Dig Live, until September 9

"Is this Roman or Medieval?" a volunteer asks, clutching a piece of terracotta-coloured pottery caked in mud.

Toby Kendall, the Dig Live field officer, peers at it intently for a moment. "Definitely medieval, it's too rough to be Roman" he replies.

Dig Live is hands-on archaeology. The project takes place every year at various sites across York. This year, what is normally a nondescript patch of pebbled land (possibly a car park) off a nondescript side road behind St Saviour's Church is temporarily transformed into a haven for archaeological enthusiasts and curious minds.

Kneeling volunteers scrape away in each of the four 10ft-by-10ft ditches that have been dug out especially for the project and sheltered with tents. One pit reveals the footings of a medieval house, against which a bank is visible which, Toby believes, was used as an "occupational deposit for household waste" such as "the hearth debris and sweepings".

The next two pits, only a few feet away from one another, reveal a medieval road and a medieval wall, not to mention boxes and boxes of paraphernalia dating from the Roman, Viking and medieval periods. One of the most striking items found is a five-millimetre cube cut out of bone - a Roman die whose numbers are intricately engraved concentric circles.

Next I am shown a beautiful worked-metal book clasp, possibly medieval. Another item handed to me is a very short, pointy thin splint. "That's the end of a human thumb!" Toby adds, as he sifts through the reams of plastic bags containing the miscellany of York's past.

Who the thumb belonged to, nobody knows. It was found among rubble from the Victorian age, but may be quite a bit older, Kendall says. It may have been brought to the surface from deeper down when the Victorians cleared the site themselves more than 100 years ago.

What are, at first sight, square holes in the ground reveal an historical cross section of several centuries. "This is a fantastically complicated site showing the development of York's past of 1,000 years or possibly earlier," Toby says. He likens the process of archaeological excavation to "reading a book backwards with several pages ripped out. We want to put the pages back and see whether the book can be understood reading it from front to back".

Dig Live is a great opportunity for anyone who is interested to learn about archaeology and have a go at excavating, he says. It's also a way into the profession for archaeology students and graduates.

"We have a vast bank of knowledge to draw from with extremely experienced professionals from the York Archaeological Trust (YAT), the YAT's reference collection and the experience it has built up from more than 30 years of excavating."

Non-professionals who come along to Dig Live get a power point presentation to fill them in on what is happening plus a health and safety talk before they can get their hands dirty.

There is still time for you to have a go, if you fancy finding out what it is like to be on a real archaeological dig. Dig Live runs for another two weeks, until September 9. For details, contact the website www.yorkarchaeology.co.uk If you do give it a go, chances are you will be back. "Fifty per cent of the volunteers are returning enthusiasts." says Toby.

Clive Green is among them. He has been coming on the dig every year since 2000. "I have a very stressful job as a Wing Commander and find Dig Live extremely enjoyable, relaxing and stimulating," he says. "The people are great and you learn something new every time."

They still haven't found out who that poor human thumb belonged to, though.