In a completely unexpected discovery, excited archaeologists say they have uncovered the foundation wall from a bow window designed by George Washington for the rear of the President's House on Independence Mall.

The curved window, which enlarged the house's formal State Room, is believed by architectural historians to be the model for oval rooms in the White House in Washington, and ultimately, for the Oval Office itself.

"It's the prototype of the Oval Office," Douglas Heller, a board member of the Independence Hall Association, said yesterday as he stood looking down at the ragged top of a crescent-moon wall of rock.

"When we think of the president today, we think of the Oval Office. So what we're looking at is the prototype of what we perceive as the presidency of the United States. That's pretty big."

Archaeologists excavating the site at Sixth and Market Streets, where Washington and successor John Adams conducted their presidencies, made the discovery late Monday afternoon when they broke through a 19th-century basement floor.

The President's House, first constructed in the 1760s, was demolished in 1832 and three commercial buildings fronting Market Street were erected in its stead.

"When we broke through and started uncovering [the window foundation], at first it seemed an undefined blob," said Jed Levin, an archaeologist with the National Park Service. "We were scratching our heads. But as more of it was uncovered, an edge emerged and we thought, 'What is this?' "

Douglas Mooney, archaeologist with URS Group, the private firm conducting the dig in concert with the park service and the city, said, "The possibility of finding any foundation to the bow window was so remote to me that I didn't even consider the possibility."

As he and Levin puzzled over the feature, however, Levin said, "This [idea] is out there, but what if it's the foundation of the bow window?"

Both left the dig Monday and researched that possibility, checking plans and measurements drawn from historical records.

They returned to the excavation site yesterday, convinced that they had made a startling discovery.

"We're now certain," Levin said. "It's the bow window."

Independent historian Edward Lawler Jr. said the scholars have argued that this was the first bow window in the city, in addition to being the model for the Oval Office in the White House, which was under construction when Washington and Adams lived in Philadelphia in the 1790s.

It then became a popular feature in other buildings around the city. And it also marked the transition from straight-line Georgian architecture to a more curvilinear neoclassic style.

The current excavation is designed to determine what is left of the house - parts of its foundation have now been found about 10 feet below street level - and to learn as much as possible about its occupants, particularly the enslaved Africans held there by Washington.

The dig will continue for the next several weeks, with a public platform available for viewing, and volunteers on site to explain what is going on.