Like a roamin’ stone - boulder at Oxburgh Hall, near Swaffham, is 165m years old and was brought to Norfolk by a glacier

The mysteries behind an unusual boulder at Oxburgh Hall – known locally as the Roman Oyster Stone – are being revealed thanks to geological detective work from the University of East Anglia.

Prof Julian Andrews, from the university's School of Environmental Sciences, has been investigating the stone for the National Trust.

While it is composed of hundreds of flat oysters, his research shows that it dates back much further than Roman times.

The boulder is in fact some 165m years old. It dates back to the Middle Jurassic age - a time when dinosaurs roamed the earth.

Prof Andrews was able to date the boulder, which can be found tucked away in the far north-west corner of Oxburgh Hall's gardens, by looking at the species of oyster fossils within it.

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And he says it could more accurately be described as a 'roamin' stone' – because it appears to have strayed some 60 miles from its original home near Sleaford.

The closest similar boulders can be found exposed in a narrow band in the East Midlands – on a gently curved line between Northampton in the south and Lincoln in the north, with known oyster bearing beds in the Grantham and Sleaford area.

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'It is highly likely the boulder originated from hereabouts and this tells geologists it was moved by ice flowing broadly south-east, crossing modern day Fenland in the Spalding-Wisbech area.

'The local bedrock geology in this part of West Norfolk is Cretaceous Chalk, about 90 million years old - a much younger geological unit than the boulder.

'However, Norfolk is world-famous for its Pleistocene glacial geology, especially the 450,000-year-old 'Anglian Glaciation' tills - sediments transported and left behind when glaciers melt - which mantle much of central East Anglia.

'Glaciers often 'pick up' fragments of the bedrock over which they flow and some of these fragments are large boulders.

'As the ice flows, the boulders are transported and then dumped when the ice melts. A boulder which is moved from its place of origin and then dumped, perhaps miles away, often on quite unrelated bedrock, is known as an 'erratic'.

'So perhaps not so much a 'Roman stone' but a 'roamin' stone',' he added.

It is not known exactly when the boulder moved from its original location.

'We can only say it was probably during the 'Anglian Glaciation' which most people believe happened about 450,000 year ago,' said Prof Andrews. 'But there is some controversy over this.'

The new evidence contributes to a debate on ice movement in the Wash-Fenland area which has been raging for over a century.

Teresa Squires, National Trust property manager for Oxburgh Hall, said: 'Prof Andrews' report makes for fascinating reading - not only does it dispel the myth that the stone is of Roman origin from the local river, it opens up a whole new geographical and geological story which we never suspected.

'It is now officially the oldest artefact at Oxburgh and we hope it will be of great interest to our visitors.'

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