The backhoe arrived on Oronoco Street in Old Town at noon, rumbling into position atop a pile of dirt before tearing into the earth.

Soon crews will begin building a 169-unit luxury condominium complex on the 2.5-acre site in northwest Old Town. But the heavy equipment pushing dirt around the property recently was not there to shape the land's future. It was there to uncover the past.

It has been about 75 years since the historic mansion that once stood on the property was removed and shipped, brick by brick, to New Jersey, where it was restored. Although the house is gone, historians have records that trace in detail the physical evolution of the property long ago named Colross, as well as its previous owners -- merchants, city mayors and, perhaps most eminent among them, Thomson Mason, grandson of George Mason of Gunston Hall.

While that wealth of knowledge might have been enough to satisfy planners in other cities, it was not enough for officials in Alexandria, where historical preservation is a priority; the city employs a full-time archaeological staff, which runs the oldest and largest city-funded program of its kind in the nation.

So, in March, a major excavation of the site was begun at the behest of the city, which believed the land had enough historical significance to merit a closer look before the complex was erected.

So far, the project's developer, Diamond Properties LLC, has paid a consultant about $100,000 to explore the land for historical artifacts and ensure that burial plots have been removed.

While not much has been recovered in the way of artifacts, historians say the long dig has given them a clearer view of life on Colross. For example, they discovered an underground cistern that served as a water-filtration system, and evidence that slaves, known to have been kept at the property, lived in outbuildings on the land.

The mission is more than just a kind gesture on the developer's part -- it's a city zoning requirement.

In 1989, city leaders passed an archaeological protection ordinance to safeguard the city's history from ground-disturbing excavations. Alexandria is one of a few cities nationwide to have such legislation, which requires that construction sites of a certain size and in certain areas be examined before building begins.

Sometimes the inspections are brief. Other times they involve months of digging, as is the case at Colross, where the developer hired R. Christopher Goodwin & Associates Inc. of Frederick, a cultural resource management firm, to conduct the archaeological review.

Although the ordinance does not stop developers from building new structures, it does prevent a site with historical significance from being overrun by bulldozers before it can be examined for artifacts.

"A lot of developers view [the cost of archaeology] just as something they have to add on" to the project, said Steven J. Shephard, assistant city archaeologist. "It's not that they're happy about the extra expense, but when they build in Old Town, they know it is part of the process."

As a result, archaeological inspection has become a staple of development in a town that counts George Washington and Robert E. Lee among its former residents and believes that preserving its history is key to tourism and a healthy economy.

Alexandria started the nation's first urban archaeological program when, in the mid-1960s, members of a committee of 100 residents each pledged $10 a month to hire specialists to examine sites being excavated for construction.

That committee evolved into the current Alexandria Archaeology office, which has a fiscal 2006 budget of $487,000. Developers hire outside firms to do the archaeological work, and officials from Alexandria Archaeology oversee each of the estimated 15 digs conducted in the city each year. Officials say only a few of those are comparable in size to the Colross project.

The digs have produced many finds over the years: Hundreds of thousands of pieces of Colonial and 19th-century pottery, a 1759 wharf, the remains of communities of freed blacks -- one of the largest of which, known as Hayti, is near the Wilkes Street tunnel -- as well as a tide lock on the old Alexandria canal that once linked the city with Georgetown.

The Colross property, bounded by North Fayette, Oronoco, Henry and Pendleton streets, was purchased two years ago by Diamond Properties with plans to build the Monarch Condominiums complex. The luxury units are scheduled to be ready for buyers in the summer of 2007.

Kevin Palka, project manager for the development, described the archaeological process as "costly" but said his company built time into the construction plans to accommodate the historical excavation and so keep the project on schedule.

"It's a well-known fact you have to go through these hoops," Palka said. "But we were prepared, and we knew what we had to do. The city has cooperated, and we've cooperated in return. It's been a fairly pleasant process."

What was left of Colross's brick foundation was buried for a half-century beneath a reinforced concrete slab. On top of the slab were Andy's Car Wash, which opened in 1951, a Dominion Virginia Power substation and the sprawling Hennage Creative Printers facility.

Before archaeologists could go to work, the existing structures had to be torn down and the concrete foundation lifted. Archaeologists discovered Colross's original basement floor, its brick herringbone pattern still largely intact.

Evidence of the property's exterior walls was visible along with signs of a smokehouse, stables and burial vault. Historians say the discoveries will help them further understand the property's evolution from a rural plantation to a more urban estate.

Historians say Colross was initially developed as a residence by John Potts, a prominent Alexandria merchant, who started construction of a brick house on the property in 1800. Potts ran into financial difficulties and offered the unfinished house for sale. The buyer was Jonathan Swift, a noted merchant and Freemason, who purchased the house in December 1803 for $9,000. Swift presided over the City Council from 1822 to 1823.

After Swift's death in 1824, Colross was purchased by Thomson Mason, a respected lawyer, judge of the Criminal Court of the District of Columbia and mayor of Alexandria. Mason made extensive modifications and additions to the house before his death in 1838.

William Smoot, a lumber merchant and mayor in the 1880s, lived in the house with his family from 1885 until 1917, when William Hoge, also a lumber merchant, acquired it. A warehouse complex and other structures associated with Alexandria Hay & Grain were built on the northern half of the block, beginning the land's conversion from residential to industrial.

Both the house and the warehouse suffered significant damage from a tornado in 1927. Shortly thereafter the house was bought by John Munn, who transported it to New Jersey. There it was refurbished, and today it serves as the main building of Princeton Day School.

Given the extensive development on the property in the 20th century, archaeologists were surprised to find architectural elements of the house still visible, such as a basement drainage system and evidence of an ice chamber under the front of the house.

"You just don't know what's there until you look," said Lori Ricard, who is Goodwin's site superintendent for the Oronoco excavation.

Standing in the hot sun last week, dust covering her hiking boots, cargo pants and T-shirt, Ricard guided a backhoe onto the lot to help fellow archaeologists remove the last of the soil from the foundation.

The excavation is scheduled to be completed this month. It will take weeks to analyze the information, which will be added to the home's historical record, officials said.

Then Colross will be committed to memory forever, its remains dug up and disposed of to make way for the condo complex.

The house may be gone, but Shephard said the site's excavation will have a lasting impact

"The bottom line here is we're not destroying the knowledge of the city's past," he said.

Archaeologists Steven J. Shephard and Lori Ricard confer at the Colross property site.