Hundreds of years ago, on a cliff overlooking the St. Mary's River, a building burned down. Archaeologists have found broken tools from Native Americans who lived on the site, 17th-century colonists' nails, and the buttons, pottery and fragments of medicine bottles from slave quarters that were there as recently as the mid-20th century.

But they still do not know what the building was, how it was used or why it was built there, right at the edge of the ravine.

This weekend, visitors to Historic St. Mary's City can take part in an archaeological dig. They can shake soil off artifacts found, look at pieces of the past and learn to read the layers of dirt like a history book -- or a detective story.

"We hope they will be able to understand the way archaeology is conducted," said Henry Miller, director of research for Historic St. Mary's City, "but also get a sense of the thrill of discovery."

Tidewater Archaeology Days bring visitors from across the country and from just around the corner. Some children started coming as toddlers and have grown up making this an annual tradition.

"We get them up on bales of hay, and they find artifacts," Miller said. "Even a brick fragment -- knowing it's 350 years old is interesting for people."

It is not just for children, said Tim Riordan, chief archaeologist there. "The kids just love finding things. For adults it's something more -- a way of touching the past."

From 10 a.m. to 4 p.m., archaeologists will be working on excavations at the site and leading tours. For $7.50 (less for children and seniors) visitors can not only watch but help find things. They can help shake the dirt off pieces that archeologists will take back to the lab for study.

At the Print House site -- where the mystery building will be explored this weekend -- archaeologists have found charcoal and well-preserved nails rather than the typical lumps of iron found at 17th-century sites. The nails probably got so hot during the fire they turned from iron to steel, Riordan said. Archaeologists also have found bits of plaster, even one with what looks like a plant inscribed into it.

They are hoping to find the postholes that would show them exactly where the building stood. Those show up in layers of bright-orange dirt as darker areas, where organic material mixed in with the soil and changed its appearance.

Riordan said they will show off the layers of soil that illustrate how the land was filled in when the structure was built and how more fill was piled on top after it burned. "It's multicolored and layered -- it's pretty spectacular," he said. "There are very distinct layers that tell us something about the history of the site. Right at the contact between those two layers we find all the burned material."

Visitors can also take a rare tour of the archaeologists' lab and see things such as tobacco pipes, seals from bales of cloth that were essentially tax stamps and weights that were used to determine whether gold and silver had been shaved off coins, making them less valuable. Visitors also can see a lead coffin and a sheep's skull uncovered just last week in the lab. "I suspect this was from a flock of sheep given to Lord Baltimore by Virginia's governor in 1639," Miller said.

One year a piece of German pottery was found during Tidewater Archaeology Days, with a stamp on the bottom that said "1646." "That's pretty cool," Miller said.

This year, because the excavation site was used over so many centuries, he guaranteed people will find things.

"This is one of the best places in the country to be an archaeologist," Riordan said.