery near Oxford (1775). And it has caused many another imagination to roam. In the 19th century, poet Robert Southey spoke for many when wrote of pacing the chapel ruins: With sensations such as the site of Troy or Carthage would inspire; a spot so famed by our minstrels, so celebrated by tradition, and so memorable in the annals of legendary, yet romantic truth. Poor Rosamund!' In April 1944, as Allied preparations were being made for the D-Day landings in Normandy, some children playing by the river at Godstow discovered a stone coffin.

It lay four-feet down, protruding from under the towpath in the west bank of a new cutting between the wall of the old nunnery and the garden of the Trout Inn.

The foot of the coffin pointed east and its top was about six inches above water level. Inspection revealed that the lower half of the skeleton had been removed by the children, but enough bones survived for them to be identified as those of an adult female. The rest of the coffin lay under the towpath and could not be excavated. The bones were replaced in the coffin and the excavation filled in.

Coffins have more than once been seen in the river bank at Godstow nunnery. Mediaeval floor tiles have also been found at the site. Both inlaid and printed, often bearing Fleurs-de-lys motifs, the tiles are now in the Ashmolean Museum. But the coffins, presumably mediaeval burials at the convent, have proved more of a challenge to investigators. They appear through erosion, perhaps especially when the water level is low. But with the ceaseless flow of the Isis and its cloudy sediment they appear to vanish again.

Some years ago, when the Environmental Agency was carrying out work on the lock at Godstow., divers went down to look for the missing coffins. They probed the riverbed and searched with underwater cameras, but nothing was detected.

Once dislodged by the natural force of the river, coffins may have sunk further into the river, or fallen apart and been carried away. It remains possible though that more will be revealed at the whim of the capricious river.