Nothing pleases archeologist Pamela Cressey more than a vintage trash pit, but rapid real estate development in Old Town Alexandria keeps Cressey on her toes as much as on her knees.

As Alexandria's full-time urban archeologist - "a professional who elects to dig in parking lots," as she says - Cressey, 32, had to win countless delays to postpone excavation for a new federal courthouse in 1977 and '78 until she had dug for relics on the site.

Now Cressey''s Alexandria Archeology Center, which runs on a $140,000 yearly budget furnished by the city and the Interior Department, is digging on an Old Town site occupied continuously by black residents since well before the Civil War.

But as in the case of the courthouse site - which yielded thousands of objects used and discarded by Alexandria's white 19th century merchant class - the diggers are one step ahead of the builders.

The city-owned site near the intersection of S. Alfred and Gibbon streets, the edge of town when Civil War-era blacks lived there, will be turned into town houses starting in late August.

"We knew it would be destroyed soon," Cressey said of the site, which she chose because city archives showed that the original occupants of some tracts were free blacks.

"Black people in Alexandria have roots 200 years old. There was a slave market on Duke Street up until the Civil War. By 1890, Alexandria was one-third black.

"Since blacks were settling on the edge of town (by 1800), we knew this was a black neighborhood. It's perfect for a long-term study that includes archeology and history."

Cressey and her colleagues combed through city archives for tax records, deeds, and blacks' manumission documents (the papers legally freeing slaves) to give themselves the detailed map they needed to begin work.

Working with 80 volunteer assistants, they started digging June 2 - except that digging is not exactly what archeologist do. Mostly it's a "a very laborious scraping process with trowels," Cressey said.

They began by trying to locate old wells, trash dumps and privies - all rich repositories of archeological finds because they were used as trash bins. Most of the discoveries are shards of glass or fragments of china, although Cressey said she was shocked to find the bones of a one-month-old infant (c. 1830) at the courthouse site last year.

Seeds, pits, bones and oyster shells are expected to provide information about the diets of the early Gibbon Street residents. Archeologist Paul Davidson already has noticed that oyster shells found on Gibbon Street are smaller than those unearthed at the courthouse site on King Street where white merchants once lived.

Meanwhile, the archeologists, both professional and amateur, work happily to rescue almost anything from the unsavory surroundings. "People love dirt," says Davidson." Some people couldn't wait to get down in the privies - they jump in to their knees." CAPTION: Picture 1, Alexandria's chief archeologist, Pamela Cressey, 32, is leading an effort to uncover relics of the city's past - such as the 19th-century dish she is holding - but has to scramble to stay ahead of developers. By douglas Chevalier - The Washington Post; Picture 2, Alexandria architect Paul Davidson digs in Old Town site where free blacks lived before the Civil War. By Douglas Chevalier - the Washington Post*