For one brief, glittering moment yesterday, the ruins of the Alexandria waterfront as it was during George Washington's lifetime were exposed to 20th century air.

But by last evening, the dig again was covered with earth, the victim of an oversight by city archeologists and a construction schedule for an office complex being erected on the site at Union and Queen streets in Old Town.

Frank Reeb, an archeologist with a private firm in the District of Columbia, was strolling in Founder's Park at the edge of the building site on Thursday when he spotted pilings from old wharves and oyster shells. When Reeb investigated, he also found old timber and bricks.

The remains, said Reeb, were a waterfront building that stood sometime between 1750 and 1780. "This is probably the most significant site on the waterfront," said Daniel Koski-Karell, head of Karell Archeological Services and Reeb's employer.

A plea by Koski-Karell to the developer, Peter N.G. Schwartz Companies Inc., brought a brief reprieve -- the daylight hours of yesterday -- to unearth any artifacts. Schwartz employes cited a need to stick to their construction schedule, but also noted that city archeologists had surveyed the site earlier and dismissed it as insignificant.

"We hadn't seen anything except the remains of a 20th century structure. We had no indication until we heard this," said Steven Shephard, assistant archeologist for the city.

Although Reeb said he notified the city of his find, officials were absent yesterday as Reeb and others frantically sifted through the ruins.

Jean Federico, director of the city's Office of Historic Alexandria, said yesterday the city can't afford to be at every construction site. "They'll build over the top of [the 18th century structure]; nothing will be lost," Federico said. "I think the bottom line is how much can be done."

Yesterday, Koski-Karell, Reeb and Luis Ortiz, another archeologist with Koski-Karell's company, were wedged in a three-foot-wide ditch, between the concrete foundation of the new office building and the walls of what they believe was the basement of an old warehouse. A fireplace also was exposed.

The firm, which worked on a volunteer basis, contracts with developers to do archeological surveys before the digging begins.

The site of the dig, eight feet below Union Street, was unearthed Thursday afternoon by construction workers digging a ditch around the foundation of the 77,000-square-foot Harbor Center, a four-story office, retail and restaurant complex scheduled to open in June.

"There are at least five strata of cultural episodes above the original foundation," Koski-Karell said.

George Washington surveyed the Alexandria waterfront in 1749, two years after the city was founded. At that time, the waterfront extended just past Union Street, which was known then as Water Street. As larger ships began plying the river, Alexandria merchants gradually filled in the old waterfront to reach the deeper water, until the riverbank extended 100 yards from its original site