In muddy boots and a hard hat, Pam Cressy does not exactly evoke the image of Indiana Jones, the dashing archaeologist in ''Raiders of the Lost Ark.''

But she is not without her adventures, tramping over a modern-day Alexandria construction site with a respected dendrochronologist (who studies tree rings), trying to glean quickly all information possible from a newly discovered 18th-century wharf before it is paved over with a condominium parking lot.

"All of this below North Fairfax Street used to be under water," she said, pausing to survey the rippling waves on the Potomac a block away. With her finger punctuating the air, Cressy, a professional archaeologist, sketches how Old Town Alexandria looked in the 1700s -- a bay curved here, a street there -- while pointing out where recent digs have been made and what was uncovered. It is an archaeologist's-eye view of prime real estate.

"Old Town is rich in history," she said. "It is one big site."

Northern Virginia is rich in history, archaeologists say, both as a center of activity in the 1700s and as a native American camping ground as many as 12,000 years ago. But the development of the Washington area in recent decades has turned their job into a game of hide-and-seek, hunting out potential archaeologically rich sites from under parking lots and behind 20th-century homes.

They may be consoled somewhat by community and local government support of their search for historical artifacts. The Fairfax County government employs two professional archaeologists and Alexandria has three, rare for a city its size. New York City has only one archaeologist.

There also are volunteers: Hundreds of them throughout Arlington and Fairfax counties and Alexandria devote long hours to digging and researching Northern Virginia's past.

In the Alexandria Archaeology Research Center on King Street, the early morning sunlight washed over volunteer Peggy Weiss' table, covered with bits of rat bones and battered pottery shards, broken glass and de cayed metal. All are treasures to Weiss.

Carefully excavated last summer from a well in Old Town, the items are stored and numbered, then carefully pieced together like an intricate puzzle to form a story, a tale of a 19th-century family with rats and cats, old dishes from Europe and empty whiskey bottles, nails that bent when hammered and worn-out tools.

"You feel very close to history here," said Weiss, fingering a shard of pottery. "Once I started coming here, I got hooked. It's an engrossing business. I like to think about the people who used to live here and the people who live here now and how alike we all are."

During summer months, volunteers sift through the dirt of an estimated 350 sites in Northern Virginia looking for items the uninitiated might term junk.

"Okay, we've never found a King Tut tomb out here in Fairfax, but we've found things that are significant to the people who live here," said Ed Chatelain, one of Fairfax County's two full-time archaeologists.

Chatelain has spent the past three years excavating the site of a 19th-century farm in Centreville owned by the Walney family. Mike Johnson, the county's other archaeologist, is working on a site in Springfield that was the home base of native Americans thousands of years ago.

"A lot of sites used to be ignored and destroyed," Chatelain said. "But people are becoming prouder of their American history. I don't think anyone wants to stop all construction. But they do want to know what was there before they build on top of it."

By law, artifacts belong to the owner of the property on which they are found; only the discovery of human bones must be reported to authorities.

That is why Alexandria archaeologists worked closely with the private construction workers who uncovered the wharf in the 100 block of Cameron Street. The wharf backs up to the historic home of John Carlyle, who is believed to have built the wharf about 1760.

"We knew it was there because we had a sketch of it," Cressy said.

The dirt over the site has been sifted for artifacts and the wharf studied. It will be buried again, probably this week, as construction progresses on the condominiums, which will replace two buildings of a World War II torpedo factory.

"Alexandria is very good about preserving its history," she said. "They agree to keep some and change some. There is always a balance."

Alexandria, where all of Old Town has been designated a historic district, started its archaeology program in 1972. Fairfax's began in 1977. Before that, residents who stumbled over artifacts called in professionals from local universities or amateurs with good reputations.

Martha Williams, a history teacher at Fairfax County's Marshall High School, had just completed an archaeology seminar at Colonial Williamsburg in 1973 when Fort Belvoir officials told her they had discovered some old stone foundations. The old buildings turned out to be those of William Fairfax, who owned a 2,000-acre estate next to Mount Vernon.

"Talk about trial by fire," Williams said.

The site was turned over to professionals and placed on the National Register of Historic Places. Williams, her colleague John Hiller and her students have since helped excavate an estate that was owned by a relative of Robert E. Lee and the home of a farmer in Falls Church.

"Historical archaeology is different from what people perceive it to be," Williams said. "Half of your research is in the library looking up old papers, the other half is digging in the hot sun.

"You have to love it," she said, leaving no doubt that she does.

Archaeologists say they are lucky when local residents or workers ask them to check a site.

Earlier this fall, for example, Temple and Marty Moore called the Alexandria center after renovators discovered turn-of-the-century artifacts in the basement of their North Fairfax Street home. Archaeologist Barbara Magid and volunteers spent a week digging up a variety of bottles, toys, plates and other items.

But archaeologists say some construction workers simply stash away guns, silver and bottles they find while renovating old buildings. It is worse for archaeologists who specialize in native American artifacts.

"You would not believe the thousands of years of history a bulldozer can ruin in a few seconds," said Scott Silsby of Arlington, an amateur archaeologist.

Silsby combs the county for spearheads, soapstone tools and bowls. He said Arlington was frequented by native American tribes because the fishing near Great Falls was good. Still, the county has less historical significance than its neighbors, he said, because Alexandria was more a center of colonial activity.

"Why do I love it? Because it is a mystery," he said. "You can be your own Ellery Queen. You have a chisel, a bowl and some rocks and from that you must construct a society. There are no written records, only clues."

Cressy concedes that "a lot of people would consider us a luxury. But I think most are proud of this program. We are a role model for other communities, and you don't destroy something unique you have created."

"We've never found a real museum piece in that sense," said Cressy, surveying the patched-together bowls and colored bottles displayed in the archaeology center.

"But those things never interested me as much as the common things. I get a real feeling when I find a shoe with a worn-down heel because it makes that person back in time very real to me. I think that's important for us as a society to see where we've come from."