Edward Johnson, a senior archeologist with Thunderbird Archeology scrapes mud from a section of timber in preparation to cut the piece on Monday. After removal from the site, large beams are impregnated with a chemical to remove the water then placed in a freeze drier. Archaeologists working on the site for a new hotel/condo project on river front have uncovered big timber remains of the first warehouse in Alexandria from 1750s. (Kate Patterson/For The Washington Post)

Construction crews preparing Alexandria’s waterfront for its 21st-century future have discovered a major remnant of its 18th-century past: the timbered foundation of a warehouse believed to be the city’s first public building.

The huge beams, intact floor planks and what might be a repurposed ship’s mast emerged from damp clay and sand eight feet below ground at 220 S. Union St., a site that is slated to become the 250-room Hotel Indigo, owned by Carr Hospitality.

Francine Bromberg, the city’s archaeologist, called the remnants “one of the most significant finds” uncovered along the Potomac Riverfront.

“We are in­cred­ibly excited,” said Bromberg, wearing a hard hat and bearing copies of early land plats as she watched workers Monday scrape excess soil from the wood and prepare it for removal and preservation. “It gives us a sense of what the city was like in the 18th century.”

Nichole Doub, Head Conservator with the Maryland Archeological Conservation Laboratory prepares a sling to lift out one of the large section of timber in Alexandria on Tuesday. (Kate Patterson/For The Washington Post)

The discovery thrusts observers back to the early years of Alexandria’s long and rich history.

John Carlyle, a Scottish immigrant, town trustee and major landowner, was tasked with building a warehouse on the Port Lumley sand flats, one of two spots where the deep-water channels of the Potomac River approached the city’s shoreline, and to extend Duke Street to the site.

The 100-by-24-foot warehouse was constructed in 1755. Carlyle later submitted a bill for 259 pounds, 11 shillings and a half-pence in expenses, but made clear that he had already collected 198 pounds, 4 shillings and 2 pence from renters.

As the city grew, Alexandria became a major port, including for the slave trade. Exports, according to a city history, included flour, hemp and tobacco. By the end of the 18th century, the town was one of the 10 busiest ports in the new nation.

The last known tenant at the warehouse — a brewer — was recorded as holding a 10-year lease in the 1770s and 1780s. Bromberg speculated that the building may have been destroyed not too long after that.

The foundation lay underground for years — not quite forgotten because it was on old plat maps, but not quite remembered, either, because no one expected that the wood beams would have survived in such good condition.

The Union Army stored grain and hay in a later building on the same site during the Civil War, and a fertilizer company bought the property in 1893. In 1897, fire swept through the building and other wooden structures along the waterfront, burning many of them down.

Modern warehouses eventually were built. One set was demolished earlier this year in preparation for construction of the hotel, part of the city’s long-running waterfront redevelopment plan. The hotel is slated for completion in March 2017.

The timbers may not be the last important archaeological discovery made during the transition of the riverfront from warehouses to parks, hotels and homes.

Across Duke Street is Robinson Terminal South, a huge warehouse that sits atop an 18th-
century shipbuilding site and is scheduled to be replaced by a mixed-use condo and retail project.

Six blocks north, the Robinson Terminal North warehouse will also be demolished for a hotel-residential-retail complex. That warehouse is on a site known as West’s Point, where a tobacco inspection warehouse was built in 1732 and where British General Edward Braddock came ashore during the French and Indian War.

Early Monday morning, at the muddy Union Street location, backhoes beeped warnings and workers stepped carefully over the 260-year-old pieces of wood, each of which had been tagged, mapped and photographed for posterity.

The timbers, which measure 12 inches high and 12 inches across, were cut into 12-foot lengths so they can fit into the largest treatment chamber at the Maryland Archaeological Conservation Laboratory in St. Leonard, Md.

They will be soaked in water and polyethylene glycol to stabilize the cell structure of the wood, then freeze-dried to remove the moisture, enabling the timbers to retain their shape, lab director Patricia Samford said. The process could take two to three years.

Bromberg said she does not yet know whether, how or where the foundation will be displayed afterward.

“It couldn’t be left in place,” she said. “We know it’s so important and significant to the city that it should be conserved. We’ll figure out where and how after we get this out.”