National Park Service archeologists have unearthed the remains of a house built in Prince George's County in the 1690s, which they say is the oldest structure ever discovered in the Washington metropolitan area.

The wooden structure, which largely burned down at the end of the 17th century, was discovered last fall in the southern part of the county near Oxon Hill.

It wasn't until the end of last week, however, that excavators could date the structure and clearly define its dimensions.

"We used to refer to this as the national capital region's best kept secret," said Stephen Potter, regional archeologist who is overseeing the dig.

The archeologists have found more than 25,000 fragments of history -- including parts of pipes, stones used to ignite muskets, Delft stoneware and European beads -- that offer a rare glimpse of a time when the colonial frontier extended only as far west as what would later become the nation's capital, according to Potter.

"A lot of people whipping around the Beltway or across the Woodrow Wilson Bridge have difficulty imagining that this was once the American West," said Potter, "but indeed this would have been the western frontier of the colony of Maryland in 1690."

Until this discovery, according to Potter, the oldest structures found in the Washington area were the remains of the 1710 Addison House and its outbuildings recently unearthed in a corner of the county's proposed waterfront development, PortAmerica.

Park Service diggers made their latest find on the grounds of Harmony Hall, an 18th-century Georgian estate owned by the Park Service and leased last fall to a private firm. The discovery was made during a routine survey of the site to ensure that plans to lay new utility wires and pipes did not interfere with any historically significant aspect of the Harmony Hall property.

Most of the actual work at the site has been done by 26 volunteers.

They excavate units five feet square and two feet deep at a time, uncovering artifacts that are washed in a kitchen house adjoining the mansion, then dried, bagged and sent to the Park Service laboratory in Greenbelt, where they await further cataloging and analysis.

An artifact's age is determined by comparing it to other items from the same time period, said Robert C. Sonderman, site supervisor. He likened the process to dating and identifying a 1957 Chevrolet by its tailfins.

Excavators believe the structure they found was originally built about 18 feet wide by 32 feet long.

They call it an "earth-fast house," or a building without a foundation where corner poles are inserted in the earth.

Along with the fragments, they found a pit in the middle of the site that Potter said was probably once in front of a brick hearth. They also found bones of wild turkeys, boar, pigs and cows consistent with life on the Maryland frontier.

At first, the excavators were thrown off by Indian arrowheads they found near the surface of the site dating to 2,000 B.C. The arrowheads were found in dirt that apparently had been carried up from the Potomac river banks by Harmony Hall's last private owner in this century.

The land on which the early house was built was part of a 1662 patent to a Humphrey Haggett. The "We used to refer to this as the . . . region's best kept secret." -- regional archeologist Stephen Potter land was later subdivided. In 1696 the 100 acres containing the newly found structure were probated as part of the estate of a Richard Lewis, according to Park Service historian Marilyn Nickels.

After two centuries, Harmony Hall was acquired in 1928 by Charles Collins, a deputy comptroller of the currency who went on to practice law in Washington and who became one of the architects of the segregationist and states' rights Dixiecrat Party, which bolted from the Democrats in 1948.

The federal government bought the property in 1966 as part of a right-of-way for the George Washington Memorial Parkway on the Maryland side of the Potomac, which was never built. Collins' widow was allowed to remain in the house until her death in 1983.

Under a new program to care for property for which there is no public use, the Park Service leased it for 55 years last fall to the Battersea Corp., which is restoring Harmony Hall.

Plans call for opening a training school for carriage drivers this fall, according to Battersea's H. Carlton Huhn.